Christophe Malaterre

Antoine C Dussault

Ely Mermans

Gillian Barker

Beatrix E Beisner

Frédéric Bouchard

Eric Desjardins

Tanya Handa

Steven W Kembel

Geneviève Lajoie

Sophia Rousseau- Mermans

I Tanya Handa

Virginie Maris

Alison D Munson

Jay Odenbaugh

Timothée Poisot

B Jesse Shapiro

Curtis A Suttle

Functional Diversity: An Epistemic Roadmap

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Biodiversity functionalism in historical perspective

Thinking functionally about living entities has deep historical roots that still strongly impact the conceptual tools used in the investigation of functional diversity today. This way of thinking engages in the characterization and classification of entities by focusing on what they do instead of what they are made of. Aristotle (384-322 BC) already classified animals according to their modes of subsistence, e.g. as aquatic or terrestrial, and as carnivorous, graminivorous, omnivorous or special [START_REF] Lundgren | Relationships of natural enemies and non-prey foods[END_REF]. And one of his disciples, Theophrastus (371-287 BC), proposed the first functional groupings of plants, based on their height and woodiness, reflecting different resource-use strategies [START_REF] Weiher | Challenging Theophrastus: A common core list of plant traits for functional ecology[END_REF]. Thinking in terms of functions also contributes to the view that living entities exist not by themselves, but in interaction with other entities and with their environment, and thus can be seen as fulfilling certain roles within higher levels of organization such as communities or ecosystems. The naturalist Carl von Linnaeus (1707-1778)-who conceived of living entities as being fitted to one another's use [START_REF] Pearce | A Great Complication of Circumstances" -Darwin and the Economy of Nature[END_REF]-was an early proponent of this view. The work of 19 th century plant geographers, such as Alexander von Humboldt (1769-1859), August Grisebach (1814Grisebach ( -1879) ) and Eugenius Warming (1841Warming ( -1924)), who aimed to capture the relationship between environmental conditions and geographical plant distribution patterns, is another important historical influence on taxonomic classifications based on function [START_REF] Duckworth | Plant functional types: an alternative to taxonomic plant community description in biogeography?[END_REF]. This work was at the origin of subsequent functional classifications linking the traits of plants to their environmental requirements, for instance by Christen Raunkiaer or Leonty Ramenskii (1884Ramenskii ( -1953)), culminating in John Philip Grime's (1935-) still widely used C-S-R classification of plants (as competitive, stress-tolerant and ruderal) [START_REF] Garnier | Plant functional diversity: Organism traits, community structure, and ecosystem properties[END_REF].

Functional classifications used in animal ecology are also central to the genealogy of the contemporary functional diversity studies. These functional views derive from a line of research that spans from Charles Elton's (1900Elton's ( -1991) ) trophic-focused niche concept, to George Evelyn Hutchinson's (1903Hutchinson's ( -1991) ) view of linking species coexistence to resource use [START_REF] Blondel | Guilds or functional groups: does it matter?[END_REF] and to the research tradition centered around the concept of "guild", expression coined by Richard Root (1937Root ( -2006) ) to designate groups of animals that use similar resources in similar ways and are therefore more likely to compete [START_REF] Simberloff | The guild concept and the structure of ecological communities[END_REF]. The concept has thus been central in the search for assembly rules of ecological communities [START_REF] Simberloff | The guild concept and the structure of ecological communities[END_REF][START_REF] Keddy | Assembly and response rules: two goals for predictive community ecology[END_REF][START_REF] Blondel | Guilds or functional groups: does it matter?[END_REF]). Elton's niche concept has also played a significant role in thinking about organisms according to their position within food webs and their contributions to ecosystem-level nutrient and energy flows in ecosystem ecology and functional ecology (K. W. [START_REF] Cummins | Structure and function of stream ecosystems[END_REF], Naeem 2002b).

These historical streams of research are key to explaining the emergence, at the end of the 20 th century, of a "functional framework", initially used in plant ecology and now in many spheres of ecology (Naeem 2002a[START_REF] Loreau | Linking biodiversity and ecosystems: towards a unifying ecological theory[END_REF]. They are also key to putting into perspective the institutionalization of functional ecology as a sub-discipline of ecology in the late 1980swhen its eponymous journal Functional Ecology was founded by the British Ecological Society-and to understanding its foundational concepts.

"Traits" and "functions"

The functional ecology framework presupposes the identification of functional traits of entities that compose ecological communities and participate in ecological processes. Hence, the twofold conceptual challenge of defining a trait, and explaining why a trait is termed functional. While a recognized definition of trait is any morphological, physiological, phenological, or behavioral feature measured at the level of the individual [START_REF] Violle | Let the concept of trait be functional![END_REF][START_REF] Pey | Current use of and future needs for soil invertebrate functional traits in community ecology[END_REF][START_REF] Garnier | Plant functional diversity: Organism traits, community structure, and ecosystem properties[END_REF], what makes a trait "functional" varies depending on the perspective one adopts on the term "function". In order to clarify the sense in which organismal traits are understood as "functional", it is helpful to draw a distinction between concepts of functional response and effect traits on the one hand, and analyses of the concept of function by philosophers of biology on the other. One standard philosophical analysis of function in biology is the selected effect account, according to which the function(s) of the parts or traits of biological entities are the effects for which those entities were favored under past natural selection [START_REF] Millikan | In defense of proper functions[END_REF][START_REF] Neander | The teleological notion of "function[END_REF]. This account thus equates the concept of function with that of adaptation, as understood in evolutionary biology, and accordingly emphasizes the contrast between functions and evolutionary byproducts [START_REF] Williams | Adaptation and natural selection: A critique of some current evolutionary thought[END_REF].

Although some discussions in functional ecology associate the concept of function with that of adaptation-and so implicitly casts them as selected effects [START_REF] Calow | Towards a Definition of Functional Ecology[END_REF][START_REF] Laureto | Functional diversity: an overview of its history and applicability[END_REF]-it seems unclear whether all actual uses of "functional" in ecology agree with the selected effect account. In particular, functional ecology typically characterizes traits as either functional response traits or effect traits [START_REF] Lavorel | Predicting changes in community composition and ecosystem functioning from plant traits: revisiting the Holy Grail[END_REF][START_REF] Jax | Function and "functioning" in ecology: what does it mean?[END_REF]. Functional response traits are organismal features that determine organisms' responses to changes in environmental conditions [START_REF] Lavorel | Predicting changes in community composition and ecosystem functioning from plant traits: revisiting the Holy Grail[END_REF][START_REF] Violle | Let the concept of trait be functional![END_REF]. Functional response traits are termed functional insofar as they affect the survival and reproductive success of an organism in an environment. In contrast, functional effect traits are features of organisms that contribute to ecosystem processes (e.g. primary productivity, litter decomposition). These traits are deemed functional insofar as they explain or predict possible roles of organisms in the overall functioning of the ecosystem.

Functional response traits-in that they are usually defined in relation to organismal fitness (e.g. [START_REF] Mcgill | Rebuilding community ecology from functional traits[END_REF][START_REF] Violle | Let the concept of trait be functional![END_REF])-seem to conceivably accord with the selected effect account of function. However, an influential alternative to the selected effect account of function, which also links functions to fitness, is the dispositional or contribution to fitness account of function [START_REF] Bigelow | Functions[END_REF][START_REF] Walsh | Fitness and function[END_REF]. Somehow amending the selected effect account-which focuses exclusively on a trait's past contributions to fitness and how this contribution explains why an organism has this trait-the contribution to fitness account attributes functional roles to traits based on their contribution to the current propensity of an organism to survive and reproduce (therefore based on their contribution to the organism's current fitness). Whereas the selected effect account is specifically backward-looking (i.e. defines function in relation to fitness in past environments), the contribution to fitness account has been termed "forward-looking" in that it defines functionality in relation to expected survival and reproduction in current environment. Of course, "forward-looking", here, does not imply a view of traits as evolving in anticipation of future events: only that traits should be considered functional to the extent that they increase an organism's propensity to survive and reproduce. In some important respects, functional response traits as used in functional ecology better align with the contribution to fitness account. Indeed, when traits are characterized as functional response traits, the primary aim is to predict community assembly and community response to environmental changes on the basis of how specific traits suit the organisms that exhibit them to fare well in such environmental contexts [START_REF] Keddy | Assembly and response rules: two goals for predictive community ecology[END_REF]. For this purpose, what matters is how an organism's traits affect its ability to survive and reproduce in a given environment, not whether the organism has its traits because they were advantageous in its ancestor's past environment. If the environment changes, a trait that served some particular fitness use in the ancestor's environment can serve a different fitness use in the offspring's environment (as in the case of exaptations [START_REF] Gould | Exaptation-a missing term in the science of form[END_REF]); and a trait that possibly served no fitness use at all in its ancestors' environment can end-up serving one (as in the case of evolutionary by-products). However, given that the characterization of traits as response traits is often justified in terms of past contributions of these traits, the use of functional response traits also seems to partly accord with the selected effect account. Moreover, it must be recognized that the selected effect and contribution to fitness accounts will converge in cases where the focus is on recent selection and where predicted future or hypothetical selection pressures resemble historical ones (Godfrey-Smith 1994).

The notion of functional effect traits seems to accord less with the selected effect account of function. Conceiving of functional effect traits as selected effect functions would entail a view of organisms and their traits as being shaped by natural selection for their roles in ecosystem processes. This would require defending the claim that natural selection customarily operates at the level of ecosystems, contrary to the standard view that natural selection mainly operates at organismal or gene levels [START_REF] Maclaurin | What is biodiversity?[END_REF][START_REF] Odenbaugh | On the very idea of an ecosystem[END_REF]). The use of functional effect traits in functional ecology, however, does not seem committed to such claims [START_REF] Dussault | Functional ecology's non-selectionist understanding of function[END_REF]. This suggests that functional effect traits better accord with philosophical theories of function that do not draw connections between functions and evolutionary concepts of fitness and selection (though see [START_REF] Dussault | A persistence enhancing propensity account of ecological function to explain ecosystem evolution[END_REF]. Accordingly, several philosophers of ecology (e.g., [START_REF] Maclaurin | What is biodiversity?[END_REF][START_REF] Odenbaugh | On the very idea of an ecosystem[END_REF] in the philosophy of biology. The causal role account defines functions as contributions of the parts of a system to a higher-level capacity of that system. In ecology, this capacity may refer to various dimensions of ecosystem functioning (e.g. primary productivity, litter decomposition).

There is a third notion of function which is distinct from-but related to-the previous notions, namely functions as ecosystem services [START_REF] Jax | Function and "functioning" in ecology: what does it mean?[END_REF][START_REF] Kareiva | Natural Capital: Theory and Practice of Mapping Ecosystem Services[END_REF]). An ecosystem service can be defined as a contribution of an ecosystem in part or as a whole to the well-being of humans [START_REF] Costanza | The value of the world's ecosystem services and natural capital[END_REF][START_REF] Daily | Nature's Services: Societal Dependence On Natural Ecosystems[END_REF]. In this case, the (fitness-related) functional response traits and the (causal role) functional effect traits of organisms are considered from the perspective of how they indirectly are useful to humans. For example, insects pollinate flowers, mangrove trees can mitigate the effects of sea level rise, bacteria decompose waste, etc. Each of these improve human lives and accomplish something we would otherwise have to do at great expense. In this respect, ecosystem services are distinct from other notions of function in that other notions of function are not indexed to the contribution of single species well-being. This also adds a value component, which is not necessarily found in other notions of functions. Thus, since it is also concerned with the responses and effects of organisms as pertaining to the wellbeing of humans, the notion of ecosystem service is related to-but distinct from-the previous selected effects and causal role accounts of function.

The current discussion indicates that the basis on which a trait is termed functional can be diverse. Functional attributions are intimately linked to the research questions pursued, be they assembly or ecosystem-focused. Clearly defining what we mean by "functional trait" is therefore important to ensure a consistent conceptual grounding in functional ecology that makes it possible to understand, compare, and aggregate findings across multiple studies.

"Functional diversity"

Functional diversity can be understood as representing one of the many dimensions of biodiversity. While biodiversity can loosely be defined as the diversity of life at all its levels of organization, from genes to whole ecosystems [START_REF] Wilson | The Diversity of Life[END_REF][START_REF] Harper | Preface[END_REF][START_REF] Gaston | Biodiversity: an introduction[END_REF], functional diversity offers a characterization of this diversity of life in terms of a diversity of functions. Identifying and listing these functions, sorting them out into functional groups and categories, measuring and quantifying them through indices of functional diversity, all of these provide perspectives on biodiversity that are complementary to existing ones, be they genetic, taxonomic or ecosystemic. Functional diversity can therefore be thought as providing a novel vantage point of biodiversity, apprehending the diversity of life in terms of a diversity of organismal functional traits, complementing diversity framed, for instance, in terms of species, genes or ecosystems. In this way, the concept contributes to broadening our understanding of the multiple facets of biodiversity, thereby supplementing taxonomic approaches and revealing another dimension of the complexity of life.

But the concept of functional diversity is more than just a way of depicting the diversity of life: it also enables different predictions and explanations of the ways in which the diversity of life interacts, evolves and responds to change. Indeed, functional diversity perspectives allegedly offer a deeper mechanistic and more integrated perspective onto biodiversity than traditional taxonomic approaches [START_REF] Norberg | Biodiversity and ecosystem functioning: A complex adaptive systems approach[END_REF][START_REF] Petchey | Functional diversity: back to basics and looking forward[END_REF][START_REF] Gagic | Functional identity and diversity of animals predict ecosystem functioning better than species-based indices[END_REF], including the degree to which community constituents are redundant or complementary in their functional contributions [START_REF] Micheli | Low functional redundancy in coastal marine assemblages[END_REF][START_REF] Bracken | Functional consequences of realistic biodiversity changes in a marine ecosystem[END_REF]). Such a perspective is possible by focusing on the different interactions of living forms and the ways in which these interactions are mediated via the functional traits observed. In particular, trait values represented within communities-and their mean and distribution among other statistical measures-can provide complementary insights into ecosystem processes, including the productivity of focal trophic levels and adjacent ones, as well as nutrient cycling [START_REF] Litchman | Trait-based community ecology of phytoplankton[END_REF][START_REF] Handa | Consequences of biodiversity loss for litter decomposition across biomes[END_REF][START_REF] Hébert | Linking zooplankton communities to ecosystem functioning: toward an effect-trait framework[END_REF]. Estimates of community functional diversity and examination of the most responsive traits can also reflect both selective pressures and changes in the biotic and abiotic environment [START_REF] Vogt | Using functional traits to investigate the determinants of crustacean zooplankton community structure[END_REF]).

One such perspective on community functional diversity is that of "functional structure,"

which is described by the community weighted mean of functional traits (CWM, using species abundances in a given community as weights for the trait values exhibited by these species).

Another perspective is that of "functional diversity," which is used to describe trait value variability in terms of richness, evenness and divergence [START_REF] Mouillot | Functional structure of biological communities predicts ecosystem multifunctionality[END_REF], richness referring to the extent of the trait value distribution-i.e. difference between maximum and minimum values-and evenness and divergence to the regularity or the inequalities in distribution along the axis of trait values, respectively [START_REF] Garnier | Plant functional diversity: Organism traits, community structure, and ecosystem properties[END_REF]). These two perspectives (structure and diversity) underlie the two main hypotheses relating community structure and ecosystem processes, that is the importance of dominance of particular species and traits that control function (importance measured in terms of biomass hypothesis, evaluated by CWM) versus the importance of complementarity, evaluated by trait value variability or divergence [START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF][START_REF] Tilman | The ecological consequences of changes in biodiversity: A search for general principles[END_REF]. They also underlie investigations of the roles played by keystone and rare species and their specific traits, of trait redundancy (e.g. questions about determinants of community assembly [START_REF] Nock | Functional Traits[END_REF], the community being seen as the result of different filters that exclude those phenotypes that do not have adequate trait values [START_REF] Keddy | Assembly and response rules: two goals for predictive community ecology[END_REF]).

An epistemic challenge comes from the plurality of indices that have been developed, based on different types of functional data and different mathematical expressions that can be sensitive to context. In this respect, [START_REF] Garnier | Plant functional diversity: Organism traits, community structure, and ecosystem properties[END_REF] suggest that one option might be to consistently apply more simple measures of trait distributions (means, SD, skewness, curtosis), that have well-understood mathematical properties. The quantification of functional diversity-with specific metrics and indices that aggregate raw measurements-remains an area where more work is needed; in particular, the standardization of statistical approaches could contribute to overcome current difficulties for comparing and integrating across different studies.

Empirical challenges: identifying and measuring functional traits

The epistemic role of functional traits depends on their clear identification and measurement in the field. This, however, is not straightforward. A first challenge consists in identifying the relevant functional traits with regard to the phenomena to be explained. Because organismal traits are extremely numerous, researchers must find ways to focus on those traits that matter for explaining or predicting the target features of the ecological phenomena under investigation. For instance, over the last decades, numerous studies have attempted to explain community structure and dynamics as affected by specific environmental conditions through changes in response traits-traits that respond to abiotic, biotic and disturbance filters [START_REF] Lavorel | Predicting changes in community composition and ecosystem functioning from plant traits: revisiting the Holy Grail[END_REF][START_REF] Barnett | Zooplankton Biodiversity and Lake Trophic State: Explanations Invoking Resource Abundance and Distribution[END_REF]. In this case, the focus is on organismal traits that are related to constraining environmental or biotic variables. For example, an abiotic filter linked to low nutrient availability will result in a restricted range of plant trait values linked to the physiological tolerance to low fertility (traits related to nutrient conservation). In this case, the distribution of community plant traits would converge, corresponding to species that respond similarly to these conditions. When the objective of the study is to relate traits to ecosystem processes, the traits are selected based on their role in the underlying mechanisms of those processes (such traits are referred to as effect traits in that they have an effect onto ecosystem processes). Though these effect traits are harder to assess in practice since they require singling out-within an extremely large set of possibilities-the most significant activities of organisms that might affect ecosystem processes, experimental approaches have been developed, as in the case of zooplankton or arthropods [START_REF] Hébert | Linking zooplankton communities to ecosystem functioning: toward an effect-trait framework[END_REF], Brousseau et al. 2018a). For plants, one Preprint -Published in BioScience Volume 69, Issue 10, October 2019, Pages 800-811 https://doi.org/10.1093/biosci/biz089 https://academic.oup.com/bioscience/article-abstract/69/10/800/5556011?redirectedFrom=fulltext -10 -of the most-studied ecosystem processes in relation to leaf traits is that of decomposition. A number of effect traits, both morphological and chemical, have been related to decomposition, but in many studies, the trait of leaf dry matter content seems to be the single trait that explains the most variation in leaf decomposition rate [START_REF] Garnier | Plant functional diversity: Organism traits, community structure, and ecosystem properties[END_REF]. Needless to say, still only a few of all potentially relevant organismal functional traits are known today.

A second challenge concerns the level and granularity at which functional traits should be defined and measured [START_REF] Mcgill | Rebuilding community ecology from functional traits[END_REF]. In particular, should such traits be considered at the level of species? Should intra-species genetic variation also be considered? Or should functional traits be aggregated at a higher granularity level of species functional groups or of other evolutionary taxa (e.g. family, order)? For instance, plant trait information is available in data banks such as TRY (Kattge et al. 2011), measured at the level of individual species; whether this is aggregated at another level will depend on the purpose and the particular questions of the study. For example, global vegetation models integrate information at the much higher level of plant functional type, and new approaches are being tested to integrate and aggregate the species-level trait data in these models [START_REF] Verheijen | Variation in trait trade-offs allows differentiation among predefined plant functional types: implications for predictive ecology[END_REF]. The importance of trait variation at the level of the individual can be addressed by questions about intraspecific variability, which represents a current frontier gaining importance in the context of adaptation to global change [START_REF] Violle | The emergence and promise of functional biogeography[END_REF]. A number of studies indicate that intraspecific variability is most important in systems with lower species richness, such as boreal ecosystems [START_REF] Kumordzi | variability of dominant species across contrasting island ecosystems[END_REF], and the importance varies with scale, being more important at more local scales [START_REF] Albert | When and how should intraspecific variability be considered in trait-based plant ecology? Perspectives in Plant Ecology[END_REF]).

However, measuring traits at a coarse granularity may be the only viable option for studies that span trophic levels, involving large numbers of taxa whose traits are difficult to measure at the species level, such as plankton or microbial species [START_REF] Torsvik | Microbial diversity and function in soil: from genes to ecosystems[END_REF][START_REF] Krause | Trait-based approaches for understanding microbial biodiversity and ecosystem functioning[END_REF].

A third and related challenge concerns the practical measurability of certain functional traits and their substitution by easier-to-measure "proxy traits". For example, in plant functional ecology, certain easy-to measure traits are used and also widely accepted as proxies of traits that are more difficult to assess. A good example is that of leaf dry matter content (LDMC) which is considered a good proxy for both leaf tissue density [START_REF] Garnier | A standardized protocol for the determination of specific leaf area and leaf dry matter content[END_REF], and also for flammability of plant tissue [START_REF] Garnier | Plant functional markers capture ecosystem properties during secondary succession[END_REF]. The fact that it is very easy to measure (two masses-saturated fresh mass and dry mass), makes it subject to lower error than is likely the case for the measures that it substitutes. In the case of microbial communities, species or taxa are often taken as a proxy for microbial traits that appear to be phylogenetically constrained (e.g., salinity preference, methanogenesis). However other traits (e.g., phosphate utilization, phage resistance) appear to be fast-evolving and poorly correlated with phylogeny [START_REF] Martiny | Microbiomes in light of traits: A phylogenetic perspective[END_REF]. In such cases, assessing their diversity through taxonomic proxies will fail, which leaves open the question of their measurement in practice. More generally, it is important to be aware that, because proxies are less directly linked to ecosystem processes than the traits they replace, their use in practice may be significantly limited. Microbial ecologists are increasingly turning to shotgun metagenomics rather than purely taxonomic surveys to provide a read-out of gene functions present in an environment [START_REF] Sunagawa | Structure and function of the global ocean microbiome[END_REF][START_REF] Quince | Shotgun metagenomics, from sampling to analysis[END_REF]. The extent to which microbial taxa are good proxies for traits (understood as gene functions) is currently an active area of debate [START_REF] Galand | A strong link between marine microbial community composition and function challenges the idea of functional redundancy[END_REF][START_REF] Louca | Function and functional redundancy in microbial systems[END_REF]) and it appears that functional redundancy may be a feature of broad metabolic traits, but not of finer-grained traits related to biotic interactions.

A fourth challenge to measuring functional traits in the field stems from their spatio-temporal variability: measured traits may vary intra-specifically depending on environmental or regional differences to which individuals have been exposed. For instance, in the case of evergreen needle chemistry, studies have found that intraspecific plasticity contributed to significant differences in forest productivity and carbon cycles-and therefore ecosystem functioning [START_REF] Reich | Biogeographic variation in evergreen conifer needle longevity and impacts on boreal forest carbon cycle projections[END_REF]. Note that intra-specific genetic variability often adds to this plasticity, the two not being distinguished in most studies. Although some studies identify at which scales and under which conditions intra-specific variability matters-notably alongside environmental gradients- [START_REF] Auger | Inter-specific and intra-specific trait variation along short environmental gradients in an old-growth temperate forest[END_REF][START_REF] Kumordzi | variability of dominant species across contrasting island ecosystems[END_REF][START_REF] Siefert | A global meta-analysis of the relative extent of intraspecific trait variation in plant communities[END_REF], measurement at the scale of the individual adds another dimension to functional diversity empirical approaches. In addition, many traits can show variability over time as an organism develops-such as changes in leaf traits, leaf and wood chemistry-as observed, for instance, by [START_REF] Martin | Size-dependent changes in leaf and wood chemical traits in two Caribbean rainforest trees[END_REF] for tropical trees of various life stages. In aquatic phytoplankton, rapid generation times and large overall variation in traits has led to a different approach to the spatiotemporal variability challenge: examination of the adaptation of the community composition as a whole-as represented by the constituent traits observed, for instance coloniality, cell size (surface area-to-volume ratio, as indicator of nutrient uptake and protection against predators), motility and nutritional mode-to local or regional environmental differences (e.g., [START_REF] Leonilde | Individual trait variation in phytoplankton communities across multiple spatial scales[END_REF], with consequences for ecosystem processes (e.g., [START_REF] Follows | Emergent biogeography of microbial communities in a model ocean[END_REF]. This approach considers that trait variation conforms to the ubiquity hypothesis according to which "everything is everywhere, and the local environment selects" for microbes [START_REF] Baas-Becking | Geobiologie of Inleiding Tot de Milieukunde[END_REF]. Trait variation across regions then occurs via selection of organisms best adapted to prevailing Preprint -Published in BioScience Volume 69, Issue 10, October 2019, Pages 800-811 https://doi.org/10.1093/biosci/biz089 https://academic.oup.com/bioscience/article-abstract/69/10/800/5556011?redirectedFrom=fulltext -12 -temperature, mixing, light, ice cover or predation regimes (e.g., [START_REF] Schwaderer | Eco-evolutionary differences in light utilization traits and distributions of freshwater phytoplankton[END_REF][START_REF] Acevedo-Trejos | Mechanisms shaping size structure and functional diversity of phytoplankton communities in the ocean[END_REF][START_REF] Thomas | Environment and evolutionary history determine the global biogeography of phytoplankton temperature traits[END_REF][START_REF] Özkundakci | Winter severity determines functional trait composition of phytoplankton in seasonally ice-covered lakes[END_REF].

Empirical challenges: extending the domain of functional diversity studies

Functional diversity studies have-so far-only addressed a very limited spectrum of the biosphere's total biological diversity and biomass. Under-represented but vastly abundant and diverse organisms include, in particular, arthropods and microbial organisms-such as protists, fungi, bacteria, and viruses-that have been largely ignored as compared to plants, birds, stream insects, fish and plankton communities as the focus of functional diversity studies [START_REF] Devictor | Functional biotic homogenization of bird communities in disturbed landscapes[END_REF][START_REF] Nock | Functional Traits[END_REF]. Evidently, these organisms interact with plants and with each other to influence ecosystem functioning. A key empirical question for future research is the degree to which the functional framework initially developed for some macroscopic species can be applied to other kinds of organisms. This includes both the question of which functional traits can and should be measured across the whole range of living organisms, and to what extent the general concept of function will need to be modified.

For instance, microorganisms such as bacteria and archaea present significant challenges owing to limited morphological differences among microorganisms and because most microbial life cannot easily be grown in culture and observed in controlled settings (Martiny et al. 2006[START_REF] Green | Microbial biogeography: From taxonomy to traits[END_REF][START_REF] Malaterre | The challenges of microbial diversity[END_REF]. The identification of microbial functional traits has largely focused on two types of traits [START_REF] Martiny | Microbiomes in light of traits: A phylogenetic perspective[END_REF]: traits that can be measured directly on microbial organisms in the laboratory (e.g., ability to fix nitrogen or growth rate in culture) and traits that can be inferred based on the presence or expression of genes in an organism or in the metagenome or metatranscriptome of a microbial community (e.g., presence of genes for nitrogen fixation in a metagenomic sample). Several empirical challenges remain: (i) the potential disconnection between the presence of a gene in a genome or community versus the expression of that gene due to microbial dormancy [START_REF] Lennon | Microbial seed banks: the ecological and evolutionary implications of dormancy[END_REF]; (ii) the assumptions associated with inferring functions from phylogenetic relatedness when relative abundances of microbial taxa are estimated only using barcoding approaches such as sequencing of the bacterial 16S gene to detect the presence of microbial taxa in a sample instead of direct observations of individual organisms and their functions; (iii) the difficulty of inferring function from gene sequences, especially for complex or polygenic traits such as behavior and environmental niche tolerances [START_REF] Keeling | Marine protists are not just big bacteria[END_REF] and (iv) the lack of empirical data to Preprint -Published in BioScience Volume 69, Issue 10, October 2019, Pages 800-811 https://doi.org/10.1093/biosci/biz089 https://academic.oup.com/bioscience/article-abstract/69/10/800/5556011?redirectedFrom=fulltext -13 -identify correlations among microbial functional traits, and between traits and ecosystem functions [START_REF] Fierer | Seeing the forest for the genes: using metagenomics to infer the aggregated traits of microbial communities[END_REF]).

To take another example that concerns arthropods, empirical challenges have included accounting for strong sexual dimorphism or highly distinct juvenile and adult life forms within a single species (Brousseau et al. 2018a) and identifying a core set of traits across the staggering diversity of morphologies, life histories, resource acquisition and predator avoidance strategies [START_REF] Moretti | Handbook of protocols for standardized measurement of terrestrial invertebrate functional traits[END_REF]. While progress has been made, a majority of existing studies lack clear hypotheses associated with environmental filters, traits or functions, limiting the capacity to use the existing conceptual framework predictively (Brousseau et al. 2018a).

Considering traits of organisms of newly studied communities opens the door to the emerging field of interaction ecology. The challenge in this case is to acknowledge the complexity of interactions and incorporate all their different consequences on ecosystem functioning. For example, viruses as free entities may be considered functionally inert, as they have no metabolism and gene expression outside of their host; yet, through strain-specific lysis they influence community structure and are major players in nutrient cycling [START_REF] Suttle | Marine viruses -major players in the global ecosystem[END_REF]. Moreover, metabolic pathways in viruses can be distributed across many individuals, rather than constrained to a single genome [START_REF] Hurwitz | Metabolic reprogramming by viruses in the sunlit and dark ocean[END_REF], further complicating the relationship between traits and functions. Another use of traits in the study of interactions is to match traits of consumers to that of their resources in order to predict trophic interactions [START_REF] Bartomeus | A common framework for identifying linkage rules across different types of interactions[END_REF]. Trait matching of predatory traits (e.g. mandible strength of a carabid beetle) to prey traits (toughness of prey) has been successfully used to predict feeding interactions, providing a technique useful to anticipate consequences of novel communities and ecosystems (Brousseau et al. 2018b). But needless to say, much remains to be investigated.

Epistemic roles of functional diversity

The concept of functional diversity and its empirical assessment fulfill diverse epistemic roles in ecological research. Disentangling these roles sheds light on the various ways by which knowledge is produced and justified in this domain of ecology. As alluded to above (section "Functional diversity"), functional traits and their measures of diversity can first be used as explanatory factors in models that aim at explaining why and how certain ecosystem processes and properties obtain. For instance, ecosystem productivity measured in terms of biomass can be well explained by models based on functional trait diversity (e.g., [START_REF] Paquette | The effect of biodiversity on tree productivity: from temperate to boreal forests[END_REF] for temperate to boreal forests, or , [START_REF] Finegan | Does functional trait diversity predict above-ground biomass and productivity of tropical forests? Testing three alternative hypotheses[END_REF] for tropical forests). Conversely, Preprint -Published in BioScience Volume 69, Issue 10, October 2019, Pages 800-811 https://doi.org/10.1093/biosci/biz089 https://academic.oup.com/bioscience/article-abstract/69/10/800/5556011?redirectedFrom=fulltext -14 -functional diversity can also be conceptualized as the element that requires explaining. For instance, this is the case when modeling diversity in functional traits as a function of certain environmental gradients (e.g., variation in plant leaf size as a function of environmental water availability and temperature [START_REF] Wright | Global climatic drivers of leaf size[END_REF]). This dual role of functional diversity-as something that explains and as something needs explaining-shows notably in the distinction made by ecologists between functional effect traits and functional response traits.

The explanatory role of functional diversity is often linked to a predictive role. For instance, identifying trait values that favor survival in a given environment makes it possible to predict which species have better chances of persisting or are more at risk of becoming extinct given specific environmental changes (Frenette- [START_REF] Frenette-Dussault | Trait-based climate change predictions of plant community structure in arid steppes[END_REF]. Functional diversity also facilitates the development of testable predictions of broader ecological hypotheses. For example, in asking whether assembly of plant communities was mostly driven by environmental filtering or by competitive interaction, [START_REF] Weiher | Assembly rules, null models, and trait dispersion: New questions from old patterns[END_REF] predicted the former should lead to low variance in the functional traits of a plant community, while the latter should lead to a more even distribution of traits around the community trait mean. Such predictions were indeed corroborated in field studies (e.g., [START_REF] Cornwell | Community assembly and shifts in plant trait distributions across an environmental gradient in coastal California[END_REF]. Similar questions concern microbial community structure where environmental filtering also appears to be an important driver (Horner-Devine & Bohannan 2006[START_REF] Silverman | A phylogenetic transform enhances analysis of compositional microbiota data[END_REF], although the relative importance of environmental filtering vs. competitive effects varies by trait [START_REF] Martiny | Microbiomes in light of traits: A phylogenetic perspective[END_REF][START_REF] Tromas | Niche Separation Increases With Genetic Distance Among Bloom-Forming Cyanobacteria[END_REF]. Another instance where predictions from functional diversity models can inform broader ecological hypotheses concerns the unified neutral theory of biodiversitywhich postulates that functional differences among species are unimportant in generating and maintaining the diversity of communities [START_REF] Hubbell | The unified neutral theory of species abundance and diversity[END_REF]). The neutral theory provides a starting point from which the importance of non-neutral processes might be investigated [START_REF] O'dwyer | Cross-scale neutral ecology and the maintenance of biodiversity[END_REF]. In specific cases where the neutral theory alone could not account for the empirical data, dual approaches incorporating also functional diversity have been shown to provide more accurate predictions (e.g., [START_REF] Gravel | Reconciling niche and neutrality: the continuum hypothesis: Reconciling niche and neutrality[END_REF][START_REF] Yıldırım | Contrasting patterns of neutral and functional genetic diversity in stable and disturbed environments[END_REF]. Interestingly, the neutral theory has also pushed functional ecologists to seek to demonstrate the importance of functional differences, instead of simply assuming their importance (e.g., [START_REF] Gotelli | Null versus neutral models: what's the difference?[END_REF][START_REF] Rosindell | The unified neutral theory of biodiversity and biogeography at age ten[END_REF].

Functional diversity can also play a role in further enabling greater generalization and unification in ecology. Indeed, this may be achieved through the pragmatic operationalization of ecology by the use of a common and reduced set of functional traits representing major Preprint -Published in BioScience Volume 69, Issue 10, October 2019, Pages 800-811 https://doi.org/10.1093/biosci/biz089 https://academic.oup.com/bioscience/article-abstract/69/10/800/5556011?redirectedFrom=fulltext -15 -resource-use strategies within a given taxon or trophic level. As well, by facilitating the scaling of ecological observations across levels of biological organization, functional diversity may also contribute to the conceptual and methodological unification of fields such as population, community and ecosystem ecology, a major epistemic ideal according to some authors [START_REF] Pickett | Functional trait niches of North American lotic insects: traits-based ecological applications in light of phylogenetic relationships[END_REF][START_REF] Vellend | Drawing ecological inferences from coincident patterns of population-and community-level biodiversity[END_REF].

Finally, functional diversity plays a heuristic role in the current development of ecological theory. Although functional generalizations in ecology remain limited by the small number of studies conducted using a shared set of traits with standard measurement methodologies, several sub-fields of ecology have been fruitfully guided by the functional diversity framework [START_REF] Krause | Trait-based approaches for understanding microbial biodiversity and ecosystem functioning[END_REF][START_REF] Moretti | Handbook of protocols for standardized measurement of terrestrial invertebrate functional traits[END_REF]. For example, functional approaches can facilitate the description and study of complex adaptive patterns [START_REF] Peters | Operationalization of terms and concepts[END_REF][START_REF] Odenbaugh | Idealized, Inaccurate but Successful: A Pragmatic Approach to Evaluating Models in Theoretical Ecology[END_REF]) by leading researchers to focus on specific variables and relations-such as traits that can be linked to functions-over other variables-e.g. traits without function-and by shaping the ways ecological hypotheses and models are developed.

The role of functional diversity in ecological management

Research on functional diversity is often considered relevant for ecological management [START_REF] Cadotte | Beyond species: functional diversity and the maintenance of ecological processes and services[END_REF]. For instance, the knowledge that functional redundancy and response diversity within functional groups affects ecosystem resilience [START_REF] Gunderson | Ecological resilience in theory and application[END_REF][START_REF] Walker | Resilience Thinking: Sustaining Ecosystems and People in a Changing World[END_REF][START_REF] Desjardins | Promoting resilience[END_REF], or that certain traits co-vary with the presence of some ecosystem properties [START_REF] De Bello | Towards an assessment of multiple ecosystem processes and services via functional traits[END_REF], could be used to influence ecosystems by intervening on functional trait assemblages. Intervening on functional effect traits could thus be a way to influence ecosystem processes, for instance to optimize specific functions and services [START_REF] Isbell | Linking the influence and dependence of people on biodiversity across scales[END_REF], or even promote targeted aspects of biodiversity [START_REF] Walker | Biodiversity and Ecological Redundancy[END_REF][START_REF] Tilman | Future threats to biodiversity and pathways to their prevention[END_REF].

Conversely, changes in ecosystemic properties-including changes in spatial and temporal heterogeneity or habitat connectivity-known to affect species diversity in some contexts [START_REF] Tews | Animal species diversity driven by habitat heterogeneity/diversity: the importance of keystone structures[END_REF][START_REF] Fahrig | Functional landscape heterogeneity and animal[END_REF] could also be sought to induce specific changes on some selected characteristics of functional trait diversity.

Nonetheless, inquiries on the relationships between functional diversity and ecosystem functioning and services are yet to provide clear and well-corroborated answers as to how these relationships are to play any significant role in ecological management [START_REF] Steffen | Planetary boundaries: Guiding human development on a changing planet[END_REF][START_REF] Garnier | Plant functional diversity: Organism traits, community structure, and ecosystem properties[END_REF]. Although a few studies have successfully linked trait combinations to processes, scaling up to relevant management scales [START_REF] Orwin | Linkages of plant traits to soil properties and the functioning of temperate grassland[END_REF][START_REF] Lavorel | Using plant functional traits to understand the landscape distribution of multiple ecosystem services[END_REF] both in grasslands), many challenges remain to extend this approach to a range of different ecosystems, notably due to a lack of sufficient local trait data [START_REF] Garnier | Plant functional diversity: Organism traits, community structure, and ecosystem properties[END_REF]. Furthermore, due to the complexity of ecosystems and the non-linear dynamics of ecological variables, it is likely that the optimization of one function could be detrimental for other functions in any given ecosystem. A multifunctional perspective appears therefore necessary [START_REF] Wuethrich | Reconstructing Brazil's atlantic rainforest[END_REF], which adds a layer of complexity in the prediction of a specific intervention's outcome. The greatest care should thus be taken if one wants to ground current management action on a functional account of biodiversity.

It is also important to note that functional approaches to ecological management and conservation contrast with the more traditional approaches, such as those that focus on natural heritage or that attribute a non-instrumental value to species, naturalness or wilderness (Wuerthner et al. 2014). In particular, by putting the emphasis on functional traits independently of the species, the functional approach to conservation may open the door to a type of management where "anything goes" as long as some ecosystem services are provided, thereby increasing the risk of overlooking species [START_REF] Simberloff | Novel ecosystems" are a Trojan horse for conservation[END_REF]. Moreover, under a changing climate, conservationists and ecosystem managers may require new services that are more adaptive, in order to prevent ecosystem degradation [START_REF] Lavorel | Mustering the power of ecosystems for adaptation to climate change[END_REF], thus requiring further analyses of underlying processes and controlling traits. Much, therefore, hinges on the choice of normative objectives to pursue, be they framed in terms of sustainable agriculture, ecological restoration and engineering, or biodiversity and nature conservation. Goals that concern ecosystem functioning and services (e.g., biomass production, resilience, carbon fixation) are well served by functional approaches, whereas other objectives (e.g., historical continuity, naturalness, conservation of the entities themselves rather than what they do) may require a different focus and axiology [START_REF] Perring | Advances in restoration ecology: rising to the challenges of the coming decades[END_REF]. Such ethical considerations reinforce the need to adopt not only an adaptive but also a pluralistic and collaborative management framework, where various stakeholders and different social and economic goals are integrated and reevaluated along the way [START_REF] Maris | From adaptive management to adjustive management: a pragmatic account of biodiversity values[END_REF]. Finally, another relevant aspect to consider is the scale at which functional diversity should be investigated. To effectively guide ecological management, studies should range from a local piece-meal approach to a broader "geofunctional" perspective that aim at capturing the complexity of social-ecological networks and their interactions with the rest of nature (Hobbs et al. 2011, Desjardins et al. in press).

Beyond an epistemic roadmap

As the functional diversity framework is increasingly adopted in many branches of ecology, clarifying the meaning of concepts such as "functional trait" or "functional diversity" becomes critical, at least to ensure mutual understanding and to facilitate comparisons and integration among findings. But as the epistemic roadmap presented above shows, such conceptual clarification is but one of the many epistemic challenges that functional approaches in ecology must face. The functional diversity framework is nevertheless showing signs of maturity as its influence over explanations, theoretical models or ecosystem management options grows. Perhaps one of the strongest hopes about the framework is that it may enable integration across studies that typically focus-for the sake of dealing with complexity-on specific trophic levels, but that could and must be combined to elucidate larger-scale ecological phenomena. Because functional diversity tackles ecosystems through more mechanistic lenses than does taxonomic diversity, it holds the promise of a deeper understanding of ecosystem processes and of improved predictability of ecosystem functioning. The degree to which this promise will be fulfilled is still, of course, undetermined. To date, focusing on "functional traits" has successfully provided heuristics to identify candidate variables for ecosystem modeling. Yet, in the end, the epistemic fruitfulness of functional diversity will hinge upon its capacities to explain and predict across several domains. In this respect, it is still an open question to what extent functional diversity will account for community assembly patterns and/or predict ecosystem processes, or whether more detailed insights about the very mechanisms that link functional traits together and to local abiotic conditions will also be required. After all, having the list of components of a mechanism is always a good start, yet, knowing how these components interact often remains necessary to understand the actual operation of that mechanism, and most notably so in the case of ecosystems. Uncovering the mechanistic links among functional componentsand not just the diversity of these functional components-may henceforth prove necessary when investigating ecosystems and some of their more challenging dynamical properties. It is also important to note that even though functional diversity may be key to achieving the epistemic goals of a better understanding of ecosystem functioning and possibly of ecosystem services, it may or may not be appropriate for achieving conservation objectives, depending on the normative goals attached. Also, the extent to which conservation legislation will be shaped by functional diversity or by species diversity, possibly depending on their epistemic successes, will most certainly influence which normative goals are pursued. Knowledge and values, though

Figure 1 .

 1 Figure 1. Epistemic roadmap that highlights the main foundational assumptions of the functional diversity framework in ecology, including conceptual, methodological (measurement) and theoretical challenges that may ultimately affect environmental decision making.

  Preprint -Published in BioScience Volume 69, Issue 10, October 2019, Pages 800-811 https://doi.org/10.1093/biosci/biz089 https://academic.oup.com/bioscience/article-abstract/69/10/800/5556011?redirectedFrom=fulltext

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  have argued that functional effect traits should be interpreted through the lens of the causal role account of function (R. C.

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	Cummins 1975), arguably the most influential non-selectionist account of function developed
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,

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A search on Google Scholar with the keywords ["functional diversity" AND ecology] reveals over 8000 articles in

compared to just over 300 in 1997 (hence a 25-fold increase). A similar trend is present in the Scopus database (though with smaller absolute numbers due to a narrower journal base). When relativized to articles found with the keyword [ecology], the ratio goes from about 0.1% in 1997 to about 1% in 2017 (hence a 10-fold increase) (Source: scholar.google.ca; scopus.com; accessed Oct 20 th ,

2018).