Stanislao Bevilacqua
email: sbevilacqua@units.it

Laura Airoldi

Enric Ballesteros

Lisandro Benedetti-Cecchi

Ferdinando Boero

Fabio Bulleri

Emma Cebrian

Carlo Cerrano

Joachim Claudet

Francesco Colloca

Martina Coppari

Antonio Di Franco

Simonetta Fraschetti

Joaquim Garrabou

Giuseppe Guarnieri

Cristiana Guerranti

Paolo Guidetti

Benjamin S Halpern

Stelios Katsanevakis

Maria Cristina Mangano

Fiorenza Micheli

Marco Milazzo

Antonio Pusceddu

Monia Renzi

Gil Rilov

Gianluca Sarà

Antonio Terlizzi

Mediterranean rocky reefs in the Anthropocene Mediterranean rocky reefs in the Anthropocene: present status and future concerns

Keywords: algal forests, bioconstructions, coastal ecosystems, conservation, fish assemblages, global change, marine biodiversity

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Introduction

The unprecedented anthropogenic modifications of the global environment are leading the planet towards uncharted conditions, with the risk of crossing safe operating boundaries beyond which undesirable, and probably abrupt, ecological shifts could pose serious concerns about the future of our society [START_REF] Steffen | Planetary boundaries: Guiding human development on a changing planet[END_REF]. The pace of increasing human impact is particularly alarming for seas and oceans [START_REF] Halpern | Recent pace of change in human impact on the world's ocean[END_REF], especially in coastal areas, where human activities and populations concentrate and the current emphasis for blue economy might, indeed, ultimately harm marine ecosystems [START_REF] Bennett | Toward a sustainable and equitable blue economy[END_REF].

Reef habitats are highly sensitive to global change and have undergone a drastic deterioration worldwide, which still continues without signs of slowing down [START_REF] Hughes | Rising to the challenge of sustaining coral reef resilience[END_REF][START_REF] Krumhansl | Global patterns of kelp forest change over the past half-century[END_REF]. They are among the most productive and diverse marine environments, largely contributing to the functioning of much wider ecosystems that comprise vast volumes of marine space [START_REF] Boero | The Cells of Ecosystem Functioning: towards a holistic vision of marine space[END_REF] and are, therefore, crucial for the provision of goods and services by the ocean.

In the Mediterranean Sea, hard bottoms are of paramount ecological importance for coastal ecosystems. More than half of the shores are rocky [START_REF] Furlani | The rock coast of the Mediterranean and Black Seas[END_REF] and biogenic hard substrates may extend for hundreds of thousands hectares in shelf areas [START_REF] Martin | Coralligenous and maërl habitats: predictive modelling to identify their spatial distributions across the Mediterranean Sea[END_REF]. The variety of coastal rocky habitats is extremely high, spanning from the mediolittoral to the infralittoral and circalittoral zone, and comprising macroalgal and animal forests, rock beds and cliffs, submarine caves, and several types of bioconstructions (Fig. 1) that, along with seagrass beds, host most of the vast and endemism-rich Mediterranean marine biodiversity [START_REF] Ballesteros | Mediterranean coralligenous assemblages: a synthesis of present knowledge[END_REF][START_REF] Coll | The Biodiversity of the Mediterranean Sea: Estimates, Patterns, and Threats[END_REF][START_REF] Ingrosso | Mediterranean Bioconstructions Along the Italian Coast[END_REF]. Their complexity and the associated species, phylogenetic, and functional diversity place rocky reefs among the habitats that are often represented within Marine Protected Areas (MPAs) [START_REF] Agnesi | Spatial Analysis of Marine Protected Area Networks in Europe's Seas III[END_REF]. However, shoreline and nearshore reefs are experiencing a widespread degradation over the entire basin [START_REF] Sala | The Structure of Mediterranean Rocky Reef Ecosystems across Environmental and Human Gradients, and Conservation Implications[END_REF]Bevilacqua et al., 2020), so that the ecological condition of 1/3 to 2/3 of them might be classified as moderate to poor (Bevilacqua et al., 2020).

Understanding causes and mechanisms underlying this trend and the ensuing detrimental effects on the integrity of marine ecosystems is critical to develop future strategies that could counteract the ongoing degradation, and to build socioeconomic resilience [START_REF] Branch | Rocky intertidal shores: prognosis for the future[END_REF]Rilov et al., 2019a). In this chapter, we attempted to synthesize current information on diversity, structure and functioning of Mediterranean rocky reef ecosystems, the main human threats, and the potential implications for their integrity and persistence. We also provided an up-to-date overview of present and forthcoming conservation measures, current research needs and potential future avenues, trying to envisage what could be the fate of Mediterranean rocky reefs in the coming years.

Mediterranean rocky reef communities: structure and dynamics

Patterns and processes in benthic assemblages

Mediolittoral and infralittoral zone

The Mediterranean Sea should be classified somewhere between a subtropical (mostly the southeast basin) and a temperate sea (mostly the western basin). The seasonal variability in temperature, light availability and dissolved nutrient concentrations are similar to temperate environments but the average values are closer to subtropical seas. These features hinder the development of kelps and hermatypic corals [START_REF] Zabala | Surface-dependent strategies and energy flux in benthic marine communities or, why corals do not exist in the Mediterranean[END_REF], which, although present in the Mediterranean (i.e., Laminaria rodriguezii, Cladocora caespitosa), have a limited occurrence. Instead, rocky reefs where light intensity allows supporting photosynthetic growth are characterized by a large diversity of erect, turf, and encrusting algae [START_REF] Rodríguez-Prieto | Guía de las macroalgas y fanerógamas marinas del Mediterráneo Occidental[END_REF].

In a sea where tidal ranges are extremely small (except for the Alboran Sea and the northern Adriatic), the wide intertidal assemblages characterizing most oceanic rocky shores are almost absent with several mediolittoral assemblages alternating at different levels of the shore, usually arranged in very narrow belts. The upper mediolittoral is dominated by barnacles and littorinids, whereas the lower levels show belts of different species of erect and encrusting algae [START_REF] Chappuis | Vertical zonation is the main distribution pattern of littoral assemblages on rocky shores at a regional scale[END_REF]. In strongly exposed sites of the northern Mediterranean, the massive coralline alga Lithophyllum byssoides can produce hard calcareous concretions called "trottoirs" (Fig. 1) which form rims over the sea level. The lower part of the mediolittoral fringe is occupied by erect and turf red algae, but mussels (Mytilus galloprovincialis) and green algae (Cladophora spp., Ulva spp.) thrive in nutrient-rich environments [START_REF] Rodríguez-Prieto | Guía de las macroalgas y fanerógamas marinas del Mediterráneo Occidental[END_REF]. In the warmest regions, gregarious vermetid gastropods form dense aggregations cemented by the encrusting coralline alga Neogoniolithon brassica-florida, favouring the development of conspicuous platforms on subhorizontal surfaces, hosting diverse macroalgal assemblages (Fig. 1) (Rilov et al., 2020a).

The horizontal to almost vertical rocky bottoms in the infralittoral zone are dominated by macroalgae, although extensive meadows of seagrass Posidonia oceanica can also be found on shallow horizontal hard substrates [START_REF] Rodríguez-Prieto | Guía de las macroalgas y fanerógamas marinas del Mediterráneo Occidental[END_REF]. At the upper infralittoral level, belts of Fucales (Ericaria, Gongolaria, Cystoseira, Sargassum) develop both in exposed and sheltered areas (Fig. 1). The eastern Mediterranean also hosts concretions of coralline algae Tenarea tortuosa and Titanoderma trochanter in the shallows. In areas below 2 m depth, photophilic assemblages composed mostly by Sphacelariales, Dictyotales, Gigartinales or Bryopsidales are extremely common, although the best preserved sites are usually occupied by stands of Fucales [START_REF] Rodríguez-Prieto | Guía de las macroalgas y fanerógamas marinas del Mediterráneo Occidental[END_REF]. In the Alboran Sea and the Strait of Messina, Saccorhiza polyschides (Tylopteridales) makes luxurious underwater beds [START_REF] Giaccone | Struttura, ecologia e corologia dei popolamenti a laminarie dello stretto di Messina e del mare di Alboran[END_REF].

An interesting feature of Mediterranean macroalgal stands is their high biodiversity associated to a high miniaturization [START_REF] Coppejans | Phytosociological studies on Mediterranean Algal Vegetation: rocky surfaces of the photophilic infralittoral zone[END_REF]; more than one hundred different algal species can coexist in a small surface not larger than a sheet of paper (600 cm 2 ). This means that the so-called Mediterranean marine forests dominated by Fucales are miniaturized representatives of the huge oceanic temperate forests of Laminariales, with canopies not exceeding 20 to 40 cm but with a similar structural complexity. The complex three-dimensional structure of such canopies allows very speciose animal assemblages to develop in the understory or as epiphytes, whose diversity and distribution is strongly influenced by spatio-temporal variations of macroalgal stands (Fraschetti et al., 2006;[START_REF] Piazzi | Biodiversity in canopy-forming algae: Structure and spatial variability of the Mediterranean Cystoseira assemblages[END_REF].

Most Mediterranean shallow macroalgal assemblages show a marked seasonality. In the mediolittoral zone, midwinter is the period of maximum growth, with the maximum development showing up from early spring to midsummer [START_REF] Ballesteros | Structure and dynamics of Northwestern Mediterranean marine communities: a conceptual model[END_REF]. In autumn and early winter, Mediterranean vegetation is usually in a resting phase, although invasive species such as Lophocladia lallemandii, Womersleyella setacea or Caulerpa cylindracea can attain high growth rates and biomass in autumn. Measured average yearly production reaches 900 g C m -2 year -1 in the upper part of the infralittoral fringe from exposed environments with dense populations of Fucales, whereas figures around 100-300 g C m -2 year -1 are common in infralittoral communities with canopy-forming algae [START_REF] Ballesteros | Production of seaweeds in Northwestern Mediterranean marine communities: its relation with environmental factors[END_REF].

Aside from anthropogenic drivers (see section 3), the spatial distribution of macroalgae and associated invertebrate assemblages reflects a combination of different bottom-up and top-down processes [START_REF] Hereu | Multiple controls of community structure and dynamics in a sublittoral marine environment[END_REF]. The distribution of different species of Fucales along the depth gradient mainly depends on each species' photosynthetic efficiency [START_REF] Sant | Depth distribution of canopy-forming algae of the order Fucales is related to their photosynthetic features[END_REF], whereas the interplay among changes in vegetation, life cycles, feeding habits and competitive abilities shape species composition and abundances of invertebrate assemblages (e.g., [START_REF] Terlizzi | Molluscs on subtidal cliffs: patterns of spatial distribution[END_REF]. Biomineralogy plays an important role in community assembly, influencing settlement and recruitment processes of different species associated with rocky reefs, from algae to fishes, depending on rocky lithology [START_REF] Guidetti | Living on the rocks: substrate mineralogy and the structure of subtidal rocky substrate communities in the Mediterranean Sea[END_REF]. Other factors, such as regimes of exposure to air and nutrient availability are pivotal for mediolittoral and upper infralittoral assemblages respectively. Catastrophic events such as huge storms or thermal anomalies also regularly shape shallow assemblages (e.g., [START_REF] Navarro | Spatial and temporal variability on deepwater algal assemblages in the Northwestern Mediterranean: insights into the effects of an exceptional storm[END_REF][START_REF] Verdura | Localscale climatic refugia offer sanctuary for a habitat-forming species during a marine heatwave[END_REF]. Amongst top-down processes, overgrazing by sea urchins such as Paracentrotus lividus and Arbacia lixula (but also by herbivorous fishes, see paragraph 2.2), could strongly reduce algal biomass, availability and export of algal detritus, and simplify the associated community structure [START_REF] Sala | Fishing, trophic cascades, and the structure of algal assemblages: Evaluation of an old but untested paradigm[END_REF][START_REF] Sala | The Structure of Mediterranean Rocky Reef Ecosystems across Environmental and Human Gradients, and Conservation Implications[END_REF][START_REF] Yeruham | Ocean warming and tropical invaders erode the fitness of a key herbivore[END_REF].

Circalittoral zone

Although extremely limited geographically, most impressive algal beds in the circalittoral zone are those with kelp Laminaria ochroleuca from the Alboran Sea and the Strait of Messina [START_REF] Giaccone | Struttura, ecologia e corologia dei popolamenti a laminarie dello stretto di Messina e del mare di Alboran[END_REF]. Much more widespread are other canopy-forming algal assemblages dominated by Fucales, Dictyotales, Sphacelariales, Ceramiales or Gigartinales [START_REF] Rodríguez-Prieto | Guía de las macroalgas y fanerógamas marinas del Mediterráneo Occidental[END_REF]. Worth mentioning are the assemblages dominated by the endemic kelp Laminaria rodriguezii in eastern Mediterranean and the Adriatic Sea, which can be found down to almost 100 m depth (Žuljević et al., 2016). Erect members of the order Bryopsidales and Peyssonneliales are usually dominant in the upper circalittoral zone but a huge number of red algae are also common and characteristic of these environments (UNEP-RAC/SPA, 2003).

The most widespread Mediterranean deep-water macroalgal assemblages are coralligenous outcrops, biogenic reefs built by calcareous algae of the genus Lithophyllum, Mesophyllum, Neogoniolithon, Lithothamnion and Peyssonnelia [START_REF] Ballesteros | Mediterranean coralligenous assemblages: a synthesis of present knowledge[END_REF]. Above the basal layer made by calcareous algae, a rich flora and fauna develop, ranking the coralligenous bioconstructions amongst the most biodiverse assemblages in the Mediterranean Sea, with a rough estimate of species richness being far above 1500 species [START_REF] Ballesteros | Mediterranean coralligenous assemblages: a synthesis of present knowledge[END_REF]. These bioconstructions, which are made by living benthic organisms that overgrow the calcareous remnants of previous generations, change the primary geological substrate creating a secondary biogenic hard substrate, representing a preferred habitat for the settlement, spawning, recruitment, feeding and shelter of a wide variety of marine species. The habitat provision through bioconstruction can be mediated by either vegetal (e.g., coralline algae, coralligenous sensu stricto) [START_REF] Ingrosso | Mediterranean Bioconstructions Along the Italian Coast[END_REF] and/or animal organisms such as corals, sponges, bryozoans, molluscs and polychaetes among others [START_REF] Corriero | A Mediterranean mesophotic coral reef built by non-symbiotic scleractinians[END_REF][START_REF] Cardone | Massive bioconstructions built by Neopycnodonte cochlear (Mollusca, Bivalvia) in a mesophotic environment in the central Mediterranean Sea[END_REF]. The general pattern of coralligenous allows to recognize four main levels of organization due to i) cryptic/boring, ii) encrusting, iii) massive, and iv) arborescent organisms (Fig. 2A).

In coralligenous bioconstruction (Fig. 2Bv), the three-dimensional complexity created by both boring and erect organisms influences the biodiversity of vagile benthic fauna with gastropods, polychaetes, crustaceans, echinoderms, and fishes representing the most common vagile organisms inhabiting bioconcretions [START_REF] Ballesteros | Mediterranean coralligenous assemblages: a synthesis of present knowledge[END_REF][START_REF] Valisano | Temporal variability of sedimentation rates and mobile fauna inside and outside a gorgonian garden[END_REF]. When the primary geological substrate is granitic, volcanic or not calcareous, the contribution of bioeroders is limited; epilithic coralline algae and calcified animals show lower capability to build thick concretions on granite compared to limestone [START_REF] Canessa | Coralligenous assemblages differ between limestone and granite: A case study at the Tavolara-Punta Coda Cavallo Marine Protected Area (NE Sardinia, Mediterranean Sea)[END_REF]. In this case, the bioconstruction can be very limited (Fig. 2Bvii) and the general structure of the community is less complex, confirming the pivotal role of biomineralogy in structuring benthic assemblages [START_REF] Bavestrello | Bio-mineralogy as a structuring factor for marine epibenthic communities[END_REF]. Recently, a new type of bioconstruction has been described along the Italian coast of Apulia [START_REF] Cardone | Massive bioconstructions built by Neopycnodonte cochlear (Mollusca, Bivalvia) in a mesophotic environment in the central Mediterranean Sea[END_REF]. Main builders of this bioconstruction are scleractinians and the bivalve Neopycnodonte cochlear, which make mass aggregations on the sea bottom on both soft and hard substrates, supporting the development of a rich benthic associated fauna (Fig. 2Bvi).

Light availability drives the dynamics of circalittoral assemblages, mediating the depth distribution of the algal components [START_REF] Sant | Depth distribution of canopy-forming algae of the order Fucales is related to their photosynthetic features[END_REF]. Seasonality is also important in determining temporal variations of macroalgae in the circalittoral zone, with the period of maximum growth ranging between late spring and early summer [START_REF] Ballesteros | Structure and dynamics of Northwestern Mediterranean marine communities: a conceptual model[END_REF] and average yearly production ranges from 30-150 g C m -2 year -1 in upper circalittoral coralligenous environments [START_REF] Ballesteros | Production of seaweeds in Northwestern Mediterranean marine communities: its relation with environmental factors[END_REF]UNEP-RAC/SPA, 2003). Competition for food and space is the main process affecting species distribution and abundance, the species being forced to develop a highly specialized diet and niche to coexist in the same habitat [START_REF] Morganti | Trophic niche separation that facilitates coexistence of high and low microbial abundance sponges is revealed by in situ study of carbon and nitrogen fluxes[END_REF]. However, cooperation among species is common, with some species facilitating the settlement of others [START_REF] Gribben | Facilitation cascades in marine ecosystems: A synthesis and future directions[END_REF]. In the absence of human impacts, the development of slow-growing and long-living species, especially sponges and anthozoans is favoured [START_REF] Montero-Serra | Strong linkages between depth, longevity and demographic stability across marine sessile species[END_REF], mediating long-term stability conditions [START_REF] Cerrano | Gold coral (Savalia savaglia) and gorgonian forests enhance benthic biodiversity and ecosystem functioning in the mesophotic zone[END_REF]2019). Stability and longevity enhance intra and inter-specific interactions facilitating coevolutionary pathways, which, in turn, further contribute to the persistence of the ecosystem itself [START_REF] Pola | Updating the current knowledge on the relationships between Haplosyllis chamaeleon Laubier, 1960 (Annelida, Syllidae) and Paramuricea clavata (Risso, 1826) (Cnidaria, Plexauridae) in the Mediterranean Sea[END_REF].

The role of fish

Out of the approximately 650 fish species inhabiting the Mediterranean Sea, rocky reefs host about 200 species belonging to 40 families, covering a wide range of trophic levels [START_REF] Sala | The Structure of Mediterranean Rocky Reef Ecosystems across Environmental and Human Gradients, and Conservation Implications[END_REF][START_REF] Guidetti | Large-scale assessment of Mediterranean marine protected areas effects on fish assemblages[END_REF][START_REF] Bussotti | Fish Assemblages of Mediterranean Marine Caves[END_REF]. Species richness has been suggested to be driven by biogeographic patterns with the highest values recorded between 10°-20°E longitude [START_REF] Guidetti | Large-scale assessment of Mediterranean marine protected areas effects on fish assemblages[END_REF], and be positively correlated with depth [START_REF] Harmelin-Vivien | The littoral fish community of the Lebanese rocky coast (eastern Mediterranean Sea) with emphasis on Red Sea immigrants[END_REF]. The highly diversified fish assemblages of Mediterranean rocky reefs support a large variety of ecological traits and functions [START_REF] Albouy | FishMed: traits, phylogeny, current and projected species distribution of Mediterranean fishes, and environmental data[END_REF].

Rocky reef fish act as trophic vectors connecting different Mediterranean habitats. This is the case of small-sized and gregarious fish like the widespread damselfish Chromis chromis and the cardinal fish Apogon imberbis, whose nycthemeral migrations between the water column and the bottom and in/out of marine caves, transfer organic mattermostly via faecal pelletsinto depleted rocky reef habitats (Pinnegar and Polunin, 2005;[START_REF] Bussotti | Fish mitigate trophic depletion in marine cave ecosystems[END_REF].

High-level predator fishes exert top-down controls triggering trophic cascades directly and indirectly affecting a large number of species, finally shaping the structure and functioning of the entire coastal rocky ecosystems [START_REF] Prato | The importance of highlevel predators in marine protected area management: Consequences of their decline and their potential recovery in the Mediterranean context[END_REF]. These large-sized and long living animals, like as sharks and groupers, nowadays are generally depleted in the Mediterranean Sea due to overfishing. The reduction in populations of high-level predators has determined the disruption of the ecological functions that these species perform especially in shallow rocky habitats. Here, the decreased functional redundancy at high trophic levels has left medium-sized fish like seabreams alone to control ecosystem structure by regulating the grazing pressure exerted by sea urchins on erect macroalgae, increasing the chance of drastic shifts in coastal ecosystems (see paragraph 3.3).

Contrary to what has been thought until a decade ago, even the very few macro-herbivorous fish species present in the Mediterranean Sea heavily contribute to regulating the functioning of shallow rocky ecosystems [START_REF] Sala | Alien marine fishes deplete algal biomass in the eastern Mediterranean[END_REF]. Both the native salema Sarpa salpa, widely distributed in the western Mediterranean, and the invasive rabbitfishes Siganus luridus and S. rivulatus, that entered via the Suez Canal between 1927 and 1956 and more recently replacing the native salema in the eastern Mediterranean, may represent a threat to macroalgal canopies and indirectly to the high diversity they support (Verges et al., 2014;[START_REF] Gianni | Threats to large brown algal forests in temperate seas: the overlooked role of native herbivorous fish[END_REF], Rilov et al., 2018).

In this regard, a more general reshuffling of the Mediterranean reef fish assemblages has occurred in the last few decades. This was mainly attributed to changes in the relative abundances of native species as a result of geographical range contractions and deepening of cool-water fishes, rapid range extensions of alien warm-water and tropical fishes, and changing interspecific interactions [START_REF] Milazzo | Climate change exacerbates interspecific interactions in sympatric coastal fishes[END_REF][START_REF] Azzurro | Climate change, biological invasions, and the shifting distribution of Mediterranean fishes: a large-scale survey based on local ecological knowledge[END_REF]. Model projections suggest a dramatic reduction of Mediterranean endemic fish by the end of this century [START_REF] Rais Lasram | The Mediterranean Sea as a 'cul-de-sac' for endemic fishes facing climate change[END_REF].

The contribution of rocky reefs to coastal ecosystem functioning and services

Mediterranean rocky reefs, from the shore to shelf areas, are highly dynamic, in both time and space, and productive systems. They receive abiotic and biotic inputs from both the open sea and the mainland, through water mass movements that locally may have seasonal fluctuations and that are able to connect adjacent ecosystems. Ecological connectivity can be very high at a regional scale [START_REF] Rattray | Geographic distance, water circulation and environmental conditions shape the biodiversity of Mediterranean rocky coasts[END_REF][START_REF] Bevilacqua | Nestedness and turnover unveil inverse spatial patterns of compositional and functional β-diversity at varying depth in marine benthos[END_REF] being the outcome of community assembly processes, which are mainly driven by the local species pools [START_REF] Briton | High-resolution modelling of ocean circulation can reveal retention spots important for biodiversity conservation[END_REF].

However, spatial patterns of compositional and functional diversity in Mediterranean rocky reefs can overlap or diverge due to the complex interplay among functional redundancy of species, environmental filtering and biogeography [START_REF] Bevilacqua | Nestedness and turnover unveil inverse spatial patterns of compositional and functional β-diversity at varying depth in marine benthos[END_REF]. Due to their high habitat heterogeneity and diversity, which offer a variety of niches available from the upper mediolittoral to the deeper circalittoral habitats, the biological components of the Mediterranean rocky reefs express a lot of different ecological functions (de la [START_REF] De La Torriente | Habitat forming species explain taxonomic and functional diversities in a Mediterranean seamount[END_REF].

The large variety of rocky reef species and the associated functional traits is responsible for a range of benefits to humans in terms of ecosystem services, which are at the base of the human wellbeing of most Mediterranean coastal countries. For example, despite the mediolittoral being limited to a few tens of centimetres, assemblages in this zone play a fundamental role in the overall functioning of coastal ecosystems. Trottoirs formed by vermetid platforms, distributed across the warmest coastlines of the basin, provide several ecosystem services associated with production functions (e.g., the provision of food and baits), to regulation functions by influencing water column quality (e.g., filtering) and enhancing anthropogenic disturbance prevention, to support functions, through the provision of food and organisms to adjacent systems, and to cultural functions, by offering space for recreation/leisure, exploration and relaxation [START_REF] Milanese | Climate change, marine policy and the valuation of Mediterranean intertidal ecosystems[END_REF]. In the lower mediolittoral fringe, and where trophic conditions are more enriched, mussel beds are mainly built by Mytilus galloprovincialis and represent a crucial element of the rocky shore biodiversity. Their local loss or reduction in density generates changes in the associated assemblages [START_REF] Maggi | Effects of changes in number, identity and abundance of habitat-forming species on assemblages of rocky seashores[END_REF] with a net loss of services such as water quality regulation, habitat and refugia, and enhancement of habitat heterogeneity.

At the other depth extreme, in the circalittoral zone, coralligenous habitats are so structured and biodiverse that are able to provide a number of ecosystem services (UNEP-RAC/SPA, 2003), from provisioning (e.g., food, red coral and species for aquariums), regulating (e.g., carbon sequestration and water filtration), cultural (e.g., recreational diving and intellectual/spiritual inspiration) to supporting (e.g., species nursery and refugia) services (de Ville d' Avray et al., 2019).

In the middle of these two depth extremes, in the infralittoral zone, vegetated rocky substrates certainly provide the most important contribution to the functioning of coastal ecosystems. They are primary sources and sinks of blue carbon, participate in nutrient cycling, provide essential support for marine coastal biota by providing food, nursery and shelter [START_REF] Mineur | European seaweeds under pressure: Consequences for communities and ecosystem functioning[END_REF][START_REF] Zunino | Cultural ecosystem services provided by coralligenous assemblages and Posidonia oceanica in the Italian seas[END_REF], and foster the diversity of important ecological compartments, such as microbiota and meiofauna [START_REF] Bianchelli | Impairment of microbial and meiofaunal ecosystem functions linked to algal forest loss[END_REF].

Recent attempts to quantify energy flows in Mediterranean algal forests and coralligenous outcrops revealed a prominent input of these habitats to the energetic budget of coastal ecosystems (De la Fuente et al., 2019a;[START_REF] Buonocore | Assessing natural capital value in marine ecosystems through an environmental accounting model: A case study in Southern Italy[END_REF]. Nonetheless, an accurate and synthetic evaluation of ecosystem services provided by rocky reefs across the Mediterranean Sea is still not addressed although it would be of paramount importance for decision-makers and environmental practitioners in order to set appropriate conservation and management measures.

Human threats to Mediterranean reefs: status and trends

Rocky reef assemblages have shown critical changes due to anthropogenic activities in most of the Mediterranean Sea [START_REF] Claudet | Human-driven impacts on marine habitats: A regional metaanalysis in the Mediterranean Sea[END_REF][START_REF] Rilov | Alien turf: Overfishing, overgrazing and invader domination in south-eastern Levant reef ecosystems[END_REF]. The prevalent and widely reported modification concerns the structure of macroalgal stands, shifting from canopies to less complex communities dominated by turf or encrusting algae (e.g., [START_REF] Thibaut | Decline and local extinction of Fucales in the French Riviera: The harbinger of future extinctions?[END_REF][START_REF] Rilov | Alien turf: Overfishing, overgrazing and invader domination in south-eastern Levant reef ecosystems[END_REF]. The ensuing reduction in primary production and habitat complexity largely turned into profound changes of the animal component of rocky reef assemblages [START_REF] Peleg | Tropicalization may invert trophic state and carbon budget of shallow temperate rocky reefs[END_REF]. Several benthic invertebrates and fish species of Mediterranean rocky reefs have also been threatened by a long history of intense exploitation. In some cases, such as for collection of commercial sponges and red coral, highly damaging fishing tools have been adopted in the last centuries [START_REF] Cattaneo-Vietti | An overexploited Italian treasure: past and present distribution and exploitation of the precious red coral Corallium rubrum (L., 1758) (Cnidaria: Anthozoa)[END_REF], deeply altering the structure of benthic assemblages of many rocky reefs. However, one major driver of change cannot be unequivocally identified and most studies infer several interacting anthropogenic stressors in determining the ongoing alterations of reefs (Bevilacqua et al., 2018;[START_REF] Rilov | Alien turf: Overfishing, overgrazing and invader domination in south-eastern Levant reef ecosystems[END_REF], including pollution [START_REF] Ivesa | Long-term fluctuations in Cystoseira populations along the west Istrian Coast (Croatia) related to eutrophication patterns in northern Adriatic Sea[END_REF], urbanization [START_REF] Mangialajo | Loss of fucoid algae along a gradient of urbanisation, and structure of benthic assemblages[END_REF], destructive fishing [START_REF] Rodríguez-Prieto | Guía de las macroalgas y fanerógamas marinas del Mediterráneo Occidental[END_REF], increase in sedimentation rates [START_REF] Airoldi | The effects of sedimentation on rocky coast assemblages[END_REF], overfishing [START_REF] Sala | Fishing, trophic cascades, and the structure of algal assemblages: Evaluation of an old but untested paradigm[END_REF][START_REF] Sini | Smallscale coastal fishing shapes the structure of shallow rocky reef fish in the Aegean Sea[END_REF], biological invasions and disease outbreaks [START_REF] Sala | Alien marine fishes deplete algal biomass in the eastern Mediterranean[END_REF][START_REF] Vezzulli | 16SrDNA pyrosequencing of the Mediterranean gorgonian Paramuricea clavata reveals a link among alterations in bacterial holobiont members, anthropogenic influence and disease outbreaks[END_REF][START_REF] Rilov | Alien turf: Overfishing, overgrazing and invader domination in south-eastern Levant reef ecosystems[END_REF].

In the last twenty years, the increase in intensity and frequency of thermal anomalies and extreme marine weather events is causing destructive disturbance episodes and mass mortalities, leading to very marked alterations of density and size of the epibenthic structuring species [START_REF] Teixidó | Impacts on coralligenous outcrop biodiversity of a dramatic coastal storm[END_REF][START_REF] Sarà | The impact of climate change on Mediterranean intertidal communities: losses in coastal ecosystem integrity and services[END_REF][START_REF] Garrabou | Collaborative database to track mass mortality events in the Mediterranean Sea[END_REF], or mining their reproductive potential, and therefore, their persistence [START_REF] Bevilacqua | Climatic anomalies may create a long-lasting ecological phase shift by altering the reproduction of a foundation species[END_REF]. Acidification may also deeply influence benthic assemblages, affecting the structural component represented by coralline algae and hard shelled invertebrates [START_REF] Martin | Response of Mediterranean coralline algae to ocean acidification and elevated temperature[END_REF][START_REF] Rastelli | A high biodiversity mitigates the impact of ocean acidification on hard-bottom ecosystems[END_REF], thus altering the balance between bioconstruction and bioerosion processes of rocky reefs [START_REF] Cerrano | The role of sponges bioerosion in Mediterranean coralligenous accretion[END_REF]. Climate change, therefore, will exacerbate human impacts [START_REF] Gissi | A review of the combined effects of climate change and other human stressors on the marine environment[END_REF][START_REF] Verdura | Localscale climatic refugia offer sanctuary for a habitat-forming species during a marine heatwave[END_REF], with additive effects on the degradation of reef assemblages (e.g., [START_REF] Rilov | Alien turf: Overfishing, overgrazing and invader domination in south-eastern Levant reef ecosystems[END_REF][START_REF] Peleg | Tropicalization may invert trophic state and carbon budget of shallow temperate rocky reefs[END_REF], further eroding their resilience potential.

Pollution

A closed basin like the Mediterranean Sea, which receives multiple inputs from the mainland, from maritime transportation and aquaculture, and has little turnover, is particularly exposed to pollution.

On shallow rocky reefs, the emblematic outcome of organic and inorganic pollution from waste discharges is the proliferation of tolerant, opportunistic species, over sensitive, habitat-structuring species (e.g., [START_REF] Terlizzi | Multivariate and univariate asymmetrical analyses in environmental impact assessment: a case study of Mediterranean subtidal sessile assemblages[END_REF], which in turn results in biodiversity loss of the understorey and associated fish assemblages [START_REF] Azzurro | Sewage pollution impact on Mediterranean rocky-reef fish assemblages[END_REF]. Cystoseira s.l. species and other habitat formers, like vermetids, can be particularly affected by surface pollutants such as hydrocarbons, surfactant detergents, and herbicide [START_REF] Badreddine | Threatened biogenic formations of the Mediterranean: Current status and assessment of the vermetid reefs along the Lebanese coastline (Levant basin)[END_REF][START_REF] De Caralt | Differential effects of pollution on adults and recruits of a canopy-forming alga: implications for population viability under low pollutant levels[END_REF], suggesting that pollution could have contributed to their decline in the basin.

In the last years, although contamination levels in coastal areas have decreased, concerns on emerging pollutants have been raised, especially on plastic litter (UNEP/MAP, 2017). Marine litter has been found in the Mediterranean Sea in similar quantities to those observed in the five oceanic garbage patches [START_REF] Fossi | Assessing and mitigating the harmful effects of plastic pollution: the collective multistakeholder driven Euro-Mediterranean response[END_REF], and the presence of microplastic (i.e., fragment size 1μm-5 mm) has been widely documented throughout the basin [START_REF] Martellini | A snapshot of microplastics in the coastal areas of the Mediterranean Sea[END_REF]. The average concentration of microplastic in the water column in the Mediterranean Sea is 2.3 kg km -2 , ranging between 6.5 kg km -2 and 1.5 kg km -2 (Soto- [START_REF] Soto-Navarro | 3D hotspots of marine litter in the Mediterranean: a modeling study[END_REF]. Since many invertebrates on rocky reefs are suspension-filter feeders, the increasing amount of microplastics is particularly alarming due to their potential bioaccumulation, and biomagnification through the trophic web.

Several studies have suggested that microplastics can carry toxic chemicals, as well as microscopic organisms, that animals inadvertently swallow (e.g., van Der Hal et al., 2020). The transport of microplastics in the environment is accompanied by the immigration of contaminants (e.g., polychlorinated biphenyls, brominated flame retardants, polycyclic aromatic hydrocarbons, bisphenol A) via sorption/desorption, known as the "Trojan-Horse effect" [START_REF] Zhang | Transport of micro-and nanoplastics in the environment: Trojan-Horse effect for organic contaminants[END_REF].

This phenomenon significantly changes the potential health risks of microplastics by enhancing the bioavailability of contaminants for living organisms [START_REF] Li | Plastic waste in the marine environment: a review of sources, occurrence and effects[END_REF]. Also, many organisms, such as benthic diatoms and microalgae, adhere to the fragments and can be involuntarily swallowed causing also dietary alterations or the introduction of potentially pathogenic or toxic species [START_REF] Cole | Microplastics as contaminants in the marine environment: A review[END_REF]. Microplastics are only one aspect of the problem, since plastic fragments smaller than microplastics, the so-called nanoplastics (i.e., fragment size 1-1000 nm), are likely to represent a more subtle hazard for marine organisms, since they can have a higher ability to adsorb and carry toxic compounds if compared to microplastics [START_REF] Rios Mendoza | Micro(nanoplastics) in the marine environment: Current knowledge and gaps[END_REF]Piccardo et al., 2021).

Other emerging environmental pollutants for the marine environment are pharmaceutical residues, which have been detected in different marine organisms, from algae to bivalves, crustaceans and fish [START_REF] Fabbri | Human pharmaceuticals in the marine environment: focus on exposure and biological effects in animal species[END_REF][START_REF] Almeida | Anti-inflammatory drugs in the marine environment: Bioconcentration, metabolism and sub-lethal effects in marine bivalves[END_REF]. These substances may interfere with subcellular and cellular processes of marine organisms, threatening the survival of the marine microbiota, with consequences that can potentially propagate to the highest levels of the trophic web, posing a health risk also for humans [START_REF] Fabbri | Human pharmaceuticals in the marine environment: focus on exposure and biological effects in animal species[END_REF].

The long-term effects that old and new pollutants could have on the marine environment are not yet fully understood, especially in light of potential synergies with other drivers of global change, increasing the uncertainty on future scenarios. Global warming and the ocean acidification can have direct consequences on the balance of ecosystems, acting on the basis of microbial components [START_REF] Coelho | Interactive effects of global climate change and pollution on marine microbes: the way ahead[END_REF], and causing a greater dispersion and availability of pollutants, intensifying their effects on the marine biota. Considering the current state, and that measures such as the local protection of species and areas are not fully effective to mitigate (Soto- [START_REF] Soto-Navarro | Impact of the marine litter pollution on the Mediterranean biodiversity: A risk assessment study with focus on the marine protected areas[END_REF] or reverse the trends of pollution, the implementation of containment/prevention measures for all kinds of pollutants, mainly through regulatory instruments and by vast plans of decontamination, are urgently required.

Direct physical habitat disruption and artificialisation

In the past decades, the European date mussel (Lithophaga lithophaga) fishery has been the main anthropogenic source of direct physical damage on shallow rocky reefs (e.g., [START_REF] Fanelli | Human predation along Apulian rocky coasts (SE Italy): desertification caused by Lithophaga lithophaga (Mollusca) fisheries[END_REF][START_REF] Bevilacqua | Mitigating human disturbance: can protection influence trajectories of recovery in benthic assemblages?[END_REF]. This destructive fishing practice, which requires breaking rocky surfaces and removing the whole benthic assemblage to collect the endolithic bivalves, has led to the destruction of large extensions of rocky reefs in the whole basin, from Morocco to Israel [START_REF] Colletti | The date mussel Lithophaga lithophaga: Biology, ecology and the multiple impacts of its illegal fishery[END_REF]. Though banned since the end of 1980s in most Mediterranean countries, the date mussel fishery still continues illegally in several regions [START_REF] Katsanevakis | Protected" marine shelled molluscs: thriving in Greek seafood restaurants[END_REF][START_REF] Colletti | The date mussel Lithophaga lithophaga: Biology, ecology and the multiple impacts of its illegal fishery[END_REF] and the current extent of damaged areas remains largely unknown. Other activities associated with human visitation, such as trampling, anchoring, and diving can cause direct damage to shallow benthic communities inhabiting rocky reefs [START_REF] Milazzo | Patterns of algal recovery and smallscale effects of canopy removal as a result of human trampling on a Mediterranean rocky shallow community[END_REF][START_REF] Guarnieri | Increasing heterogeneity of sensitive assemblages as a consequence of human impact in submarine caves[END_REF], although these impacts are often limited and locally concentrated in crowded sites.

For hard substrates at greater depths, and especially for coralligenous outcrops and maërl, bottom trawling is primarily responsible for habitat degradation and loss. Trawling may have either a direct physical impact caused by otter boards and nets that destroy the three-dimensional structure of arborescent corals colonies or an indirect impact due to sediment resuspension and deposition on benthic habitats [START_REF] Savini | Mapping cold-water coral habitats at different scales within the Northern Ionian Sea (Central Mediterranean): an assessment of coral coverage and associated vulnerability[END_REF][START_REF] D'onghia | Anthropogenic impact in the Santa Maria di Leuca cold-water coral province (Mediterranean Sea): observations and conservation straits[END_REF]. Altered sedimentary regime due to bottom trawling for crustaceans (i.e., Aristeidae shrimps, Norway lobster) is one of the main threats for fragile cold-water corals (Latras et al., 2016). In the Mediterranean Sea, fishing efforts by trawling are intensifying especially at depth <200 m [START_REF] Ferrà | Mapping change in bottom trawling activity in the Mediterranean Sea through AIS data[END_REF], suggesting a future increase in the extent and magnitude of deterioration and loss of rocky habitats in the circalittoral zone.

Coastal habitats in the Mediterranean Sea are experiencing profound modifications also due to the continuous expansion of coastal anthropization since the 1950s [START_REF] Airoldi | Loss, status and trends for coastal marine habitats of Europe[END_REF][START_REF] Bugnot | Current and projected global extent of marine built structures[END_REF]. From 1990 until 2000, the surface of coastal artificial structures within 1 km from the shoreline increased on average by 3%, whereas coastal armouring interested 8% of coasts (EEA, 2006). Recent estimates projected an increase in urbanized areas below 20 m of elevation ranging between 67%-104% in many Mediterranean countries [START_REF] Wolff | Future urban development exacerbates coastal exposure in the Mediterranean[END_REF]. Coastal development may involve the introduction of artificial substrates but also massive land reclamation, which causes the irreversible destruction/loss of original habitats. Recent data concerning the Monaco Principality (see http://www.medam.org), for instance, show that >90% of the coastline and >90% of shallow marine habitats (between 0 and 10 m depth, including rocky reefs) have been destroyed/artificialized, with extensive land reclamation allowing the small country to gain more and more "emerged surfaces" to build. These massive interventions of coastal engineering may appear punctual and irrelevant at the Mediterranean scale, but they deeply and irreversibly impact local marine biodiversity with the potential risk of scaling-up effects at regional or even larger scale with future expansion of these practices.

Besides the direct effects, indirect effects on rocky reef communities due to the introduction of artificial substrates, coastal urbanization, coastline modifications and ensuing alterations of sedimentary regimes are even more harmful [START_REF] Bulleri | The introduction of artificial structures on marine soft-and hard-bottoms: Ecological implications of epibiota[END_REF][START_REF] Airoldi | Corridors for aliens but not for natives: effects of marine urban sprawl at a regional scale[END_REF]. The presence of breakwaters, sea walls, jetties, and other man-made structures in coastal environments, as well as offshore oil and gas platforms could affect species populations and communities from native hard substrates by modifying natural patterns of ecological connectivity, either enhancing or dampening dispersal and trophic connections [START_REF] Bishop | Effects of ocean sprawl on ecological connectivity: impacts and solutions[END_REF]. Assemblages on artificial structures are generally less diverse and complex than those on natural hard substrates, which make them more prone to be colonized by non-indigenous species (NIS) and to act as stepping stones disproportionately favouring the spread of alien species over native rocky reef species [START_REF] Airoldi | Corridors for aliens but not for natives: effects of marine urban sprawl at a regional scale[END_REF]. Moreover, coastal engineering often relates to sediment resuspension and dislodgement as a direct consequence of interventions (e.g., dredging) or indirectly by modifying hydrodynamic coastal regimes, causing pulse or chronic increases in water turbidity and sedimentation. Burial, abrasion, and limitation of photosynthetic activity due to increased sediment suspension and deposition may drastically curb settlement, recruitment and survival abilities of sessile organisms, causing dramatic changes in rocky reef communities, which typically exhibit a shift from complex and highly diverse algal canopies towards homogenized assemblages dominated by a few sedimenttolerant turf-forming algae and suspension feeders [START_REF] Airoldi | The effects of sedimentation on rocky coast assemblages[END_REF][START_REF] Ballesteros | Mediterranean coralligenous assemblages: a synthesis of present knowledge[END_REF][START_REF] Claudet | Human-driven impacts on marine habitats: A regional metaanalysis in the Mediterranean Sea[END_REF].

Eco-engineering could be of help in mitigating potential impact of man-made marine structures and, in some cases, restoration actions could effectively sustain the recovery of damaged habitats.

Reversing this trend should necessarily imply a radical rethinking of the use of marine space, especially in urban and periurban coastal areas [START_REF] Airoldi | Emerging solutions to return nature to the urban[END_REF], along with a consistent reduction of direct physical impacts.

Overfishing

Mediterranean rocky reefs along with their food webs are well known to be diverse, complex and, to some extent, fragile. Modern and traditional ecological knowledge can be paired with the long tradition of natural history studies in the Mediterranean to better understand temporal trajectories of changes attributable to human activities. Mediterranean rocky reefs, from this perspective, have been exploited by humans for millennia [START_REF] Sala | The past and present topology and structure of Mediterranean subtidal rocky-shore food webs[END_REF], with many species of vertebrates (e.g., fishes) and invertebrates (e.g., sea urchins, molluscs, crustaceans) that have been fished/harvested since ancient times for food and decorative uses.

Fishing, both professional and recreational, is widely considered one of the major causes of community-wide alterations of Mediterranean rocky reefs [START_REF] Font | Biological and ecological impacts derived from recreational fishing in Mediterranean coastal areas[END_REF][START_REF] Sini | Smallscale coastal fishing shapes the structure of shallow rocky reef fish in the Aegean Sea[END_REF].

Fishing methods can be multi-specific (e.g., fixed nets) or target one or few species (e.g., spearfishing), but the overall result is often the alteration of trophic relationships through the depletion of ecologically important species, and mostly high-level predators.

An emblematic example of the impact of fishing on large fish predators concerns the dusky grouper Epinephelus marginatus, a commercially important species and one of the largest predators on Mediterranean rocky reefs. Targeted by commercial and recreational fishing, its abundance and size dramatically decreased in recent decades. Large groupers were fished since prehistory and then in Greek and Roman times in very shallow stands, while nowadays abundant and large groupers can be observed just in fully protected zones of well enforced MPAs or deeper than 25-30 m in areas open to fishing [START_REF] Guidetti | Art serving marine conservation[END_REF][START_REF] Giakoumi | Ecological effects of full and partial protection in the crowded Mediterranean Sea: a regional meta-analysis[END_REF]. However, the paradigm of fishing impact producing community-wide consequences in the Mediterranean rocky reefs relates to sparid fishes, which thrive in these habitats and are targeted by professional and recreational fishers [START_REF] Guidetti | Community-wide effects of marine reserves in the Mediterranean Sea[END_REF]. Some sparids of high commercial value belonging to the genus Diplodus (especially the seabreams Diplodus sargus and D. vulgaris) actively feed on sea urchins. These seabreams have the potential of shaping the structure of entire communities associated with subtidal rocky reefs [START_REF] Sala | Fishing, trophic cascades, and the structure of algal assemblages: Evaluation of an old but untested paradigm[END_REF][START_REF] Guidetti | Marine reserves reestablish lost predatory interactions and cause community changes in rocky reefs[END_REF] as they might control the abundance of sea urchins and, indirectly, of erect macroalgae. Overfishing these seabreams may trigger a trophic cascade: whenever fishing reduces seabream abundance and size, sea urchin populations are released from predation and then dramatically increase, erect macroalgal forests disappear due to overgrazing and are replaced by the so-called barren grounds (Fig. 3). This transition implies a dramatic loss of biodiversity (of fish, invertebrate and algal species) and consequently of important ecosystem functions and services (e.g., reduced carbon sequestration, decreased vegetal biomass and detritus available for consumers, reduced three-dimensional habitat available for juvenile/adult fishes and invertebrates; [START_REF] Sala | Fishing, trophic cascades, and the structure of algal assemblages: Evaluation of an old but untested paradigm[END_REF][START_REF] Guidetti | Marine reserves reestablish lost predatory interactions and cause community changes in rocky reefs[END_REF][START_REF] Cheminée | Nursery value of Cystoseira forests for Mediterranean rocky reef fishes[END_REF][START_REF] Prato | The importance of highlevel predators in marine protected area management: Consequences of their decline and their potential recovery in the Mediterranean context[END_REF]. Such species-to community-wide consequences of overfishing in subtidal rocky reefs can be effectively reversed with the creation of well enforced MPAs, especially if large enough to include species' home range [START_REF] Guidetti | Large-scale assessment of Mediterranean marine protected areas effects on fish assemblages[END_REF], Di Franco et al., 2018).

Overfishing is not limited to infralittoral rocky reefs, but there is clear evidence of the impact of unregulated harvesting on mediolittoral limpets. In particular, the giant Mediterranean limpet Patella ferruginea, an endemic species harvested by man since prehistory [START_REF] Fa | Effects of tidal amplitude on intertidal resource availability and dispersal pressure in prehistoric human coastal populations: the Mediterranean-Atlantic transition[END_REF], is the most endangered marine macroinvertebrate in the Mediterranean Sea. Presently, its distribution is restricted to only a few sites along the western Mediterranean rocky shores where, although officially protected (Annex IV of the Habitats Directive and Annex II of the Berne and Barcelona Conventions), limpet populations are declining at an alarming rate due to poaching in MPAs [START_REF] Coppa | Is the establishment of MPAs enough to preserve endangered intertidal species? The case of Patella ferruginea in Mal di Ventre Island (W Sardinia, Italy)[END_REF]. Circalittoral rocky communities in some areas of the Mediterranean have been since long impacted by the red coral Corallium rubrum harvesting, performed using the destructive trawling gear called St. Andrew Cross, nowadays banned and replaced by the employment of specialized divers and ROVs [START_REF] Cattaneo-Vietti | An overexploited Italian treasure: past and present distribution and exploitation of the precious red coral Corallium rubrum (L., 1758) (Cnidaria: Anthozoa)[END_REF]. Clear and widespread impacts from lost debris/gears ascribable to both professional and recreational deep-water fishing activities on rocky substrates are reported to affect deep-water habitats and sessile organisms [START_REF] Bo | Fishing impact on deep Mediterranean rocky habitats as revealed by ROV investigation[END_REF].

Finally, a quite often neglected point is the virtual elimination of large consumer species (i.e., monk seal, loggerhead turtle, sharks). These ecologically extinct megafauna were crucially important in terms of trophic interactions, and their removal certainly had strong ecological consequences on rocky reef ecosystems in the Mediterranean Sea [START_REF] Sala | The past and present topology and structure of Mediterranean subtidal rocky-shore food webs[END_REF][START_REF] Zogaris | Sharks, blast fishing and shifting baselines: insights from Hass's 1942 Aegean expedition[END_REF].

Bioinvasions

Νon-indigenous species (NIS) are introduced by human activities, accidentally or intentionally and through multiple vectors [START_REF] Mack | Biotic invasions: Causes, epidemiology, global consequences, and control[END_REF]. In the last decades, the number of NIS in the Mediterranean Sea has continuously increased and the most recent studies have documented the presence of >1000 marine NIS, more than half of which are established and widespread, with almost 120 species being definitively invasive [START_REF] Zenetos | Mediterranean non indigenous species at the start of the 2020s: Recent changes[END_REF]. Most NIS entered the Mediterranean Sea through the Suez Canal (namely Lessepsian species; [START_REF] Rilov | Marine bioinvasions in the Mediterranean Seahistory, distribution and ecology[END_REF][START_REF] Katsanevakis | Invading the Mediterranean Sea: biodiversity patterns shaped by human activities[END_REF][START_REF] Galil | The enlargement of the Suez Canal and introduction of non-indigenous species to the Mediterranean Sea[END_REF]. Other important pathways of marine NIS introduction in the Mediterranean include maritime transport (ballast waters, or hull fouling), aquaculture, and the aquarium trade [START_REF] Zenetos | Alien species in the Mediterranean Sea by 2012. A contribution to the application of European Union's Marine Strategy Framework Directive (MSFD). Part 2. Introduction trends and pathways[END_REF]. Not all NIS, however, exert negative ecological effects as those caused by invasive species. In fact, only a fraction of NIS, thanks to combinations of favourable abiotic and biotic factors in the recipient ecosystem, become invasive alien species (IAS), and may severely affect its functioning [START_REF] Butchart | Global biodiversity: Indicators of recent declines[END_REF]. IAS are, indeed, among the most pernicious outcomes of global change [START_REF] Geraldi | Ecological effects of non-native species in marine ecosystems relate to co-occurring anthropogenic pressures[END_REF], so that their spread has been suggested as being a case of "biopollution" (Occhipinti Ambrogi, 2021).

Bioinvasions are not only associated with IAS but can also be the result of native species outbreaks that, due to contingent conditions, proliferate well above the carrying capacity of their system. In the Mediterranean Sea, examples of the former include outbreaks of the NIS algae Ostreopsis cf.

ovata [START_REF] Mangialajo | Trends in Ostreopsis proliferation along the Northern Mediterranean coasts[END_REF], the medusa Rhopilema nomadica [START_REF] Giallongo | Long-term changes in population genetic features of a rapidly expanding marine invader: implication for invasion success[END_REF], the fish Pterois miles [START_REF] Dimitriadis | Updating the occurrences of Pterois miles in the Mediterranean Sea, with considerations on thermal boundaries and future range expansion[END_REF] and Siganus rivulatus [START_REF] Renanel | Habitat utilization by an invasive herbivorous fish (Siganus rivulatus) in its native and invaded range[END_REF]; examples of the latter include outbreaks of the dinoflagellate Noctiluca scintillans [START_REF] Hallegraeff | Global harmful algal bloom status reporting[END_REF], the medusa Rhizostoma pulmo [START_REF] Basso | Jellyfish summer outbreaks as bacterial vectors and potential hazards for marine animals and humans health? The case of Rhizostoma pulmo (Scyphozoa, Cnidaria)[END_REF] and the fish Pomatomus saltatrix (Villegas-Hernandez et al., 2015). Some invaders have large direct structural or consumptive impacts on the local communities, and sometimes both. One of the best examples is the invasion of the two Indopacific herbivorous rabbitfish in the previous century, that, by overgrazing, have depleted lush macroalgal stands in the eastern Mediterranean [START_REF] Sala | Alien marine fishes deplete algal biomass in the eastern Mediterranean[END_REF]Verges et al., 2014). This overgrazing most probably also contributed (in combination with sea warming) to the collapse of sea urchin populations on the southeastern Levant reefs through competition for food that reduced population viability [START_REF] Yeruham | Collapse of the echinoid Paracentrotus lividus populations in the Eastern Mediterranean-Result of climate change?[END_REF]. The recent rapid invasion of the lionfish (P. miles) [START_REF] Dimitriadis | Updating the occurrences of Pterois miles in the Mediterranean Sea, with considerations on thermal boundaries and future range expansion[END_REF], a voracious predator that has already caused extensive ecological impacts in the Caribbean Sea [START_REF] Albins | Invasive Pacific lionfish Pterois volitans reduce abundance and species richness of native Bahamian coral-reef fishes[END_REF], is also expected to cause extensive ecological impacts on reefs in the region.

IAS can also exert indirect, and often elusive, effects on biological and behavioural traits of native species. A good example of an invader with such a potential is the green alga Caulerpa cylindracea, native to southwestern Australia, reported for the first time in the Mediterranean Sea along the coasts of Tunisia in 1990 and then rapidly spreading in the whole basin (e.g., [START_REF] Piazzi | The invasion of Caulerpa cylindracea in the Mediterranean: the known, the unknown and the knowable[END_REF][START_REF] Rizzo | Potential effects of an invasive seaweed (Caulerpa cylindracea, Sonder) on sedimentary organic matter and microbial metabolic activities[END_REF]. The basin-scale outbreak of this alga is likely due to its capacity to thrive in either hard or soft substrates at depths ranging from 0 to >70 m [START_REF] Klein | The Caulerpa racemosa invasion: A critical review[END_REF] and to easily invade ecosystems already degraded by other anthropogenic stressors [START_REF] Ceccherelli | The role of disturbance in promoting the spread of the invasive seaweed Caulerpa racemosa in seagrass meadows[END_REF][START_REF] Piazzi | The invasion of Caulerpa cylindracea in the Mediterranean: the known, the unknown and the knowable[END_REF].

At the same time, C. cylindracea is among the most paradigmatic examples of chemical pollution mediated by molecules produced by the living biota. Caulerpenyne, caulerpine and caulerpicin, are the most abundant and investigated secondary metabolites of C. cylindracea, and all possess biological properties such as allelochemical, neurotoxic and cytotoxic activities [START_REF] Raniello | Phytotoxic activity of caulerpenyne from the Mediterranean invasive variety of Caulerpa racemosa: A potential allelochemical[END_REF], which, by interfering native species, may contribute to the invasion potential of the alga.

Despite the toxicity of these metabolites, C. cylindracea has become an important food item in the diet of the common Mediterranean seabream Diplodus sargus, whereas recent evidence indicates the presence of the alga in stomach contents of both other native Mediterranean fishes (i.e., the sparids Spondyliosoma cantharus, Sarpa salpa, Diplodus vulgaris, and the scarid, Sparisoma cretense) and the siganid Lessepsian migrant, Siganus luridus [START_REF] Felline | Preliminary observations of caulerpin accumulation from the invasive Caulerpa cylindracea in native Mediterranean fish species[END_REF]. Such a 'new' diet has been related to the appearance of cellular and physiological alterations in fish, which include enhanced oxidative stress, the onset of genotoxic damage and the possible appearance of neurotoxic damage and anomalous behaviour that can lead to higher mortality rates [START_REF] Felline | Subtle effects of biological invasions: cellular and physiological responses of fish eating the exotic Pest Caulerpa racemosa[END_REF]. Fish consuming C. cylindracea show important changes in the gonadosomatic index (GSI) and a significant induction of the vitellogenin gene expression, altogether indicating the alteration of gross gonad morphology and possible adverse implications on the fish reproductive performance [START_REF] Gorbi | Could molecular effects of Caulerpa racemosa metabolites modulate the impact on fish populations of Diplodus sargus?[END_REF]. Also, the ingestion of C. cylindracea causes a sort of behavioural relaxation in fish, which could hamper, especially in juveniles, their ability to escape predators.

The tropicalization of the Mediterranean by the interaction of rapid warming and the invasion of thermophilic species, and especially macroalgae [START_REF] Israel | Alien seaweeds from the Levant basin (Eastern Mediterranean Sea), with emphasis to the Israeli shores[END_REF], is already leading to the complete restructuring of the reef communities in the southeast Levantine basin (Rilov et al., 2019a), which evidently also transforms reef ecosystem functions [START_REF] Peleg | Tropicalization may invert trophic state and carbon budget of shallow temperate rocky reefs[END_REF]. Concern is also raising because either NIS and IAS could also favour the introduction and spread of new pathogens, potentially harmful to native species and, through cascading effects, on the entire ecosystem.

In contrast to the terrestrial realm, in marine ecosystems, where most of NIS have larval (and sometimes adult) dispersion stages that are practically impossible to counteract, and/or possess peculiar biological traits (like the ability to produce new propagules even from fragments; Bulleri et al., 2018a;[START_REF] Gribben | Below-ground processes control the success of an invasive seaweed[END_REF], eradication is impossible, unless at a very early stage of introduction. Nevertheless, for some invasive species, control of their populations can be achieved through dedicated management measures, including commercial exploitation, which could mitigate the negative effects of IAS on native communities and ecosystems [START_REF] Giakoumi | Management priorities for marine invasive species[END_REF][START_REF] Rotter | Non-indigenous species in the Mediterranean Sea: turning from pest to source by developing the 8Rs model, a new paradigm in pollution mitigation[END_REF].

Climate change

Anthropogenic climate change is a major driver of global biodiversity change (Poloczanska et al., 2013), and projections of many ecosystems seem grim under business-as-usual or milder emission scenarios [START_REF] Gattuso | Contrasting futures for ocean and society from different anthropogenic CO2 emissions scenarios[END_REF]. Multiple climate-related pressures are emerging in ocean systems including warming, acidification, deoxygenation, increased storminess, sea level rise and changes in circulation. Consequently, many marine ecosystems are rapidly restructuring (or even collapsing, [START_REF] Burrows | Ocean community warming responses explained by thermal affinities and temperature gradients[END_REF]Garrabou et al., in press) under the effects of climate change, which often interact with non-climatic stressors [START_REF] Gissi | A review of the combined effects of climate change and other human stressors on the marine environment[END_REF].

The Mediterranean Sea is considered a hotspot for climate change [START_REF] Cramer | Climate change and interconnected risks to sustainable development in the Mediterranean[END_REF]. Satellite sea surface observations indicate that the Mediterranean has warmed by 1.48°C on average for the entire basin over the 1982-2018 period [START_REF] Pisano | New evidence of Mediterranean climate change and variability from sea surface temperature observations[END_REF], with a rate 3-6 times faster than the warming rate of oceans globally [START_REF] Cramer | Climate change and interconnected risks to sustainable development in the Mediterranean[END_REF]. The increase of mean temperature has been related to the increase of the frequency and intensity of extreme temperature conditions, i.e. marine heat waves (MHW; [START_REF] Darmaraki | Past variability of Mediterranean Sea marine heatwaves[END_REF]. However, the warming trend is uneven across Mediterranean marine ecoregions, showing higher warming in eastern ecoregions than in the Western ones [START_REF] Skliris | Decadal scale variability of sea surface temperature in the Mediterranean Sea in relation to atmospheric variability[END_REF][START_REF] Salat | Forty-five years of oceanographic and meteorological observations at a coastal station in the NW Mediterranean: a ground truth for satellite observations[END_REF].

The increase in temperature particularly affects water masses from surface down to 80 m depth causing major impacts to the reef assemblages through two main processes: the shift in abundance and distribution of indigenous species and NIS and the occurrence of unprecedented mass mortality events (MMEs). Current warming is driving the decline in abundance of cold-water affinity species [START_REF] Azzurro | Climate change, biological invasions, and the shifting distribution of Mediterranean fishes: a large-scale survey based on local ecological knowledge[END_REF] in favour of warm-water affinity species. These changes are affecting different taxa, including keystone herbivores like sea urchins [START_REF] Yeruham | Collapse of the echinoid Paracentrotus lividus populations in the Eastern Mediterranean-Result of climate change?[END_REF], and are rapidly reshaping the composition of reefs across the entire basin. Two main movement patterns can be recognized: (i) native warm-affinity species are moving northward, while the range of geographic distribution of cold-affinity species is progressively shrinking, and (ii) warm-adapted NIS that entered through the Suez Canal are moving westwards replacing and competing with native species. MMEs have been associated with intense MHWs, which have affected a wide range of rocky reefs from the infralittoral and circalittoral zones by causing severe impacts (with mortality rates up to 80-90%) on at least 100 macrobenthic species (macroinvertebrates, macroalgal and seagrasses), encompassing 9 phyla [START_REF] Garrabou | Collaborative database to track mass mortality events in the Mediterranean Sea[END_REF]. The most severe ones occurred in 1999 and 2003 when more than 30 macrobenthic species along thousands of kilometres of coastlines were affected [START_REF] Garrabou | Mass mortality in Northwestern Mediterranean rocky benthic communities: effects of the 2003 heat wave[END_REF]. Besides these events, several other large scale MMEs as well minor episodes which are usually more restricted in geographic extent and/or number of affected species have been reported [START_REF] Garrabou | Collaborative database to track mass mortality events in the Mediterranean Sea[END_REF]. Most evidence comes from the coldest areas of the Mediterranean (e.g., the northern Adriatic Sea), especially in shallow coralligenous reefs where gorgonians and sponges suffered extensive impacts [START_REF] Garrabou | Collaborative database to track mass mortality events in the Mediterranean Sea[END_REF].

Evidence of impacts related to other climate-related alterations, such as acidification, sea-level rise and deoxygenation, are currently lacking in the scientific literature. Ocean acidification is occurring in the Mediterranean Sea [START_REF] Kapsenberg | Coastal ocean acidification and increasing total alkalinity in the northwestern Mediterranean Sea[END_REF], but it does not seem to be at levels that may cause changes to rocky reefs and associated communities yet. However, in-situ surveys and experiments along natural CO2 gradients as well as laboratory experiments have shown that acidification may strongly affect reef communities by 2100 [START_REF] Milazzo | Ocean acidification impairs vermetid reef recruitment[END_REF][START_REF] Linares | Persistent natural acidification drives major distribution shifts in marine benthic ecosystems[END_REF]. Furthermore, sea level rise is expected to affect mediolittoral reefs through the reduction of the associated biodiversity and the alteration of metabolic rates of marine organisms [START_REF] Rilov | Sea level rise can severely reduce biodiversity and community net production on rocky shores[END_REF].

Climate change is already severely transforming Mediterranean reefs across all ecoregions. There is mounting evidence of multiple collapse of cold-affinity, habitat-forming and keystone reef species, which will be exacerbated by future reduction in pH of sea water and sea-level rise [START_REF] Sala | Alien marine fishes deplete algal biomass in the eastern Mediterranean[END_REF][START_REF] Sarà | The impact of climate change on Mediterranean intertidal communities: losses in coastal ecosystem integrity and services[END_REF]Rilov, 2016;[START_REF] Azzurro | Climate change, biological invasions, and the shifting distribution of Mediterranean fishes: a large-scale survey based on local ecological knowledge[END_REF]. The interaction of these processes will likely promote the shift of Mediterranean reef communities from temperate-Atlantic affinity towards Indopacific affinity through both regional extinctions of native species and the establishment of NIS (see also paragraph 3.4), first in the Levantine basin and in the future, further westward and northward, with profound changes in the functioning of reef ecosystems.

Cumulative human pressures

The Mediterranean Sea ecosystems are under high and escalating pressure from multiple uses and stressors (Lesjeune et al., 2010;[START_REF] Coll | The Mediterranean Sea under siege: spatial overlap between marine biodiversity, cumulative threats and marine reserves[END_REF][START_REF] Micheli | Cumulative human impacts on Mediterranean and Black Sea marine ecosystems: assessing current pressures and opportunities[END_REF], which have led to major shifts in marine ecosystems and widespread conflict among marine users [START_REF] Airoldi | Loss, status and trends for coastal marine habitats of Europe[END_REF][START_REF] Abdulla | Challenges facing a network of representative marine protected areas in the Mediterranean: prioritizing the protection of underrepresented habitats[END_REF][START_REF] Ferretti | Loss of large predatory sharks from the Mediterranean Sea[END_REF][START_REF] Claudet | Human-driven impacts on marine habitats: A regional metaanalysis in the Mediterranean Sea[END_REF]. Because of such intense pressure, the Mediterranean is characterized as a sea 'under siege' [START_REF] Coll | The Mediterranean Sea under siege: spatial overlap between marine biodiversity, cumulative threats and marine reserves[END_REF].

The combination of multiple pressures from fishing, pollution, warming, acidification, direct habitat alteration and biological invasion makes the Mediterranean Sea one of the most impacted marine ecoregion globally [START_REF] Halpern | A global map of human impact on marine ecosystems[END_REF][START_REF] Micheli | Cumulative human impacts on Mediterranean and Black Sea marine ecosystems: assessing current pressures and opportunities[END_REF]. A recent global analysis showed that cumulative impacts to marine ecosystems are increasing in 60% of the ocean, including the Mediterranean Sea [START_REF] Halpern | Recent pace of change in human impact on the world's ocean[END_REF]. In the Mediterranean, the Levantine Sea, the Sicily Channel, parts of the Ionian, Alboran Sea and the western Mediterranean, particularly along the coastlines of Morocco and Algeria, have undergone significant increase in cumulative impacts over a period of only a decade (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013), and no region has seen a decline in cumulative impact over this time period (Fig. 4A).

Mediterranean rocky habitats are subject to multiple pressures (Fig. 4B,C). Summarizing the evidence reported in the previous paragraphs, they are affected by overfishing, which reduces biomass and diversity, alters food web structure and interactions -in some cases triggering trophic cascades and shifts to low-diversity "barren" states [START_REF] Sala | Fishing, trophic cascades, and the structure of algal assemblages: Evaluation of an old but untested paradigm[END_REF][START_REF] Micheli | Cascading human impacts, marine protected areas, and the structure of Mediterranean reef assemblages[END_REF] -or directly impacts populations and alters habitat through removal of habitat forming species (e.g., red coral) and destructive practices, such as the seafloor and benthic assemblage removal associated with illegal date mussel harvesting [START_REF] Guidetti | The destructive date-mussel fishery and the persistence of barrens in Mediterranean rocky reefs[END_REF]. Increased sediment and nutrient loadings lead to benthic community shifts and eutrophication of the water column [START_REF] Airoldi | The effects of sedimentation on rocky coast assemblages[END_REF]. Biological invasions, including by macroalgae and herbivorous rabbitfishes (Siganus spp.) have also resulted in invasive monocultures (e.g., of Caulerpa spp.) or desertification (Cebrian and Rodriguez-Prieto, 2012;[START_REF] Sala | Alien marine fishes deplete algal biomass in the eastern Mediterranean[END_REF]. Major, escalating threats to Mediterranean rocky reefs are temperature warming and MHWs, which are causing species range shifts, reef tropicalization, and mass mortalities [START_REF] Garrabou | Mass mortality in Northwestern Mediterranean rocky benthic communities: effects of the 2003 heat wave[END_REF].

Climate drivers are the main contributors to increased cumulative impact to rocky reefs, but overfishing and pollution are also key drivers of increased impact (Fig. 4B,C). Compared with other habitats, rocky reefs are impacted by the largest suite of different stressors, even though the magnitude of impact for some is lower than for several other ecosystem types (Fig. 4C). Expanding and strengthening rocky reef protection through the establishment and improved management of marine protected areas, and promoting climate resilience by reducing cumulative impact are key priorities for reversing these escalating trends of reef degradation.

Conservation achievements and next targets

Current conservation measures

Rocky reef ecosystems are in peril at basin scale, with increasing evidence of ongoing regime shifts across habitats [START_REF] Claudet | Human-driven impacts on marine habitats: A regional metaanalysis in the Mediterranean Sea[END_REF][START_REF] Benedetti-Cecchi | Chapter 8. Species Interactions and Regime Shifts in Intertidal and Subtidal Rocky Reefs of the Mediterranean Sea[END_REF]. Measuring progress to the European 2020 Biodiversity Strategy, [START_REF] Gubbay | European Red List of Habitats. Part 1[END_REF] documented that a critical proportion of rocky reef ecosystems are either vulnerable or endangered. They included macroalgaldominated communities and circalittoral rocks, mussel and oyster beds and biogenic concretions such as those of the red algae Lithophyllum byssoides, platforms formed by the algae Neogoniolithon brassica-florida and the gastropod Dendropoma petraeum in the European Red List of Habitats. However, the report highlighted that insufficient quantitative data often prevent determining the status of most marine ecosystems, suggesting that the list might be even longer than currently assumed to be.

There is a strong need for prioritizing conservation initiatives for these productive, diverse and vulnerable environments. Addressing the status of Mediterranean infralittoral rocky reefs, Bevilacqua et al. (2020) reported that two-thirds of sites included in their study were classified to be in moderate to poor conditions. While it should be recognized that >50% of investigated sites for infralittoral reefs were within MPAs and/or the European Conservation Network (i.e., Natura 2000 sites), care should be given in not overestimating the potential benefits of those conservation strategies to preserve the integrity of rocky reef ecosystems (Fig. 5).

Applying the recently developed Regulation-Based Classification System for MPAs [START_REF] Horta E Costa | A regulationbased classification system for Marine Protected Areas (MPAs)[END_REF], [START_REF] Claudet | Underprotected marine protected areas in a global biodiversity hotspot[END_REF] showed that while the 1062 MPAs in the Mediterranean Sea cover 6% of the Mediterranean Basin, 95% of the total protected area lacks regulations to reduce human impacts on biodiversity. Unevenly distributed across political boundaries and eco-regions, effective levels of protection for biodiversity conservation represent only 0.23% of the Mediterranean Basin [START_REF] Zupan | Marine partially protected areas: drivers of ecological effectiveness[END_REF]. In the French Mediterranean, while infralittoral rock and other hard substrata are the habitats most covered by full or high protection, only 2.6% of these habitats receive such levels of protection [START_REF] Claudet | Critical gaps in the protection of the second largest exclusive economic zone in the world[END_REF]. It is clearly evident that current efforts are insufficient at managing human uses of nature at sea and protection levels should be increased to deliver tangible benefits for biodiversity conservation in general and for these habitats in particular. There are few exceptions. A systematic review along the Italian coasts about the occurrence and conservation status of Cystoseira s.l. concluded that while only a negligible proportion of their populations was protected (3%) before 2000, the proportion of "protected" populations has increased in recent years to 77.8%, with nearly two thirds of the populations included in MPAs (Tamburello et al., unpublished data). MPAs might guarantee protection to Cystoseira s.l., with recent evidences showing higher coverage inside MPAs compared to unprotected areas [START_REF] Franco | Inconsistent relationships among protection, benthic assemblage, habitat complexity and fish biomass in Mediterranean temperate rocky reefs[END_REF], by limiting coastal urbanization and overfishing, including illegal and destructive practices [START_REF] Colletti | The date mussel Lithophaga lithophaga: Biology, ecology and the multiple impacts of its illegal fishery[END_REF], and avoiding overgrazing allowing the recovery of high-level predators, controlling herbivores populations, or directly limiting the discard of dismissed fishing gears.

Despite the European Union having progressed in implementing an imponent legal framework for marine resources protection and management, there are still important gaps [START_REF] Fraschetti | Light and shade in marine conservation across European and Contiguous Seas[END_REF] and intertidal and subtidal rocky reef ecosystems suffer from the lack of specific conservation interventions. Currently, there are no specific conservation actions for macroalgal habitats and the action plans on Protecting Coralligenous and other calcareous BioConcretions in the Mediterranean developed by Mediterranean institutions are not legally binding. The consequence is that, for most of the Mediterranean Sea, the status of the category Reef (1170) of the Habitat Directive is insufficient in terms of both 'structure and functioning' and 'future prospects'.

Future conservation needs

The failure of past conservation measures to achieve large-scale outcomes for Mediterranean reefs stresses the urgent need for holistic, integrated, effective and more ambitious conservation strategies to improve their status (Fig. 6). Shifting from the existing paper-park-dominated networks of MPAs [START_REF] Claudet | Underprotected marine protected areas in a global biodiversity hotspot[END_REF] to well-connected networks with extended no-take areas and wellmanaged multi-use MPAs that can effectively contribute to the recovery of reef ecosystems is crucial [START_REF] Bevilacqua | Mitigating human disturbance: can protection influence trajectories of recovery in benthic assemblages?[END_REF][START_REF] Sala | The Structure of Mediterranean Rocky Reef Ecosystems across Environmental and Human Gradients, and Conservation Implications[END_REF][START_REF] Thibaut | An ecosystem-based approach to assess the status of Mediterranean algae-dominated shallow rocky reefs[END_REF]. The ambitious new European Biodiversity Strategy for 2030 and its targets for 30% protection -10% strict protectionoffer an unprecedented opportunity to achieve large scale conservation of rocky reefs, substantially upgrading the current low level of the Mediterranean being effectively protected. Towards these targets, it is critical to apply a systematic conservation planning process [START_REF] Katsanevakis | Twelve Recommendations for Advancing Marine Conservation in European and Contiguous Seas[END_REF], through which the full compositional and functional variability and diversity of rocky reef ecosystems is adequately represented [START_REF] Bevilacqua | Nestedness and turnover unveil inverse spatial patterns of compositional and functional β-diversity at varying depth in marine benthos[END_REF].

More than that, reduction of stressors also outside MPAs is critical. Better fisheries management and a strong overall reduction of fishing effort also in non-protected areas are needed to improve the status of rocky reef ecosystems at a large scale. A reduced fishing pressure has been related to substantially increased fish abundance and biomass in non-protected areas, including that of predators [START_REF] Sini | Smallscale coastal fishing shapes the structure of shallow rocky reef fish in the Aegean Sea[END_REF], which in many cases can be essential for the recovery of reef ecosystems via cascading effects [START_REF] Sala | Fishing, trophic cascades, and the structure of algal assemblages: Evaluation of an old but untested paradigm[END_REF][START_REF] Hereu | Depletion of palatable algae by sea urchins and fishes in a Mediterranean subtidal community[END_REF]. Better integration of conservation strategies and the ecosystem approach of fisheries management in the EU Common Fisheries Policy and the regional fisheries management by the General Fisheries Commission for the Mediterranean will be beneficial for both reef conservation and the related Mediterranean fisheries, which are currently largely unsustainable [START_REF] Colloca | Recent trends and impacts of fisheries exploitation on Mediterranean stocks and ecosystems[END_REF].

The need to include biological invasions and climate change considerations in conservation strategies has been highlighted as a necessity but also a grand challenge [START_REF] Micheli | Evidence that marine reserves enhance resilience to climatic impacts[END_REF][START_REF] Katsanevakis | Twelve Recommendations for Advancing Marine Conservation in European and Contiguous Seas[END_REF]. Despite the effectiveness of protection measures within MPAs, invasive species and climate change can substantially compromise any efforts to conserve or restore reef ecosystems [START_REF] Rilov | Alien turf: Overfishing, overgrazing and invader domination in south-eastern Levant reef ecosystems[END_REF][START_REF] Dimitriadis | Long term interactions of native and invasive species in a marine protected area suggest complex cascading effects challenging conservation outcomes[END_REF], often enhancing the impacts of overfishing, pollution and other stressors [START_REF] Gissi | A review of the combined effects of climate change and other human stressors on the marine environment[END_REF]. Although there are important progresses in incorporating climate change in conservation strategies [START_REF] Johnson | Should potential for climate change refugia be mainstreamed into the criteria for describing EBSAs?[END_REF][START_REF] Katsanevakis | Twelve Recommendations for Advancing Marine Conservation in European and Contiguous Seas[END_REF], developing frameworks to incorporate biological invasions in conservation planning [START_REF] Mačić | Biological invasions in conservation planning: a global systematic review[END_REF], and prioritizing management measures for invasive species [START_REF] Giakoumi | Management priorities for marine invasive species[END_REF], current guidelines and practices for expanding the network of MPAs in the Mediterranean totally ignore the issues. Future conservation strategies (Fig. 6) need to be adaptive to account for change, identify and consider climate refugia for reef conservation and restoration, set different targets for climate change hotspots focusing on securing reef ecosystem functioning, account for both negative and positive impacts of alien species and adopt appropriate mitigation measures, and promote the identification of climate-driven eco-evolutionary changes and climate-resistant algal populations to be used for reef restoration (Rilov et al., 2019a;2020b;[START_REF] Katsanevakis | Twelve Recommendations for Advancing Marine Conservation in European and Contiguous Seas[END_REF].

Frontiers in the restoration of rocky habitats

The exacerbated speeds of global and local changes from multiple stressors are decreasing the resilience of rocky habitats globally [START_REF] Airoldi | The loss of natural habitats and the addition of artificial substrata[END_REF], hampering their ability to recover naturally following passive management efforts [START_REF] O'leary | The resilience of marine ecosystems to climatic disturbances[END_REF]. With the pace of current loss and degradation there is a need for more active intervention-based approaches that can integrate social, economic and environmental objectives [START_REF] Abelson | Upgrading marine ecosystem restoration using ecological-social concepts[END_REF]. Marine habitat restoration is an emerging field to actively reverse the degradation and loss of natural ecosystems [START_REF] Abelson | Challenges for restoration of coastal marine ecosystems in the Anthropocene[END_REF] recognized by the UN Decade on Ecosystem Restoration (2021Restoration ( -2030)). Here, we use restoration in its broadest possible meaning [START_REF] Airoldi | Emerging solutions to return nature to the urban[END_REF], encompassing multiple approaches to both actively repair the damaged marine life as well as to "green" the currently grey hard infrastructure. Examples include active interventions to restore canopy forming seaweeds in both their natural habitats (De La Fuente et al., 2019b) as well as in artificial habitats used as "gardens" where their natural habitat is under threat [START_REF] Perkol-Finkel | Conservation challenges in urban seascapes: Promoting the growth of threatened species on coastal infrastructures[END_REF].

Marine restoration practice and governance are still in their infancy both in terms of research as well as political, legal and economic support [START_REF] Abelson | Challenges for restoration of coastal marine ecosystems in the Anthropocene[END_REF][START_REF] Bayraktarov | The cost and feasibility of marine coastal restoration[END_REF]. The challenge of safeguarding ecosystem services is especially prominent for the Mediterranean Sea, where knowledge of marine ecosystems status and dynamics is still fragmented [START_REF] Airoldi | Loss, status and trends for coastal marine habitats of Europe[END_REF][START_REF] Gianni | Conservation and restoration of marine forests in the Mediterranean Sea and the potential role of Marine Protected Areas[END_REF]). Yet, significant research is emerging, aiming at identifying both the feasibility and self-sustainability of restoration efforts as well as adequate techniques and spatial layouts [START_REF] Claudet | Human-driven impacts on marine habitats: A regional metaanalysis in the Mediterranean Sea[END_REF]. Concerning rocky habitats, most of these research efforts have focused on the recovery of macroalgal stands [START_REF] Verdura | Restoration of a canopyforming alga based on recruitment enhancement: Methods and long-term success assessment[END_REF][START_REF] Guarnieri | Large-scale sea urchin culling drives the reduction of subtidal barren grounds in the Mediterranean Sea[END_REF][START_REF] Orlando-Bonaca | First restoration experiment for Gongolaria barbata in slovenian coastal waters. What can go wrong[END_REF][START_REF] Savonitto | Addressing reproductive stochasticity and grazing impacts in the restoration of a canopyforming brown alga by implementing mitigation solutions[END_REF]. Recent efforts have also focused on the restoration of populations of invertebrate species, such as the scleractinian coral Astroides calycularis [START_REF] Musco | Turning casualty into opportunity: fragmenting dislodged colonies is effective for restoring reefs of a Mediterranean endemic coral[END_REF], the bryozoan Pentapora fascialis [START_REF] Pagès-Escolà | Assessing the effectiveness of restoration actions for Bryozoans: The case of the Mediterranean Pentapora fascialis[END_REF], and the limpet Patella ferruginea [START_REF] Ferranti | Are there life-history constraints on restoration of the endangered limpet Patella ferruginea (Mollusca, Gastropoda) in the northern Mediterranean Sea?[END_REF], with many novel technologies and approaches emerging, such as or the use of electro-mineral accretion for enhancing the settlement of the red coral Corallium rubrum [START_REF] Benedetti | Applying cathodically polarised substrata to the restoration of a high value coral[END_REF]. Increasing research has also focused on the bioenhancement of artificial habitats by increasing surface topographic complexity, or by using more environmentally friendly construction materials, with some successful examples but also uncertainties about the consistency of the benefits [START_REF] Strain | A global analysis of complexity-biodiversity relationships on marine artificial structures[END_REF].

There is real interest and critical opportunities to support marine restoration actions in the Mediterranean sea for the benefit of people and nature. Large-scale, long-lasting test projects are needed to nurture the most innovative ideas, overcome the barriers to scaling and address some technical challenges. This restoration, however, will rarely, if ever, result in a return to a historical state, forcing us to actively consider what we think those ecosystems should consist of.

Next challenges and perspectives in ecological research

Understanding how rocky reef communities persist in the face of intensifying climate change and escalating cumulative anthropogenic disturbances is a research priority to preserve the structure and functioning of these systems and to promote informed conservation policies. Persistence is generally understood in terms of components of stability, such as resistance and resilience to perturbations and adaptation (Donhue et al., 2016;[START_REF] Hillebrand | Decomposing multiple dimensions of stability in global change experiments[END_REF]. Despite increasing research efforts to elucidate the effects of natural and anthropogenic disturbances on rocky reef communities, a clear understanding of the mechanisms underpinning stability and adaptation has remained elusive.

Regime shifts in macroalgal forests are a case in point. Several studies have documented the collapse of these systems in response to pollution, urban development and degrading environmental quality more generally [START_REF] Benedetti-Cecchi | Predicting the consequences of anthropogenic disturbance: large-scale effects of loss of canopy algae on rocky shores[END_REF][START_REF] Wernberg | Seaweed communities in retreat from ocean warming[END_REF][START_REF] Strain | Identifying the interacting roles of stressors in driving the global loss of canopy-forming to mat-forming algae in marine ecosystems[END_REF].

Experiments and models have shown how these responses may involve nonlinearities and thresholds, implying abrupt transitions (regime shifts) towards alternative states, often characterized by algal turfs or barren habitats [START_REF] Benedetti-Cecchi | Experimental perturbations modify the performance of early warning indicators of regime shift[END_REF][START_REF] Rindi | Direct observation of increasing recovery length before collapse of a marine benthic ecosystem[END_REF]) (see Fig. 3). The occurrence of nonlinear dynamics implies the existence of reinforcing (feedback) mechanisms that stabilize the system in one state or the other [START_REF] Scheffer | Critical transitions in nature and society[END_REF]. For example, successful recruitment is more likely to occur nearby dense stands of canopy species with limited dispersal capabilities (e.g., [START_REF] Capdevila | Effective dispersal and density-dependence in mesophotic macroalgal forests: insights from the Mediterranean species Cystoseira zosteroides[END_REF]. Hence, an expanding patch of algal canopy can reinforce itself because it will produce larger and larger numbers of propagules as it grows. Similarly, an expanding patch of canopy algae can become increasingly resistant to dislodgement owing to the coalescence of an increasing number of holdfasts, as observed in some canopy-forming algae [START_REF] Gonzáles | Morphological convergence in the inter-holdfast coalescence process among kelp and kelp-like seaweeds (Lessonia, Macrocystis, Durvillaea)[END_REF]. Reinforcing mechanisms should also be at work to stabilize the alternative states. In principle, established mats of algal turfs may persist by inhibiting the recruitment of canopy algae [START_REF] Benedetti-Cecchi | Effects of canopy cover, herbivores and substratum type on patterns of Cystoseira spp. settlement and recruitment in littoral rock pools[END_REF] and by reducing desiccation stress through water retention in intertidal habitats. Both mechanisms are expected to strengthen as algal turfs grow and develop into thick mats. These runway mechanisms have been invoked to explain the nonlinear dynamics observed in Ericaria amentacea var. stricta [START_REF] Benedetti-Cecchi | Experimental perturbations modify the performance of early warning indicators of regime shift[END_REF][START_REF] Rindi | Direct observation of increasing recovery length before collapse of a marine benthic ecosystem[END_REF]. However, the limited dispersal of zygotes in this species has been inferred mostly from laboratory studies and the extent to which coalescence of holdfasts increases stability remains unquantified [START_REF] Clayton | Propagules of marine macroalgae: structure and development[END_REF].

Similarly, the stabilizing mechanisms proposed for algal turfs have been rarely tested. All these reinforcing feedbacks involve density-dependent processes as a common underlying cause. Densitydependent processes are pervasive in ecological communities and similar feedbacks may occur generally across species and habitats [START_REF] Aubier | Positive density dependence acting on mortality can help maintain species-rich communities[END_REF]. For example, mechanisms similar to those described for algal canopies may stabilize benthic communities dominated by suspension feeders, such as gorgonian forests. To the best of our knowledge, no single study has investigated these mechanisms on rocky shores and linked them to stability. We believe the lack of empirical testing of essential stabilizing mechanisms is a critical knowledge gap that hampers our ability to anticipate the response of rocky reef communities to environmental change.

Understanding the mechanisms underpinning variation in species tolerance to changing climates will be key to preserving the biodiversity and functioning of coastal rocky systems through informed restoration and conservation (e.g., MPA networks, genetic rescue) practices. This will require the coupling of common garden and reciprocal transplant experiments with cutting-edge genomic and transcriptomic techniques [START_REF] Johnson | Reciprocal transplant gardens as gold standard to detect local adaptation in grassland species: New opportunities moving into the 21st century[END_REF]. In common garden experiments, the exposure of offspring and adults from different populations to a common set of environmental conditions, including extreme events, such as heatwaves, would allow assessing variation in their performance and fitness across populations. Aquatic mesocosms appear as the elective playground for this kind of experiments. While the advantages of a fine control of environmental conditions in artificial settings are undisputable, field experiments offer the opportunity to assess species response to changing conditions while embedded into the complex web of species interactions.

In particular, reciprocal transplant experiments in the field would represent a powerful tool to identify genetic versus phenotypic plasticity adaptation of populations in natural settings. These experiments will be particularly valuable when studied populations span broad environmental gradients [START_REF] Johnson | Reciprocal transplant gardens as gold standard to detect local adaptation in grassland species: New opportunities moving into the 21st century[END_REF]. This implies taking into account variations in key environmental and climatic variables over a hierarchy of spatial scales, from geographic to local. The relevance of assessing variations in a species tolerance to warming conditions among populations at edges of its latitudinal distribution is widely acknowledged [START_REF] Hampe | Conserving biodiversity under climate change: the rear edge matters[END_REF]Bulleri et al., 2018b).

However, latitudinal gradients are not always linearly correlated with seawater and air temperature and, even less, with the degree of mean change or the intensity and frequency of extreme events [START_REF] Helmuth | Climate change and latitudinal patterns of intertidal thermal stress[END_REF][START_REF] Burrows | The pace of shifting climate in marine and terrestrial ecosystems[END_REF]. For example, viable populations from areas that have been, historically, exposed to extreme events of larger intensity and/or more frequently can be expected to exhibit a greater tolerance to adverse climatic conditions [START_REF] Coleman | The silver lining of extreme events[END_REF]. Thus, maps of past climatic conditions may be of aid in selecting relevant populations for tolerance/adaptation studies.

Dispersal potential of target species represents another key aspect to take into account while attempting to assess variations in local adaptation since connectivity among populations is paramount for sustaining climate-readiness of restoration activities and MPA networks. Assessing local adaptation across fine-scale gradients in environmental conditions appears as crucial in shortdispersal species. In the Mediterranean, canopy-forming macroalgae are often distributed from the surface to depths below the seasonal thermocline and individuals only a few m apart are, thus, exposed to very different thermal environments. Likewise, substrate topography, orientation and slope can create a fine-scale thermal mosaic in intertidal environments. Reciprocal transplants of individuals, possibly both adults and offspring, from different depths, heights on the shore or from intertidal microhabitats characterized by benign versus adverse environmental conditions (i.e., desiccation, temperature) would provide an insight into local adaptation within populations and, hence, into their potential to persist in the face of change.

Genomic tools can complement the information generated by field and mesocosm experiments providing a mechanistic understanding of the genetic basis of local adaptation. Progressive reduction of costs of next-generation sequencing techniques has fostered the production of genomic information and is giving momentum to conservation genomics of marine forests [START_REF] Mamo | Conservation genomics of a critically endangered brown seaweed[END_REF]. For instance, [START_REF] Wood | Genomic vulnerability of a dominant seaweed points to future-proofing pathways for Australia's underwater forests[END_REF] characterized neutral and adaptive genetic diversity in the brown seaweed Phyllospora comosa and how it correlates with temperature, generating an insight into genetic vulnerability under warming climates, crucial information for climate-proof restoration of this species. In addition, improvement of transcriptomic analyses can allow assessing how relevant environmental variables influence gene expression, fostering a mechanistic understanding of species physiological response to changing climates (Li et al., 2019).

A research agenda on ecological persistence and adaptation should develop following state-of-theart approaches and methodologies. For example, distributed experiments might be used to assess the generality (or lack thereof) of dispersal limitation and density-dependent processes along geographic gradients and in contrasting environmental conditions. The Mediterranean provides an invaluable model system for this purpose. New technological tools such as environmental DNA should be calibrated and used to capture phenological events, such as massive reproductive outputs of algae and invertebrates. Novel approaches that combine observational and experimental data in the same analytical framework have been proposed to increase the scale and inferential strength of ecological studies [START_REF] Benedetti-Cecchi | Hybrid datasets: integrating observations with experiments in the era of macroecology and big data[END_REF]. The implementation of these approaches is facilitated by the increasing availability of open datasets and by joint collaborative efforts among researchers. Our final recommendation is that a research agenda on persistence and adaptation should focus primarily on habitat-forming species, since these contribute most to the structure and functioning of rocky reef communities.

The fate of Mediterranean rocky reefs

The fate of Mediterranean rocky reefs remains highly uncertain, as is the fate of the entire basin, but current trends in cumulative human pressure and climate change do not bode well for a bright future. Next years will be decisive for the persistence of Mediterranean rocky reef ecosystems as we have known them so far. Most probably, we will see a relatively fast process of oversimplification of their communities, both in terms of structure and functioning, accompanied with a depletion of biodiversity and an increased biotic homogenization driven by opportunistic native species and NIS.

Bioconstructions in the midlittoral zone, such as vermetid reefs, which already have limited extension in the basin and are highly fragile and fragmented, will drastically reduce or disappear being more exposed to climate-related disturbances. Infralittoral algal forests will continue to shrink, being replaced by less structured communities dominated by smaller erect algae, algal turfs and crusts. Deeper communities in the circalittoral zone will be less affected by climate change, at least in the short-medium term. However, acidification and other interacting human stressors, sooner or later, will start to mine the growth rates of coralligenous outcrops enhancing bioerosive processes [START_REF] Ponti | The understorey of gorgonian forests in mesophotic temperate reefs[END_REF]. MHWs and MMEs, increased sedimentation and deoxygenation will contribute to simplify coralligenous communities, making them more similar to fouling assemblages. The loss of long-living ecosystem engineers, like gorgonians and large sponges, will trigger a gradual, and probably irreversible, process of habitat degradation, increasing the opportunity for NIS to spread and leading to less complex assemblages.

Changes will not be uniform over the entire Mediterranean Sea, but slower and smaller in areas less compromised by human impacts and less affected by climate-driven alterations. Rocky reefs in the eastern basin will become more and more tropicalized, losing their Mediterranean identity, whereas this progression will be delayed in the other regions, although early signals of change might also occur in previous cold-temperate areas (e.g., the northern Adriatic Sea). However, in the western Mediterranean Sea, the higher rates of intensification of fishing effort, spread of new pollutants, and of artificialization of rocky coasts will further undermine the resilience potential of reef ecosystems, making them more exposed to bioinvasions and more prone to collapse in the face of future climatic anomalies. In both regions, the process of reorganization (sensu [START_REF] Holling | Understanding the complexity of economic, ecological, and social systems[END_REF] of rocky reef ecosystems will probably be slower than current and future rates of disruption, altering, if not dismissing, the provision of essential goods and services for a long period. This will pose serious concerns (and, indeed, they are already raising) on the ability of socioeconomic systems of Mediterranean coastal areas, which largely rely on rocky reef integrity especially for tourism and small scale fisheries (UfMS, 2017), to adapt to these ecological changes.

The substantial expansion of the coverage and improvement of effectiveness of Mediterranean MPAs through the implementation of the European Biodiversity Strategy and the Green Deal, and the development of efficient conservation strategies, which beyond the structure may also allow preserving the functional diversity of reef communities [START_REF] Bevilacqua | Nestedness and turnover unveil inverse spatial patterns of compositional and functional β-diversity at varying depth in marine benthos[END_REF], are expected to contribute in decelerating or even reversing the declining trend of the ecological status of rocky reef ecosystems. Nevertheless, a number of challenges may compromise the effectiveness of future conservation strategies, and MPAs alone cannot be a panacea [START_REF] Lubchenco | Plugging a hole in the ocean: the emerging science of marine reserves[END_REF].

Climate change and biological invasions can keep increasing stress to reef ecosystems, and forthcoming socioeconomic pathways can lead to a wide range of plausible futures, with a varying effect in shaping priorities in marine policies and conservation [START_REF] Van Vuuren | A new scenario framework for climate change research: scenario matrix architecture[END_REF][START_REF] Roura-Pascual | Alternative futures for global biological invasions[END_REF]. Progresses in ecological restoration will certainly be of help in contrasting the degradation of rocky reefs in the next years, although the intrinsic difficulties of restoration techniques and constraints to their application over large areas [START_REF] Falace | Ex situ cultivation protocol for Cystoseira amentacea var. stricta (Fucales, Phaeophyceae) from a restoration perspective[END_REF][START_REF] Guarnieri | Large-scale sea urchin culling drives the reduction of subtidal barren grounds in the Mediterranean Sea[END_REF] will probably relegate restoration to an ancillary role in their conservation or recovery.

Moreover, hysteretic behaviour of reef ecosystems could vanish the attempts to restore their integrity in the absence of management strategies aiming at enhancing the wider environmental context [START_REF] Anthony | Operationalizing resilience for adaptive coral reef management under global environmental change[END_REF].

In 2012, the European Commission launched the Blue Growth Strategy (COM/2021/0494), identifying the European seas and oceans as the frontier for future sustainable socioeconomic development. The working concept of sustainable blue growth defines it as an economy that '…promotes economic growth, social inclusion and improved livelihoods while ensuring the environmental sustainability of the natural capital of the oceans and seas' [START_REF] Cinea | European Climate, Infrastructure and Environment Executive Agency (CINEA). Sustainable criteria for the blue economy -Main report[END_REF]. This is not an easy task that requires balancing often conflicting needs within marine socioecological systems, implementing development strategies carefully tuned to match local contexts, and monitoring their effectiveness through specific indicators to ensure timely adaptations to environmental changes [START_REF] Cinea | European Climate, Infrastructure and Environment Executive Agency (CINEA). Sustainable criteria for the blue economy -Main report[END_REF]. Given the current status, the pace of ongoing degradation of Mediterranean rocky reefs and future projections, any further use of their resources seems to be unrealistic. Therefore, at least for these coastal systems, the importance of the two elements composing the concept of blue growth should be reversed, emphasizing the need for a healthy status of reefs over the rush for new economic opportunities. Major efforts in policies and regulations specifically tailored for Mediterranean rocky reefs are urgently required to make the integrity of these habitats a priority for conservation and management at European and basin scale, and try to give the chance for a less troubled future to Mediterranean coastal ecosystems. (Aegean Sea, Greece); Maërl 8 (NW Mediterranean, Spain); submarine caves 9 (Ionian Sea, Italy). Sublittoral cliffs 10 (black contour, Adriatic Sea, Croatia): the sequence of panels showed the gradient in benthic assemblages from 2-3 m until 60 m depth on the rocky wall. Photo credits: G.

Rilov 1 , L. Benedetti-Cecchi 2 , E. Ballesteros 3,4,5,8 , G. Guarnieri 6,9 , Th. Dailianis 7 , G. Guarnieri and S. barren grounds and turf-dominated assemblages. In good environmental conditions, the system is dominated by habitat-forming macroalgae (e.g., Cystoseira s.l.) (A + ). Resilience is high, and pulse disturbances, if occurring, are unable to trigger a critical transition, with perturbations of the state (dotted black lines) being rapidly absorbed. As human-driven deterioration of environmental conditions increase and cause chronic disturbance (red landscape), the system approximates to a tipping point. Diversity decreases, the trophic network starts to disrupt and the recovery potential slows down, increasing system fragility (A -). In such a situation, pulse disturbance events (e.g., organic enrichment, herbivore outbreaks) could drive the system to shift toward algal turfs (B) or barren grounds (C). Depending on changing conditions, the system could exhibit a 'flickering' behaviour between alternate stable states B and C. However, the return to the original state (A + ) is difficult to happen if not over a long period, unless a substantial improvement of environmental conditions occurs and active restoration interventions are carried out to help the system to recover.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. (A) Percentage abundance of sessile and vagile taxa associated with bioconstructions

Figure 3 .

 3 Figure 3. Conceptual representation of regime shifts from Mediterranean algal forests towards

Figure 4 .

 4 Figure 4. (A) Cumulative human impacts (up to 2013, top panel), and annual change in cumulative

Figure 5 .

 5 Figure 5. Ecological status of infralittoral rocky reefs in several sites across the Mediterranean Sea

Figure 6 .

 6 Figure 6. Improvements of management policies and conservation strategies are needed to