Clementine Leroy
email: clementine.leroy@inrae.fr

Jean-Luc Brunet

Mickael Henry

Cedric Alaux
email: cedric.alaux@inrae.fr

Using physiology to better support wild bee conservation

There is accumulating evidence that wild bees are experiencing a decline in terms of species diversity, abundance or distribution, which leads to major concerns about the sustainability of both pollination services and intrinsic biodiversity. There is therefore an urgent need to better understand the drivers of their decline, as well as design conservation strategies. In this context, the current approach consists of linking observed occurrence and distribution data of species to environmental features. While useful, a highly complementary approach would be the use of new biological metrics that can link individual bee responses to environmental alteration with population-level responses, which could communicate the actual bee sensitivity to environmental changes and act as early warning signals of bee population decline or sustainability. We discuss here through several examples how the measurement of bee physiological traits or performance can play this role not only in better assessing the impact of anthropogenic pressures on bees, but also in guiding conservation practices with the help of the documentation of species' physiological needs. Last but not least, because physiological changes generally occur well in advance of demographic changes, we argue that physiological traits can help in predicting and anticipating future population trends, which would represent a more proactive approach to conservation. In conclusion, we believe that future efforts to combine physiological, ecological and population-level knowledge will provide meaningful contributions to wild bee conservation-based research.

Introduction

Like many organisms on earth, wild bees are facing growing anthropogenic pressures, including agricultural intensification (e.g. habitat loss, lack of food resources and pesticide use), climate change and the spread of invasive species and diseases [START_REF] Potts | Global pollinator declines: trends, impacts and drivers[END_REF]. Consequently, in recent decades, a decline in bee species richness has been observed on all continents except Oceania [START_REF] Biesmeijer | Parallel declines in pollinators and insect-pollinated plants in Britain and the Netherlands[END_REF][START_REF] Cameron | Patterns of widespread decline in North American bumble bees[END_REF][START_REF] Ollerton | Extinctions of aculeate pollinators in Britain and the role of large-scale agricultural changes[END_REF][START_REF] Zattara | Worldwide occurrence records suggest a global decline in bee species richness[END_REF]. In fact, according to the International Union for Conservation of Nature and Natural Resources, for which geographic range and population size are two of the criteria used for assessing conservation status, around 9% of the wild bee species in Europe could be declared as threatened, a percentage potentially underestimated given the high data deficiency for over half of the European bee species [START_REF] Nieto | European Red List of Bees[END_REF][START_REF] Potts | The Assessment Report on Pollinators, Pollination and Food. Production[END_REF]. This is especially worrisome given that wild bees, through pollination, play a critical role in the maintenance and functioning of both natural and agricultural ecosystems [START_REF] Klein | Importance of pollinators in changing landscapes for world crops[END_REF][START_REF] Ollerton | How many flowering plants are pollinated by animals?[END_REF][START_REF] Garibaldi | From research to action: enhancing crop yield through wild pollinators[END_REF]. This crucial role in maintaining pollination services therefore provides strong arguments for promoting their conservation [START_REF] Winfree | Species turnover promotes the importance of bee diversity for crop pollination at regional scales[END_REF] in addition to the argument of their intrinsic value of biodiversity.

As a result, there is an urgent need to (i) better identify population trends at local or national scales, as well as their environmental drivers and (ii) develop agricultural practices that are as supportive as possible for bee populations. The evaluation of wild bee population status and trends generally relies on species records, giving an estimate of species abundance, distribution and richness [START_REF] Winfree | A metaanalysis of bees'responses to anthropogenic disturbance[END_REF][START_REF] Zattara | Worldwide occurrence records suggest a global decline in bee species richness[END_REF]. Combined with statistical modelling, such data can help in identifying drivers of bee decline. However, standardized (i.e. not opportunistic recordings by volunteers) and long-term surveys of pollinator occurrence are needed to reliably identify pollinator trends and therefore determine the impact of environmental changes [START_REF] Breeze | Pollinator monitoring more than pays for itself[END_REF][START_REF] Garibaldi | Crop pollination management needs flower-visitor monitoring and target values[END_REF]. Interestingly, a 10-year monitoring scheme was identified as the most suitable by bee experts for detecting long-term trends in bee populations, with longer periods for assessing the influence of climate change (30 years) or landscape complexity (15 years) [START_REF] Breeze | Pollinator monitoring more than pays for itself[END_REF]. Nevertheless, from an ecological point of view, conservation action should start before a species is endangered, i.e. before its population size declines to critical levels [START_REF] Soulé | Viable Populations for Conservation[END_REF]. Such proactive conservation approaches, besides being more successful, are also more cost effective than reactive strategies [START_REF] Drechsler | Does proactive biodiversity conservation save costs?[END_REF]. Therefore, alternative methods that are more time-efficient and costeffective would highly benefit the conservation of wild bee populations.

In this context, conservation biologists need biological metrics that are sensitive to environmental changes and/or suitable as early warning signals of bee population declines (i.e. before they become local or regional extinctions). For that purpose, physiology offers a promising framework to link bee responses to environmental alteration with populationlevel responses. First, physiology is highly sensitive and often changes quickly to allow the organism to respond to environmental disturbance. Second, physiological changes can be linked to life-history traits tightly associated with population dynamics and sustainability, such as reproductive performance and survival [START_REF] Chown | Physiological variation in insects: large-scale patterns and their implications[END_REF][START_REF] Cooke | Physiology, behavior, and conservation[END_REF]. Accordingly, physiological changes generally occur well in advance of demographic changes and before a potential population decline [START_REF] Ellis | Integrating landscape ecology and conservation physiology[END_REF], and therefore can help in characterizing population dynamics and its environmental constraints [START_REF] Chown | Trait-based approaches to conservation physiology: forecasting environmental change risks from the bottom up[END_REF].

Physiological knowledge and tools, spanning a broad range of disciplines, such as endocrinology, immunology, neurophysiology, toxicology, bioenergetics and reproductive physiology, have been used for many years in solving conservation issues [START_REF] Cooke | What is conservation physiology? Perspectives on an increasingly integrated and essential science[END_REF]. This approach integrating physiology into the challenges of conservation has contributed to several successful stories, especially in vertebrates [START_REF] Madliger | Success stories and emerging themes in conservation physiology[END_REF], attesting to its high potential and interest for delivering practical guidelines in conservation [START_REF] Birnie-Gauvin | Conservation physiology can inform threat assessment and recovery planning processes for threatened species[END_REF]. Due to its potential for identifying drivers of population decline, monitoring population status and assessing the efficiency of remedial actions, we believe this approach could also benefit the field of wild bee conservation. We outline several examples of the power of physiological metrics for better understanding the influence of anthropogenic stressors and developing conservation strategies. We then highlight the key challenges that will need to be addressed for monitoring and predicting population-level changes using physiological measurements in wild bees.

Physiology Provides a Better Understanding of the Impact and Consequences of Anthropogenic Pressures on Bees

All bees (social and solitary) have the particularity to be central-place foragers. Once adult, bees establish a nest and forage for pollen and nectar around that nest for their entire life. Contrary to animals that move from place to place for collecting resources, the foraging distance of bees is limited to a specific range around the nest, which is generally related to their body size [START_REF] Greenleaf | Bee foraging ranges and their relationship to body size[END_REF]. This has strong implications for their populations, since bee survival and fitness will be determined by the habitat quality in a specific range around their nest (e.g. floral resource availability, pollution, etc.), which makes them particularly sensitive to changes in these habitats. It is therefore crucial to better understand how anthropogenic pressures leading to changes in habitat quality affect bees, before developing management strategies to minimize or reverse current population trends of species of concern. In this regard, physiological markers measured in response to environmental stressors or habitat quality can reveal mechanisms that contribute to population decline and thereby provide a cause-and-effect relationship between environmental characteristics and bee survival [START_REF] Tracy | The importance of physiological ecology in conservation biology[END_REF].

Here we offer examples of the importance of physiology for better evaluating the risks posed by anthropogenic pressures in bees. We focus on the main threats that may act at large scales and affect most of bee taxa, such as environmental chemicals, decline of floral resources and climate change.

Environmental chemicals

Landscapes have been largely transformed by agronomic practices and one of the most striking consequences is the widespread use of agrochemicals, which have permeated the biosphere causing exposure to many non-target organisms. To investigate the potential effects of such exposure on nontarget organisms, ecotoxicology has emerged as a leading discipline and is perhaps the oldest discipline linking physiology to questions of conservation [START_REF] Stevenson | Ecophysiology and conservation: the contribution of endocrinology and immunology-introduction to the symposium[END_REF]. The most popular and precursory example is undoubtedly the discovery of the inhibition, by the chemical dichlorodiphenyltrichoroethane, of calcium carbonate deposition in eggshells of birds of prey, leading to their reproductive failure [START_REF] Grier | Ban of DDT and subsequent recovery of reproduction in bald eagles[END_REF]. Bees are also exposed to pesticides as multiple residues can be found in pollen and nectar of crops and wild flowers [START_REF] Botías | Neonicotinoid residues in wildflowers, a potential route of chronic exposure for bees[END_REF][START_REF] Rundlöf | Seed coating with a neonicotinoid insecticide negatively affects wild bees[END_REF]. To analyse the consequences of bee exposure to pesticides, ecotoxicological studies have traditionally assessed the lethal effects. However, more recently, a large body of evidence has shown that this approach underestimates the risks related to low environmental doses and current research methods generally integrate sublethal effects into toxicological studies [START_REF] Sandrock | Sublethal neonicotinoid insecticide exposure reduces solitary bee reproductive success[END_REF][START_REF] Sgolastra | Combined exposure to sublethal concentrations of an insecticide and a fungicide affect feeding, ovary development and longevity in a solitary bee[END_REF][START_REF] Brandt | Immunosuppression response to the neonicotinoid insecticide thiacloprid in females and males of the red mason bee Osmia bicornis[END_REF][START_REF] Lehmann | A systematic scoping review of the methodological approaches and effects of pesticide exposure on solitary bees[END_REF][START_REF] Strobl | No impact of neonicotinoids on male solitary bees Osmia cornuta under semifield conditions[END_REF].

Sublethal exposure to environmental chemicals can be investigated by the use of biomarkers of effects, which are defined as any measurable biochemical, physiological or genetic variation that provides evidence of exposure to and/or effects of one or more environmental pollutants [START_REF] Mouneyrac | Biomarkers of ecological relevance in ecotoxicology[END_REF]. Such biomarkers enable one to determine whether and what type of exposure took place and have proven to be promising tools for evaluating ecotoxicological health status and monitoring environmental quality of honeybees (Apis mellifera) [START_REF] Badiou-Bénéteau | exposure to xenobiotics in the honey bee Apis mellifera: application to the systemic insecticide thiamethoxam[END_REF][START_REF] Caliani | Multi-biomarker approach and IBR index to evaluate the effects of different contaminants on the ecotoxicological status of Apis mellifera[END_REF]. Applying this biomarker approach to Osmia and Bombus, which have been recently proposed as additional test surrogates to honeybees (European Food Safety Authority, 2013), but also to other bee genera, would greatly improve the evaluation of past and present exposure to anthropic contamination and determine habitat quality for wild bee populations. This was confirmed by a study, which found an increase in acetylcholinesterase expression in bumblebees (Bombus impatiens) foraging in agricultural areas using neonicotinoid crop protection [START_REF] Samson-Robert | Increased acetylcholinesterase expression in bumble bees during neonicotinoid-coated corn sowing[END_REF]. More recently, another study identified genes differentially expressed between Bombus terricola worker bees sampled in agricultural and non-agricultural sites [START_REF] Tsvetkov | Conservation genomics reveals pesticide and pathogen exposure in the declining bumble bee Bombus terricola[END_REF]. These differentially expressed genes significantly overlap with those found in honeybees exposed to insecticides, suggesting an exposure of Bombus terricola to insecticides in the studied sites. Changes in the expression of several genes upon exposure to neonicotinoids have also been identified in Osmia bicornis [START_REF] Beadle | Genomic insights into neonicotinoid sensitivity in the solitary bee Osmia bicornis[END_REF], which could potentially provide biomarkers of field exposure to pesticides for this solitary bee species. An alternative and more specific approach is the multi-biomarker approach that is widely used in various aquatic and terrestrial organisms to detect and evaluate the effects of exposure to chemical contaminants. By testing and analyzing variations of several biomarkers (often enzymatic activities) upon exposure to a chemical in the laboratory or in the field, it is then possible to calculate a simple 'Integrated Biological Response' that is specific to the chemical and its concentration [START_REF] Beliaeff | Integrated biomarker response: a useful tool for ecological risk assessment[END_REF]. This has notably been developed successfully in honeybees for discriminating exposure to different concentrations of Cadmium and the fungicide Amistar ® Xtra [START_REF] Caliani | Multi-biomarker approach and IBR index to evaluate the effects of different contaminants on the ecotoxicological status of Apis mellifera[END_REF].

Another key aspect of pesticide risk assessment is to determine whether such exposure leads to irreversible modifications of bee physiology that might translate into damage to bee populations. A consistent decline in the production of new queens by colonies of Bombus terrestris has been reported upon exposure to neonicotinoids [START_REF] Whitehorn | Neonicotinoid pesticide reduces bumble bee colony growth and queen production[END_REF]. Whether this lowered reproduction is due to reduced pollen foraging [START_REF] Feltham | Field realistic doses of pesticide imidacloprid reduce bumblebee pollen foraging efficiency[END_REF], to direct pesticide effects on queen physiological development or to both is not known, but, in any case, it could have severe consequences for bumblebee populations. Physiological impairments, such as immunosuppression and reduced thoracic temperatures in O. bicornis and alteration of mitochondrial functions in B. terrestris, have also been identified in response to pesticide exposure [START_REF] Azpiazu | Chronic oral exposure to field-realistic pesticide combinations via pollen and nectar: effects on feeding and thermal performance in a solitary bee[END_REF][START_REF] Colgan | Caste-and pesticide-specific effects of neonicotinoid pesticide exposure on gene expression in bumblebees[END_REF][START_REF] Brandt | Immunosuppression response to the neonicotinoid insecticide thiacloprid in females and males of the red mason bee Osmia bicornis[END_REF]. However, perhaps the most striking effects are the decrease in ovary development, male fertility and total offspring production, as well as a male-biassed sex ratio and higher mortality of eggs and larvae [START_REF] Sandrock | Sublethal neonicotinoid insecticide exposure reduces solitary bee reproductive success[END_REF][START_REF] Sgolastra | Combined exposure to sublethal concentrations of an insecticide and a fungicide affect feeding, ovary development and longevity in a solitary bee[END_REF][START_REF] Franke | Results of 2-year ringtesting of a semi-field study design to investigate potential impacts of plant protection products on the solitary bees osmia bicornis (Linnaeus, 1758) and Osmia cornuta (Latreille, 1805) (Hymenoptera, Megachilidae) and a proposal of a suitable test design[END_REF][START_REF] Minnameyer | Eusocial insect declines: insecticide impairs sperm and feeding glands in bumblebees[END_REF][START_REF] Strobl | No impact of neonicotinoids on male solitary bees Osmia cornuta under semifield conditions[END_REF]. These latter effects clearly suggest mechanisms that explain the observed decline of solitary bee populations in conventional agroecosystems (using pesticides) [START_REF] Rundlöf | Seed coating with a neonicotinoid insecticide negatively affects wild bees[END_REF]. Finally, it was recently found in bumblebees and fig wasps that high ozone concentrations can decrease the antennal sensitivity, and consequently, the attraction to floral volatile organic compounds (Vanderplanck et al., 2021a). This physiological impairment might directly affect plantpollinator chemical communication and, consequently, bee survival. All of these examples clearly illustrate the interest of studying physiologically related endpoints for the risk assessment of environmental chemicals in wild bees.

Landscape resources

All bees have in common a reliance on floral pollen and nectar for their growth and survival. Nectar contains carbohydrates fuelling their energetic demands, and pollen provides all of the proteins, amino acids, lipids, sterols and vitamins required for their ovary development and larval diets [START_REF] Roulston | Pollen nutritional content and digestibility for animals[END_REF][START_REF] Leach | A review of native wild bee nutritional health[END_REF]. However, a decrease in the diversity and abundance of floral resources in agricultural landscapes may generate a nutritional stress for bees [START_REF] Goulson | Bee declines driven by combined stress from parasites, pesticides, and lack of flowers[END_REF][START_REF] Vaudo | Bee nutrition and floral resource restoration[END_REF], especially for wild bees, who are generally more selective for pollen nutrition than honeybees, who collect pollen from a wide spectrum of plant species (polylecty) [START_REF] Galimberti | A DNA barcoding approach to characterize pollen collected by honeybees[END_REF][START_REF] Danner | Honey bee foraging ecology: season but not landscape diversity shapes the amount and diversity of collected pollen[END_REF]. For instance, bumblebees seem to favour pollen quality over quantity [START_REF] Leonhardt | The same, but different: pollen foraging in honeybee and bumblebee colonies[END_REF], while some solitary bees have a rather specialized pollen diet by collecting pollen from a limited number of plant species (oligolecty) [START_REF] Müller | Host-plant specialization in Western Palearctic Anthidine bees (Hymenoptera: Apoidea: Megachilidae)[END_REF][START_REF] Cane | Characterizing floral specialization by bees: analytical methods and a revised lexicon for oligolecty[END_REF][START_REF] Müller | Pollen hosts of western palaearctic bees of the genus Colletes (Hymenoptera: Colletidae): the Asteraceae paradox[END_REF]. [START_REF] Vanderplanck | How does pollen chemistry impact development and feeding behaviour of polylectic bees?[END_REF][START_REF] Roger | Diet effects on bumblebee health[END_REF]. If pollen diversity might not provide an added value per se (as compared to higher quality monofloral pollens), it might help to mitigate the negative effect of unfavourable pollens [START_REF] Moerman | Pollen nutrients better explain bumblebee colony development than pollen diversity[END_REF][START_REF] Carnell | Effect of diet on incipient colony success for two long-tongued bumblebee species in the laboratory[END_REF]. The benefit of pollen mixing was notably demonstrated by Eckhardt et al. in the pollen-generalist solitary bee Osmia cornuta [START_REF] Eckhardt | Pollen mixing in pollen generalist solitary bees: a possible strategy to complement or mitigate unfavourable pollen properties?[END_REF]. Pure Ranunculus pollen had a negative effect on larval development and survival and adult body mass of both males and females, but when not predominant (less than 50% of the pollen diet), bees remained unaffected. In addition, decreased access to resources has been shown to reduce reproductive performance of solitary bees and bumblebees [START_REF] Peterson | Impact of resource levels on sex ratio and resource allocation in the solitary bee, Megachile rotundata[END_REF][START_REF] Cane | Adult pollen diet essential for egg maturation by a solitary Osmia bee[END_REF][START_REF] Centrella | Diet diversity and pesticide risk mediate the negative effects of land use change on solitary bee offspring production[END_REF][START_REF] Requier | Limitation of complementary resources affects colony growth, foraging behavior, and reproduction in bumble bees[END_REF]. Such data might explain the lower bee abundance and diversity observed in agricultural systems embedded in simple landscapes or with low levels of semi-natural habitats [START_REF] Kennedy | A global quantitative synthesis of local and landscape effects on wild bee pollinators in agroecosystems[END_REF][START_REF] Lichtenberg | A global synthesis of the effects of diversified farming systems on arthropod diversity within fields and across agricultural landscapes[END_REF].

Finally, bee body traits (e.g. body mass and size) and nutritional health indicators (e.g. lipid content) may be useful for predicting the vulnerability of species, or at least an actual response, to changes in landscape-level floral resources. For instance, by measuring the mass, size and lipid content of several bumblebee and sweat bee species sampled in the field, [START_REF] Smith | Bee abundance and nutritional status in relation to grassland management practices in an agricultural landscape[END_REF] and [START_REF] Stein | Evaluating native bee communities and nutrition in managed grasslands[END_REF] could identify which species were affected by which grassland management practices in agricultural landscapes. Body size, which is strongly affected by pollen nutrition during larval development, has also been used as a proxy for assessing the response or sensitivity of wild bees to resource availability along landscape gradients. Results vary across bee species, diet breadth and landscape types, making it difficult to draw general conclusions on a local scale [START_REF] Renauld | Landscape simplification constrains adult size in a native ground-nesting bee[END_REF][START_REF] Banaszak-Cibicka | Body size and wing asymmetry in bees along an urbanization gradient[END_REF][START_REF] Reeth | Current and previous spatial distributions of oilseed rape fields influence the abundance and the body size of a solitary wild bee, Andrena cineraria, in permanent grasslands[END_REF]. However, two meta-analyses, performed on museum samples from the northeastern United States and the Netherlands, found that a large body size is among the traits that are negatively related to population trends [START_REF] Bartomeus | Historical changes in northeastern US bee pollinators related to shared ecological traits[END_REF][START_REF] Scheper | Museum specimens reveal loss of pollen host plants as key factor driving wild bee decline in The Netherlands[END_REF], which is probably due to the greater pollen needs of larger bee species combined with food limitation. Altogether, these experimental and survey data highlight nutritional stress as a driver of bee decline and identify, for some species, which specific diets might be unfavourable.

Climate change

Since bees, as insects, have limited ability to regulate their body temperature, their physiology, behaviour and survival are largely constrained by environmental temperatures. Determining and predicting the consequences of climate change on bees therefore has become a growing research topic. To quantify the sensitivity of bees to temperature, several studies have measured the consequences of artificial or environmental variations in temperature on physiological traits. For instance, changes in the queen body size of different bumblebee species have been linked to an increase in the mean annual minimum temperature [START_REF] Gérard | Impact of landscape fragmentation and climate change on body size variation of bumblebees during the last century[END_REF]. However, a large body of literature has focused on the effect of temperature on completion of the winter diapause, likely because insects generally rely on low temperatures to reduce their metabolism and preserve their energy storage until the next spring [START_REF] Brown | Toward a metabolic theory of ecology[END_REF][START_REF] Williams | Cold truths: how winter drives responses of terrestrial organisms to climate change: organismal responses to winter climate change[END_REF]. Climate change is therefore expected to increase energy expenditure during the diapause, which can lead to vulnerable bees in the spring. Although bee species differ in their responses [START_REF] Fründ | Response diversity of wild bees to overwintering temperatures[END_REF], this hypothesis has been verified many times, with natural or artificial pre-winter and winter conditions causing a shorter diapause duration and a decrease in fat body, body weight and consequently winter survival [START_REF] Bosch | Effect of pre-wintering and wintering temperature regimes on weight loss, survival, and emergence time in the mason bee Osmia cornuta (Hymenoptera: Megachilidae)[END_REF][START_REF] Sgolastra | The long summer: pre-wintering temperatures affect metabolic expenditure and winter survival in a solitary bee[END_REF][START_REF] Caradonna | Experimental warming in the field delays phenology and reduces body mass, fat content and survival: implications for the persistence of a pollinator under climate change[END_REF]. Characterizing the baseline of overwintering gene expression, as it was done for the small carpenter bee Ceratina calcarata, could also help to better understand adaptations to overwintering and molecular responses to climate change [START_REF] Durant | Transcriptional profiling of overwintering gene expression in the small carpenter bee, Ceratina calcarata[END_REF].

Variations in summer temperatures have been investigated as well and have been shown to affect voltinism in a solitary bee (Osmia iridis) [START_REF] Forrest | Two-year bee, or not two-year bee? How voltinism is affected by temperature and season length in a high-elevation solitary bee[END_REF]. Under controlled conditions, increased temperatures were found to decrease prepupal weight and the duration of all development phases in O. bicornis [START_REF] Radmacher | Effects of constant and fluctuating temperatures on the development of the solitary bee Osmia bicornis (Hymenoptera: Megachilidae)[END_REF], and reduce wing size in B. terrestris [START_REF] Gerard | Stressful conditions reveal decrease in size, modification of shape but relatively stable asymmetry in bumblebee wings[END_REF]. Heat shock stress also has a negative impact on bumblebee reproduction by impairing sperm viability, sperm DNA integrity and the production of pheromones involved in male attractiveness, but interestingly, this was observed only in the tested declining species (not in the widespread and warm-adapted species) (Martinet et al., 2021b). Similarly, a study showed that two invasive bee species in Fiji tend to be more resistant to thermal and desiccation stress than an endemic species, which might contribute to a higher resilience to climate change [START_REF] Da Silva | Climate change and invasive species: a physiological performance comparison of invasive and endemic bees in Fiji[END_REF]. Warming summers can also lead to changes in bee morphology as evidenced by the decrease in tongue length of two alpine bumblebee species within 40 years [START_REF] Miller-Struttmann | Functional mismatch in a bumble bee pollination mutualism under climate change[END_REF]. Interestingly, a decrease in the abundance of long-tongued bumblebees and an immigration of short-tongued species from lower altitudes have been reported during the same period [START_REF] Miller-Struttmann | Functional mismatch in a bumble bee pollination mutualism under climate change[END_REF]. While the underlying mechanisms are not clear, it shows some phenotypic plasticity in response to climate [START_REF] Keyzer | Delving deeper: questioning the decline of long-tongued bumble bees, long-tubed flowers and their mutualisms with climate change[END_REF]. Last but not least, data on the thermal physiology can be extremely useful for understanding and/or predicting how different species might respond to climate stress. This was the case with the identification of bumblebee critical thermal limits, which correspond to the temperatures at which an organism may encounter failure or loss of essential functions (e.g. motor and respiratory). Measurements of thermal tolerance helped to explain population declines and range shifts in relation to climate change [START_REF] Oyen | Altitudinal variation in bumble bee (Bombus) critical thermal limits[END_REF][START_REF] Zambra | Hyperthermic stress resistance of bumblebee males: test case of Belgian species[END_REF]Martinet et al., 2021b), as well as local adaptation in bumblebees [START_REF] Pimsler | Biogeographic parallels in thermal tolerance and gene expression variation under temperature stress in a widespread bumble bee[END_REF]. In sum, documenting variations of physiological and body traits in response to temperature changes proves to be extremely

Physiology Can Help in Designing Conservation Practices

To mitigate or combat habitat loss and therefore the decline of wild bee populations, conservation efforts have been focusing on the provision of consistent foraging and nesting resources in human-dominated areas. Large-scale actions have been developed for that purpose. In Europe, the European Agri-Environmental Schemes promote environmentally friendly practices in farmlands (e.g. implementation of flower strips and hedgerows in field margins, restoration of pastures and meadows or organic farming) [START_REF] Scheper | Environmental factors driving the effectiveness of European agri-environmental measures in mitigating pollinator loss-a meta-analysis[END_REF]. In the United States, the Conservation Reserve Program supports the conversion of croplands into long-term conservation habitats with a recent priority given to the promotion of forage for pollinators [START_REF] Osteen | Agricultural Resources and Environmental Indicators No[END_REF].

To further improve such conservation practices, highresolution data are needed on the nutritional needs of bee species. Studies first attempted to identify the minimum 'pollen budget' for maintaining a self-reproducing population [START_REF] Müller | Quantitative pollen requirements of solitary bees: implications for bee conservation and the evolution of bee-flower relationships[END_REF][START_REF] Larsson | Critical resource levels of pollen for the declining bee Andrena hattorfiana (Hymenoptera, Andrenidae)[END_REF]. However, most of the research efforts are now focusing on the determination of foraging preferences and diets that are most appropriate for bees. Among others, preferred pollen protein:lipid macronutrient ratios have been identified in B. impatiens and linked to bumblebee colony health and fitness [START_REF] Vaudo | Macronutrient ratios in pollen shape bumble bee (Bombus impatiens)[END_REF]. Some of the minimum nutrient requirements for bee development have been described as well for several species [START_REF] Vanderplanck | How does pollen chemistry impact development and feeding behaviour of polylectic bees?[END_REF][START_REF] Moerman | Interspecific variation in bumblebee performance on pollen diet: new insights for mitigation strategies[END_REF][START_REF] Barraud | Variations in nutritional requirements across bee species[END_REF] and nutritional physiology has shed light on stage-specific nutritional limitations in the bee life cycle [START_REF] Woodard | Diet and nutritional status during early adult life have immediate and persistent effects on queen bumble bees[END_REF]. Physiological markers can also provide helpful information on the interface between bee populations and their resources. For instance, by characterizing the elemental composition of bees (female, male, adult, larva) and their resources, ecological stoichiometry allows one to determine whether there is a mismatch between the elemental demands of bees and floral resources, potentially leading to an imbalanced diet [START_REF] Filipiak | A better understanding of bee nutritional ecology is needed to optimize conservation strategies for wild bees-the application of ecological stoichiometry[END_REF]. By using this approach, Filipiak et al. reported some stoichiometric mismatches between larvae of O. bicornis and particular pollen species and identified specific elements related to bee fitness [START_REF] Filipiak | Key pollen host plants provide balanced diets for wild bee larvae-a lesson for planting flower strips and hedgerows[END_REF][START_REF] Filipiak | The scarcity of specific nutrients in wild bee larval food negatively influences certain life history traits[END_REF][START_REF] Filipiak | Stoichiometric niche, nutrient partitioning and resource allocation in a solitary bee are sexspecific and phosphorous is allocated mainly to the cocoon[END_REF]. Assessing bee nutritional requirements is therefore also a promising field of research for tailoring conservation measures to bee communities. In summary, by studying the bee-nutrient floral networks, we can identify the preferred floral species as well as the nutrients or nutrient ratios important for specific bees and the plant species providing them. Such information can then be used to adjust floral enhancement schemes or support the conservation of key plant species for bees (i.e. preferred floral species or with high nutritive values for bees) [START_REF] Woodard | Bumble bee ecophysiology: integrating the changing environment and the organism[END_REF][START_REF] Parreño | Critical links between biodiversity and health in wild bee conservation[END_REF].

Besides the identification of nutritional needs, assessing the efficiency of habitats designed for promoting the sustainability of bee populations is also essential. Most of the research has focused on the attractiveness of these habitats by quantifying the potential positive effects on bee abundance and species richness [START_REF] Scheper | Environmental factors driving the effectiveness of European agri-environmental measures in mitigating pollinator loss-a meta-analysis[END_REF]. However, the creation of floral hot spots highly attractive to a broad range of bee species might have negative side effects on bee health. Indeed, in landscapes with low amounts of semi-natural habitats, wildflower fields seeded to boost bee populations in field margins might create hot spots for parasite transmission [START_REF] Piot | Establishment of wildflower fields in poor quality landscapes enhances micro-parasite prevalence in wild bumble bees[END_REF]. In addition, such wildflowers can get contaminated with pesticides (under non-organic farming practices) and thus prolong exposure to pesticides beyond the treated crop flowering [START_REF] Botías | Neonicotinoid residues in wildflowers, a potential route of chronic exposure for bees[END_REF]. Therefore, in addition to attractiveness data, physiological metrics of bee health may be used as complementary criteria to evaluate the efficiency of habitats designed to be pollinator-friendly and develop Habitat Suitability Indices (HSI) to be used as operational decision support by conservationists and landmanagers. HSIs are widely used tools in applied conservation [START_REF] Larson | Linking population viability, habitat suitability, and landscape simulation models for conservation planning[END_REF]. However, physiological factors are seldom implemented into HSI algorithms, which yet is critical regarding physiological constraints faced by central place foragers [START_REF] Chudzinska | AgentSeal: agent-based model describing movement of marine central-place foragers[END_REF]. As a prerequisite, one should objectivate the links between environment and physiology in the taxonomic group of interest. In this context, physiological measures have been successfully used in honeybees to demonstrate the beneficial effects of floral landscape enrichments and uncultivated forage (e.g. pasture, grasslands, woody areas and hedgerows) on bee health [START_REF] Smart | Linking measures of colony and individual honey bee health to survival among apiaries exposed to varying agricultural land use[END_REF][START_REF] Alaux | A 'landscape physiology' approach for assessing bee health highlights the benefits of floral landscape enrichment and semi-natural habitats[END_REF][START_REF] Dolezal | Native habitat mitigates feast-famine conditions faced by honey bees in an agricultural landscape[END_REF]. A few studies have shown that several markers of nutritional state in bumblebees could be promising for monitoring bee health and assessing habitat quality [START_REF] Smith | Bee abundance and nutritional status in relation to grassland management practices in an agricultural landscape[END_REF][START_REF] Wang | Metabolomics-based biomarker discovery for bee health monitoring: a proof of concept study concerning nutritional stress in Bombus terrestris[END_REF]. However, further field studies with wild bees are required to provide a proof-of-concept of this ecophysiological approach for evaluating how well remedial actions are working.

Physiology for Predicting Population-Level Changes

The measure of physiological traits is highly valuable in the identification of the sources and consequences of stressors on bees, which inform conservation actions. However, field monitoring, which consists of tracking the population status of species of concern, also plays a key role in conservation strategies [START_REF] Nichols | Monitoring for conservation[END_REF]. The quantitative information that originates from monitoring programs can guide decision-making for environmental management and rehabilitation, and then help to evaluate the success of management actions. The only concern is that conventional monitoring (e.g. estimating population size) 'is by its very nature post hoc-that is [START_REF] Keddy | Biological monitoring and ecological prediction: from nature reserve management to national state of the environment indicators[END_REF]. Predicting and anticipating future population changes or sustainability would represent a more powerful approach to conservation. In this context, early indicators of individual fitness, such as physiological traits, have the potential to predict organismal responses to environmental changes, and therefore anticipate changes or stability in wild bee populations. However, several challenges first need to be addressed for incorporating physiological biomarkers into conservation monitoring actions.

Linking variations in physiology to changes in population dynamics

Biomarkers or signatures of stress might not always be indicative of declining populations, in particular when physiological changes have little effects on the organism (e.g. sublethal effects), or compensatory mechanisms allow the organism to ultimately survive and reproduce or tolerate changing environments [START_REF] Van Dievel | Short-and long-term behavioural, physiological and stoichiometric responses to predation risk indicate chronic stress and compensatory mechanisms[END_REF]. Furthermore, upon exposure to high levels of stress, the population may be able to persist by relying on the most physically fit or healthiest individuals [START_REF] Fefferman | Can physiological stress alter population persistence? A model with conservation implications[END_REF]. Therefore, to determine whether a physiological biomarker can be used as a monitoring tool for the management of wild bee populations, the most suitable approach would be to confirm that such a biomarker can be linked to proxies of individual fitness or demographic changes [START_REF] Bergman | Scaling from individual physiological measures to population-level demographic change: case studies and future directions for conservation management[END_REF].

First, studies that simultaneously connect environmental, physiological and population metrics should be carried out. Several biomarkers, such as bee body conditions based on morphometric variables (e.g. weight, body size, body parts size), could be used since they are affected by both genetic and environmental factors that have operated through physiological mechanisms during larval development [START_REF] Nijhout | The control of body size in insects[END_REF]. These body variables are easy to acquire and potentially nonlethal and have been found to be related to population trends [START_REF] Bartomeus | Historical changes in northeastern US bee pollinators related to shared ecological traits[END_REF][START_REF] Scheper | Museum specimens reveal loss of pollen host plants as key factor driving wild bee decline in The Netherlands[END_REF]. However, in several cases they might be less informative about the type of stressor [START_REF] Gerard | Stressful conditions reveal decrease in size, modification of shape but relatively stable asymmetry in bumblebee wings[END_REF]. Alternatively, the assessment of biochemical and genomic markers require sacrificing several specimens but are expected to provide more specific information on the stressor and its severity [START_REF] Badiou-Bénéteau | Honeybee biomarkers as promising tools to monitor environmental quality[END_REF][START_REF] Woodard | Molecular tools and bumble bees: revealing hidden details of ecology and evolution in a model system[END_REF][START_REF] Kent | Conservation genomics of the declining north American bumblebee Bombus terricola reveals inbreeding and selection on immune genes[END_REF][START_REF] Wang | Metabolomics-based biomarker discovery for bee health monitoring: a proof of concept study concerning nutritional stress in Bombus terrestris[END_REF]. However, the lack of multidisciplinary expertise in longitudinal surveys of wild bee populations might have prevented so far the realization of studies linking ecophysiological data with population-level metrics for wild bees, as seen for honeybees [START_REF] Smart | Linking measures of colony and individual honey bee health to survival among apiaries exposed to varying agricultural land use[END_REF][START_REF] Alaux | A 'landscape physiology' approach for assessing bee health highlights the benefits of floral landscape enrichment and semi-natural habitats[END_REF]. Nonetheless, this could be overcome by sampling bees in monitoring surveys and integrating candidate biomarkers (e.g. bee size, weight or nutritional health indicators) into datasets of bee population demography (e.g. abundance, growth rate). Identifying individual physiological biomarkers that correlate to population trends may then help to evaluate species resilience and responses to environmental changes.

Secondly, studies could also determine whether physiological biomarkers that were already found to be affected by environmental variables are also related to some component of individual fitness (survival, reproductive performances). For instance, body size in bees, which is highly correlated to the amount of food ingested (during larval stage) and therefore to the abundance of floral resources, has been found to be positively related to female fecundity in different bee species [START_REF] Bosch | Relationship between body size, provisioning rate, longevity and reproductive success in females of the solitary bee Osmia cornuta[END_REF][START_REF] O'neill | Oocyte size, egg index, and body lipid content in relation to body size in the solitary bee Megachile rotundata[END_REF][START_REF] Slominski | Asynchrony between solitary bee emergence and flower availability reduces flower visitation rate and may affect offspring size[END_REF]. These relationships highlight body size as a promising biomarker for monitoring population trends. However, further research is needed given that the relationship between body size and female fecundity is not consistent across studies and may depend on species and/or local environmental contexts [START_REF] Bosch | Relationship between body size, provisioning rate, longevity and reproductive success in females of the solitary bee Osmia cornuta[END_REF].

Thirdly, comparing baseline levels of physiological biomarkers in both a decreasing and an increasing or stable population could help in determining whether a biomarker could be used to measure demographic changes [START_REF] Bergman | Scaling from individual physiological measures to population-level demographic change: case studies and future directions for conservation management[END_REF]. For instance, data about heat stress sensitivity have been especially convincing for explaining population trends and restricted habitat types in bumblebees. By measuring the physiological thermal limit of individual bees, a direct link between the thermotolerance of bumblebee species and their distribution area was found [START_REF] Martinet | A protocol to assess insect resistance to heat waves, applied to bumblebees (Bombus Latreille, 1802)[END_REF](Martinet et al., , 2021a;;[START_REF] Oyen | Altitudinal variation in bumble bee (Bombus) critical thermal limits[END_REF] and widespread species proved to be less sensitive to heat stress than declining species [START_REF] Zambra | Hyperthermic stress resistance of bumblebee males: test case of Belgian species[END_REF]Martinet et al., 2021b). Similarly, physiological thermal limits were found to be a strong predictor of bee population response to heat in urban areas, with those bees having the lowest critical thermal maximum showing the most population decline with warming [START_REF] Hamblin | Physiological thermal limits predict differential responses of bees to urban heat-island effects[END_REF]. Altogether, these studies showed that species' physiology could shape community composition, and therefore help in predicting their future trends under changing environmental conditions.

Including physiological performances into ecological niche modelling

Ecological niche modelling has become an important tool for inferring species distribution and habitat suitability. By linking the observed distribution of a species to environmental data, the correlative ecological niche models have the advantage of requiring little information on the mechanistic links between the organism and its environment and are therefore especially useful for species that are poorly studied. For instance, ecological niche modelling allowed for the prediction of overall range losses under climate change for 30 North American bumblebee species, even for species with high dispersal abilities (Sirois-Delisle and Kerr, 2018). However, such models are constrained by the availability of detailed data on species distribution, and when extrapolating species distribution to future environmental conditions (not yet experienced by species), mechanistic ecological niche models that are parametrized with physiological data are expected to be [START_REF] Kearney | Mechanistic niche modelling: combining physiological and spatial data to predict species' ranges[END_REF][START_REF] Evans | Mechanistic species distribution modelling as a link between physiology and conservation[END_REF]. Notably, the inclusion of information on physiological limits of major life history characteristics (e.g. survival, growth and reproduction) can help to make inferences on species' range limits and to understand the environmental characteristics defining the species range. This mechanistic modelling was elegantly used as a tool for the ecosystem restoration of four insect pollinators at a local scale in Australia [START_REF] Tomlinson | Incorporating biophysical ecology into high-resolution restoration targets: insect pollinator habitat suitability models[END_REF]. By projecting data of metabolic rates and thermotolerance of these pollinator species onto high-resolution topoclimatic models, the authors showed the importance of mechanistic models fitted with physiological data in providing evidence-based guidance for ecological management programs. Finally, individual-or agent-based models, by simulating the fate of individuals with respect to their behavioural and/or physiological traits, might represent an alternative to better predict population and community changes. They can incorporate any number of individual-level mechanisms and take into account their changes through time [START_REF] Grimm | Individual-Based Modeling and Ecology[END_REF][START_REF] Deangelis | Individual-based models in ecology after four decades[END_REF]. Agentbased models have notably been used for exploring potential benefits of bee-friendly farming measures on the population dynamics of six bumblebee species [START_REF] Becher | Bumble-BEEHAVE: a systems model for exploring multifactorial causes of bumblebee decline at individual, colony, population and community level[END_REF]. However, using such these modelling approaches to predict the resilience and trends of bee populations in changing habitats implies that physiological limits of species of concern are readily available. Therefore, these knowledge gaps need to be addressed in the future to strengthen the implementation of mechanistic models. For instance, data on thermal limits and energy balance would be particularly helpful to better anticipate the persistence of population under climate and resource changes.

Surrogate species for community-based monitoring

With over 20 000 known bee species in the world, it appears virtually impossible to develop a species-by-species monitoring approach for the prediction of population trends. This is also the case for the monitoring of bee communities in specific habitats, in particular within the goal here to use early indicators (physiological markers) of population dynamics. Measuring physiological traits of all species within a community can quickly become expensive and time consuming, and it might not always be possible to sample all species or a sufficient number of specimens per species for assessing population trends. In addition, the ecology and biology of most of wild bee species are still poorly understood. Therefore, one solution to fill these gaps would be to focus on some species that can be easily monitored and for which we could standardize relevant physiological traits measurement in order to inform on the community's status and thus apply them in conservation contexts.

For that purpose, conservation biologists have showed a growing interest in the identification of surrogate species indicative of the state of species communities or ecosystems [START_REF] Fleishman | A new method for selection if umbrella species for conservation planning[END_REF][START_REF] Fleishman | Using indicator species to predict species richness of multiple taxonomic groups[END_REF]. Surrogate species notably include indicator and umbrella species [START_REF] Caro | Conservation by Proxy: Indicator, Umbrella, Keystone, Flagship, and Other Surrogate Species[END_REF]; the latter focusing on species that have large area requirements and for which conservation offers protection to species that co-occur in the same habitat [START_REF] Wilcox | In situ conservation of genetic resources: determinants of minimum area requirements. New Directions in Protected Area Management[END_REF]. However, empirical tests of umbrella species validity have provided equivocal results, in part due to the focus on a large home range, which might hinder their applicability [START_REF] Ozaki | A mechanistic approach to evaluation of umbrella species as conservation surrogates[END_REF][START_REF] Branton | Assessing the value of the umbrellaspecies concept for conservation planning with meta-analysis: value of the umbrella-species concept[END_REF]. Consequently, the concept of local umbrella species, that occupy a more spatially limited area, has been introduced by [START_REF] Caro | Conservation by Proxy: Indicator, Umbrella, Keystone, Flagship, and Other Surrogate Species[END_REF]. In addition, the efficacy of putative umbrella species may depend on the co-occurring taxonomic diversity that is targeted, since umbrella species are likely more indicative of elevated biodiversity within their own taxonomic groups than across taxa [START_REF] Sattler | Selection of multiple umbrella species for functional and taxonomic diversity to represent urban biodiversity[END_REF]. Because here, the goal is to monitor and predict community changes of wild bees that inhabit relatively limited areas, efforts should rather be made on the identification of local umbrella species specifically indicative of their own community. A first attempt has been made in urban areas of the Central Swiss Plateau and lowland areas of southern Switzerland by analyzing 139 bee species [START_REF] Sattler | Selection of multiple umbrella species for functional and taxonomic diversity to represent urban biodiversity[END_REF]. By using several functional traits, such as dietary specialization (oligolectic and polylectic) and nesting habits (miner-carder, renter, carpenter, mason), they were able to identify six6 umbrella species (Andrena bicolor, Heriades truncorum, Hylaeus nigritus, Hylaeus sinuatus, Lasioglossum calceatum, Psithyrus barbutellus), which were indicative of bee functional biodiversity (i.e.i.e., dietary specialization, nesting habits).

An alternative or complementary approach is to target a species or a group of species whose status (abundance, population dynamics) rather reflects the environmental conditions [START_REF] Carignan | Selecting indicator species to monitor ecological integrity: a review[END_REF]. The selection of such species, named indicator species, is more focused on their ecology and life history traits, and as such, they can be used as a proxy to determine the health of an ecosystem and provide early warning signals about environmental changes. Interestingly, within the aim of better supporting conservation strategies for bumblebee species in China, a recent study identified, among the 125 studied species, 26 species as indicator species of the different biogeographic regions: 14, 13, 12 and 12 species were associated with the regions of South China, North China, the Mongolian Plateau and the Tibetan Plateau, respectively [START_REF] Naeem | Diagnostic indicators of wild pollinators for biodiversity monitoring in long-term conservation[END_REF]. Although no empirical test was performed, these first studies suggest that some bee species could potentially be assimilated as surrogate species to monitor bee communities and/or environmental conditions in their respective biogeographic ranges.

Detailed studies of physiological health indicators on surrogate species (see Linking variations in physiology to changes in population dynamics) should then provide valuable information on the conditions of the environment where they are found, and consequently, for other co-occurring bee species or larger bee communities. Notably, building standard biomarkers values for these reference populations should help identifying poor or declining environmental conditions (when biomarkers deviate from the baseline or average values), especially if these biomarkers can be linked to proxies of individual fitness or demographic changes. Such species could therefore facilitate monitoring programs (a single or a few species to monitor), especially within the objective to perform proactive conservation with early indicators of species community changes.

Dealing with time effects

In some cases, adverse environmental conditions can affect the survival or fitness of the studied organism in a subsequent season [START_REF] Harrison | Carryover effects as drivers of fitness differences in animals[END_REF]. If not taken into account, these carry-over effects can lead to erroneous conclusions about the effectiveness of conservation efforts (O'Connor and [START_REF] O'connor | Ecological carryover effects complicate conservation[END_REF]. Such carry-over effects have been described in honeybees, with an increase of winter mortality risks in colonies that have experienced a pollen shortage the previous season [START_REF] Requier | The carry-over effects of pollen shortage decrease the survival of honeybee colonies in farmlands[END_REF]. But they have also been documented in wild bees. By exposing, adult bees of Osmia lignaria to the neonicotinoid insecticide imidacloprid, researchers demonstrated a reduction of the offspring fitness (reproduction) the following season due to maternal and/or larval effects [START_REF] Stuligross | Past insecticide exposure reduces bee reproduction and population growth rate[END_REF]. Carry-over effects can also be observed at the level of bee communities' since their abundance is typically influenced by the floral resource availability of the previous year's [START_REF] Potts | Linking bees and flowers: how do floral communities structure pollinator communities?[END_REF][START_REF] Féon | Solitary bee abundance and species richness in dynamic agricultural landscapes[END_REF]. This phenomenon is due to the specific life cycle of most wild bees: females generally emerge in the spring, mate, and then provision their nests with pollen to feed their larvae, which develop into adults in the autumn and emerge the next spring. As a result, the number of offsprings, as well as some of their morphological and physiological traits, may be affected by the environmental conditions of the previous year. For instance, their body size should reflect the amount and quality of pollen resources ingested as a larva the previous spring/summer [START_REF] Chole | Body size variation in bees: regulation, mechanisms, and relationship to social organization[END_REF]. Similarly, diet quality may affect resource assimilation and allocation during larval development and consecutively some of the physiological processes essential to bee fitness [START_REF] Filipiak | Stoichiometric niche, nutrient partitioning and resource allocation in a solitary bee are sexspecific and phosphorous is allocated mainly to the cocoon[END_REF]. The specificity of bee species life cycle needs therefore to be taken into account to deal with such carry-over effects and better interpret the influence of environmental factors on bee health.

Finally, physiological traits can vary in their response time to environmental changes, with some markers being more sensitive or responsive over shorter timescales than others (within minutes) [START_REF] Bergman | Scaling from individual physiological measures to population-level demographic change: case studies and future directions for conservation management[END_REF]. But also, the level of some physiological traits can naturally vary over time during the organism development or maturation. For instance, ovary development and the level of vitellogenin, the precursor of major yolk proteins in insects, gradually increases after the emergence in the mason bee Osmia cornifrons to reach a peak at 6 days and then decline [START_REF] Lee | Ovarian development and secretion of vitellogenin protein during the wintering period and after emergence in the hornfaced bee, Osmia cornifrons[END_REF]. Physiological markers could therefore induce some biases in the way they reflect environmental conditions if time changes are not taken into account. For such typical traits, a specific and consistent monitoring should be performed, rather than a single or random sampling over time, so as to acquire thorough time series databases.

Conclusions

Understanding and undertaking a comprehensive mitigation of the impact of anthropogenic pressures on wild bee populations is currently a demanding research challenge due to the large gaps in evaluating bee sensitivity to environmental changes [START_REF] López-Uribe | Defining pollinator health: a holistic approach based on ecological, genetic, and physiological factors[END_REF], as well as in monitoring the status and trends of populations [START_REF] Lebuhn | Detecting insect pollinator declines on regional and global scales: detecting pollinator declines[END_REF]. However, we showed here, through several examples, that physiological metrics can help solve several issues related to the conservation of wild bees (Fig. 1). Indeed, the measure of physiological traits or performances, by providing a causeand-effect relationship between environmental characteristics and bee survival, can help in identifying stressful environmental conditions. The documentation of species' physiological needs and limits can also offer valuable information for identifying and testing the efficiency of remedial actions. Equally important are physiological metrics that can be used as early indicators of population decline. Incorporating such physiological metrics into monitoring programs is still uncommon but promising for developing a more proactive approach to conservation [START_REF] Madliger | The conservation physiology toolbox: status and opportunities[END_REF]. Future efforts should however be made in the identification of physiological metrics for assessing bee population status or dynamics. To reach this goal, the identification of generalized stress biomarkers in insects would be extremely useful. For instance, glucocorticoids have been largely used in vertebrates, through noninvasive sampling (e.g. faeces, feathers, saliva), as a general stress indicator to better understand the impact of environmental variables [START_REF] Reeder | Stress in free-ranging mammals: integrating physiology, ecology, and natural history[END_REF][START_REF] Romano | Stress in wildlife species: noninvasive monitoring of glucocorticoids[END_REF][START_REF] Fairhurst | Does environmental enrichment reduce stress? An integrated measure of corticosterone from feathers provides a novel perspective[END_REF]. If such an analog does not seem to exist in insects, several elements of a general stress response have been studied in honeybees (e.g. biogenic amines, pathways of the energetic metabolism, heat shock proteins, vitellogenin) [START_REF] Even | General stress responses in the honey bee[END_REF][START_REF] Bordier | Stress response in honeybees is associated with changes in task-related physiology and energetic metabolism[END_REF]. Whether they could be transposed to other bee species remains to be investigated. However, this should be a reasonable target since moving physiological methodologies from model species (e.g. honeybees, bumblebees) to non-model species-and including physiological markers in monitoring programs-are becoming increasingly feasible (Vanderplanck et al., 2021b).

In sum, the integration of physiological data into species distribution and abundance data has the potential to provide decidedly complementary points of view on how bees deal with environmental constraints and how their populations change. To achieve this goal, a clear reinforcement of multidisciplinary approaches (e.g. spatial and landscape ecology, physiology, taxonomy), combined with training, sharing of protocols and methods, as well as the development of standardized measures for physiological health of bees would be needed in future bee conservation studies. Therefore, in the short-term, assessing physiology might be more complex, expensive and time consuming than surveying species in the field. However, such an approach, especially with the identification of surrogate species, should lead to early indicators of bee population sensitivity to (positive or negative) environmental changes. Therefore, in the long-term, we hope the benefits will outweigh the costs, especially given that proactive conservation methods (i.e. acting before population size declines to critical levels) are considered more successful and cost effective than reactive strategies [START_REF] Drechsler | Does proactive biodiversity conservation save costs?[END_REF].

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  By investigating the influence of pollen diet abundance, quality and diversity on bee physiology, several experimental studies have attempted to identify what might represent a nutritional stress for bees. It is now well established that the protein and lipid contents of pollens have a significant impact on bumblebee immunity, development and survival ..........................................................................................................................................................

  .......................................................................................................................................................... valuable for better understanding the temporal and spatial shifts in bee distribution.

  , it can only tell us what has already .......................................................................................................................................................... happened'

  .......................................................................................................................................................... more robust

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Figure 1 :

 1 Figure 1: Physiological metrics in the conservation of wild bees. By linking environmental conditions to population responses, the measurement of bee physiological traits or performances can help in identifying stressful environmental conditions, and therefore in predicting population trends. Information on physiological needs can also guide conservation strategies. Main and sublists are inexhaustive examples of parameters that have been investigated in conservation-based research of wild bees.

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Funding

This work was supported by the 2018-2019 BiodivERsA joint call for research proposals, under the BiodivERsA3 ERA-Net COFUND programme, and with the funding organization Agence Nationale de la Recherche in France [ANR-19-EBI3-0003].

Data availability

No new data were generated or analysed in support of this article.

Conflicts of interest

The authors declare that they have no conflicts of interest with this manuscript.