Rethinking megafauna

Rethinking megafauna royalsocietypublishing.org/journal/rspb Review Cite this article: Moleón M et al. 2020 Rethinking megafauna. Proc. R. Soc. B 287: 20192643. http://dx.doi.org/10.1098/rspb.2019.2643 Marcos Moleón1,2, José A. Sánchez-Zapata3, José A. Donázar1, Eloy Revilla1, Berta Martín-López4, Cayetano Gutiérrez-Cánovas5, Wayne M. Getz6,7, Zebensui Morales-Reyes3, Ahimsa Campos-Arceiz8,9, Larry B. Crowder10, Mauro Galetti11,12, Manuela González-Suárez13, Fengzhi He14,15, Pedro Jordano1, Rebecca Lewison16, Robin Naidoo17, Norman Owen-Smith18, Nuria Selva19, Jens-Christian Svenning20,21, José L. Tella1, Christiane Zarfl22, Sonja C. Jähnig14, Matt W. Hayward23,24,25,26, Søren Faurby27,28, Nuria García29, Anthony D. Barnosky30 and Klement Tockner14,15,31 1 Department of Conservation Biology, Doñana Biological Station-CSIC, Seville, Spain Department of Zoology, University of Granada, Granada, Spain 3 Department of Applied Biology, University Miguel Hernández, Elche, Spain 4 Leuphana University, Lüneburg, Germany 5 FEHM-Lab-IRBIO, Department of Evolutionary Biology, Ecology and Environmental Sciences, University of Barcelona, Barcelona, Spain 6 Department of ESPM, UC Berkeley, Berkeley, CA, USA 7 School of Mathematical Sciences, University of KwaZulu-Natal, Durban, South Africa 8 School of Environmental and Geographical Sciences, and 9Mindset Interdisciplinary Centre for Environmental Studies, University of Nottingham Malaysia, Selangor, Malaysia 10 Hopkins Marine Station, Stanford University, Standford, CA, USA 11 Departamento de Ecologia, Instituto de Biociências, Universidade Estadual Paulista, Rio Claro, SP, Brazil 12 Department of Biology, University of Miami, Coral Gables, FL, USA 13 Ecology and Evolutionary Biology Division, School of Biological Sciences, University of Reading, Reading, UK 14 Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany 15 Institute of Biology, Freie Universität Berlin, Berlin, Germany 16 Department of Biology, San Diego State University, San Diego, CA, USA 17 WWF-US, Washington, DC, USA 18 School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa 19 Institute of Nature Conservation, Polish Academy of Sciences, Kraków, Poland 20 Section for Ecoinformatics and Biodiversity, Department of Bioscience, Aarhus University, Aarhus C, Denmark 21 Center for Biodiversity Dynamics in a Changing World (BIOCHANGE), Department of Bioscience, Aarhus C, Denmark 22 Center for Applied Geoscience, Eberhard Karls University of Tübingen, Tübingen, Germany 23 College of Natural Sciences, Bangor University, Bangor, UK 24 Centre for Wildlife Management, University of Pretoria, Pretoria, South Africa 25 Centre for African Conservation Ecology, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa 26 School of Environmental and Life Sciences, University of Newcastle, Newcastle, Australia 27 Department of Biological and Environmental Sciences, University of Gothenburg, Göteborg, Sweden 28 Gothenburg Global Biodiversity Centre, Göteborg, Sweden 29 Department of Geodynamics, Stratigraphy and Paleontology, Quaternary Ecosystems, University Complutense of Madrid, Madrid, Spain 30 Jasper Ridge Biological Preserve, Stanford University, Stanford, CA, USA 31 Austrian Science Fund FWF, Vienna, Austria 2 Received: 14 November 2019 Accepted: 11 February 2020 Subject Category: Ecology Subject Areas: ecology, evolution, palaeontology Keywords: apex predators, body size, functional traits, keystone species, large animals, megaherbivores Author for correspondence: Marcos Moleón e-mail: [email protected] MM, 0000-0002-3126-619X; JAD, 0000-0002-9433-9755; WMG, 0000-0001-8784-9354; ZM-R, 0000-0002-4529-8651; AC-A, 0000-0002-4657-4216; MG, 0000-0002-8187-8696; FH, 0000-0002-7594-8205; NS, 0000-0003-3389-201X; J-CS, 0000-0002-3415-0862; CZ, 0000-0002-2044-1335; KT, 0000-0002-0038-8151 Electronic supplementary material is available online at https://doi.org/10.6084/m9.figshare. c.4860798. Concern for megafauna is increasing among scientists and non-scientists. Many studies have emphasized that megafauna play prominent ecological roles and provide important ecosystem services to humanity. But, what precisely are ‘megafauna’? Here, we critically assess the concept of megafauna and propose a goal-oriented framework for megafaunal research. First, we review definitions of megafauna and analyse associated terminology in the scientific literature. Second, we conduct a survey among ecologists and © 2020 The Author(s) Published by the Royal Society. All rights reserved. Prehistoric art provides evidence that megafauna (literally, ‘large animals’; see electronic supplementary material, appendix S1 for the etymology and popular definitions of this term) have fascinated humans since our origins (e.g. [1]). The eminent nineteenth-century naturalist Wallace [2] referred to megafauna as ‘the hugest, and fiercest, and strangest forms’. A hundred and forty plus years later, however, megafaunal research still lacks a unifying framework for the use of this term, which has diverged in the development of disciplines as diverse as wildlife biology, oceanography, limnology, soil ecology, evolutionary biology, conservation biology, palaeontology and anthropology. Thus, definitions in the scientific literature include disparate combinations of species: from the smallest organisms readily visible in photographs to the largest vertebrates ever on earth (e.g. [3–5]; figure 1, electronic supplementary material, appendix S2). Given the great sociocultural significance of megafauna [6,7], the ubiquity of the megafauna concept in addressing profound and varied scientific questions [8–11], and the multiple threats that jeopardize large animals [12–14], a re-examination of the concept is warranted [15]. Here, we review the concept of megafauna and propose a goal-oriented framework for megafauna research, which may support scientific endeavours, improve conservation policy and practice, and strengthen the public perception. To do this, we adopt a two-pronged approach. First, we review the scientific literature to (i) examine the different definitions of megafauna and (ii) analyse the terminology commonly associated with the concept of megafauna. Second, we carry out a survey among ecologists and palaeontologists to (iii) assess the traits of the species they consider as megafauna and (iv) identify the key criteria that should define megafauna. The goal of this survey is to enhance our understanding of how researchers working with megafauna conceptualize data that already exist in the scientific literature. Based on insights gained from the review and survey, we propose a working scheme for the use of the megafauna concept, discuss pros and cons of different definitions, and provide recommendations for advancing interdisciplinary megafaunal research. 2 (a) Megafauna definitions We conducted a systematic review of existing megafauna definitions in the scientific literature (276 articles reviewed; see electronic supplementary material, appendix S3 for a complete list of references and electronic supplementary material, appendix S4 for the searching methods). The majority of megafauna articles focused on terrestrial species (55% of the papers; mainly concerned with prehistorical times) and marine ecosystems (52%; mostly referencing recent times), with very few articles dealing with freshwater megafauna (1%; figure 2 and electronic supplementary material, figure S1). Our search did not uncover any paper dealing with soil megafauna, although soil ecologists use this term as well [16]. When considering whether the reviewed papers provided definitions of the term megafauna and how such definitions were justified, strikingly, 74% of the identified articles did not provide an explicit definition of megafauna. Among the remaining 26% (i.e. the 71 articles using a definition), 45% did not provide any argument or reference to support the definition, whereas 25% provided references, 20% specified distinct arguments and 10% offered both references and arguments (figure 2). Definitions, when provided, were somewhat idiosyncratic (i.e. varied according to the study system) and relied on ad hoc size-related criteria (see electronic supplementary material, table S1 and figure 1; for a complete list of definitions, see electronic supplementary material, table S2). Definitions of the megafauna concept were primarily of two types. The first group used an explicit, albeit generally arbitrary, body-size threshold above which a species is considered megafauna. Among the definitions of this group, a distinction can be made between those that used a massbased threshold and those that used a length-based threshold. On the one hand, mass thresholds ranging from around 10 kg to 2 tons have been widely used in a terrestrial context to define megafauna [5]. Palaeontologists, for example, have often referred to the megafauna definition provided by Martin [4]: i.e. animals, usually mammals, over 100 pounds (ca 45 kg; e.g. [17–20]). Recently, this megafauna definition has also been applied to marine environments [21], and several authors have adopted a slightly lower threshold (30 kg) to define freshwater megafauna [14,22]. Some terrestrial megafauna studies (e.g. [23]) are based on the megaherbivore concept of Owen-Smith [24,25], restricted to herbivores exceeding 1000 kg in adult body mass according to distinctions from smaller herbivores in a number of ecological features. Other authors have applied guild-dependent thresholds for terrestrial megafauna (e.g. greater than or equal to 100 kg for herbivores and greater than or equal to 15 kg for carnivores) [13]. Finally, Hansen and Galetti [26] emphasized the importance of taking into account the ecological context too: ‘one ecosystem’s mesofauna is another ecosystem’s megafauna’. This means that relatively small species can also be considered megafauna, as long as they are, or were, among the largest species occurring in a given area. On the other hand, papers in which the megafauna definition relies on body length are characterized by much smaller size thresholds. These studies have been common in the context of benthic and epibenthic environments, where Proc. R. Soc. B 287: 20192643 1. Introduction 2. Literature review royalsocietypublishing.org/journal/rspb palaeontologists to assess the species traits used to identify and define megafauna. Our review indicates that definitions are highly dependent on the study ecosystem and research question, and primarily rely on ad hoc size-related criteria. Our survey suggests that body size is crucial, but not necessarily sufficient, for addressing the different applications of the term megafauna. Thus, after discussing the pros and cons of existing definitions, we propose an additional approach by defining two function-oriented megafaunal concepts: ‘keystone megafauna’ and ‘functional megafauna’, with its variant ‘apex megafauna’. Assessing megafauna from a functional perspective could challenge the perception that there may not be a unifying definition of megafauna that can be applied to all eco-evolutionary narratives. In addition, using functional definitions of megafauna could be especially conducive to cross-disciplinary understanding and cooperation, improvement of conservation policy and practice, and strengthening of public perception. As megafaunal research advances, we encourage scientists to unambiguously define how they use the term ‘megafauna’ and to present the logic underpinning their definition. 3 terrestrial prehistorical royalsocietypublishing.org/journal/rspb length-based historical soil marine, benthic freshwater marine, pelagic mass-based Proc. R. Soc. B 287: 20192643 Figure 1. A representation of several examples of megafauna according to explicit-size-based-threshold definitions that are commonly found in the scientific literature (see electronic supplementary material, table S1). Mass-based definitions are typically used in vertebrate studies in terrestrial, pelagic marine and freshwater ecosystems, while length-based definitions are typically used in invertebrate studies in benthic marine and soil ecosystems. A list of the species represented and photograph credits is provided in the electronic supplementary material, appendix S2. (Online version in colour.) marine megafauna are usually defined as animals visible on seabed photographs (normally over ca 1 cm) or caught by trawl nets (e.g. [3,27–29]). Furthermore, soil ecologists have used the term megafauna to encompass those species above 20 mm in length that exert strong influences on gross soil structure [16]. The second major group of papers included those that relied on body size only implicitly—i.e. considering megafauna as certain clades or groups of species that are relatively large-sized within the focal study system. These articles normally concerned aquatic environments. Several studies of marine benthic megafauna focused on particular taxonomic groups, such as decapods and fish [30,31]. In a marine pelagic context, some authors focused on the largest sea-dwelling species—i.e. marine mammals, sea turtles and seabirds (termed ‘air-breathing marine megafauna’) [32], along with sharks, rays and other predatory fish (e.g. [33–35]) and even polar bears and cephalopods [36]. In terrestrial marine freshwater (1; 0.4%) definition: no definition: yes (citation: yes; arguments: yes) definition: yes (citation: yes; arguments: no) definition: yes (citation: no; arguments: yes) definition: yes (citation: no; arguments: no) Figure 2. Number of megafauna publications according to ecosystem (terrestrial, marine and freshwater) and period (historical and prehistorical). For each pathway, we indicate in parentheses the number and percentage of the total reviewed articles (n = 276) that provide a definition of megafauna and those that do not provide any definition; in the former case, we indicate if the definition is supported by citations, arguments, both or none. Line width is proportional to the number of studies. When an article referred to more than one ecosystem and/or period—6% of cases—we depicted as many lines as needed. Note that some ‘terrestrial’ studies do not explain in detail the species considered and may include also freshwater-dwelling species. Only articles with the term ‘megafauna’ in the title were considered for this purpose. (Online version in colour.) freshwater ecosystems, crustaceans, amphibians and fish were classified as megafauna by some authors [37]. Other work has focused on particular functional groups, such as higher/apex marine predators [34,36]. It is noteworthy that the term megafauna has been virtually ignored for dinosaurs and, until recently, barely used for mammals other than those of the Late Pleistocene period. Instead, dinosaur experts and wildlife biologists prefer using the species, clade or group name rather than the more general term megafauna (e.g. [38–41]). (b) Terminology associated with megafauna research As demonstrated above, the megafauna definition may differ according to the studied ecosystem. In this section, we highlight the fact that definitions also differ depending on the ecological and biological questions of the study. To this end, we created semantic networks based on the terms included in the title and abstract of the 276 reviewed articles, and identified thematic clusters based on co-occurrence of these terms (see electronic supplementary material, appendix S4 for methodological details). From this, we obtained three major megafauna research clusters (electronic supplementary material, figures S1 and S2). The first cluster included articles on terrestrial megafauna and mainly corresponded to the study of the extinction of Pleistocene megafauna: its timing, causes and impacts on ecosystems (e.g. [17,42,43]). The terms included in this terrestrial cluster were related to the megafauna definitions provided by Owen-Smith [24] and, 3. Survey of researchers Given that the majority of the papers using the concept megafauna do not provide a definition of this term, we surveyed researchers working on megafauna to get a better understanding of how they understand the concept when using it. (a) Species traits associated with megafauna To understand the species traits (i.e. taxonomy, biology, ecology, behaviour, conservation status and popularity; see electronic supplementary material, tables S3 and S4 for more details) that researchers associated with megafauna, we asked ecologists and palaeontologists (n = 93 respondents) to fill in a questionnaire that included photos of 120 animal species (electronic supplementary material, table S3). In the questionnaire, respondents had to specify which species they considered as megafauna. Then we ranked species traits according to their capacity to predict the probability that the respondents would classify these species as megafauna (see electronic supplementary material, appendix S4 and tables S3–S5 for methodological details). We found that adult body mass was by far the most important trait, followed by the taxonomic group; all other traits analysed were of minor importance (electronic supplementary material, figure S3a). According to a generalized linear model (GLM), body mass and taxonomic group accurately predicted the probability that a species would be classified as megafauna (F15,104 = 72.79, p < 0.001, R 2 = 0.90). Larger species were more likely to be considered as megafauna, following a sigmoidal (logistic) relationship (figure 3a). However, the slope of this relationship varied among 4 Proc. R. Soc. B 287: 20192643 (124; 44.9%) (6; 2.2%) prehistorical (3; 1.1%) mostly, by Martin [4]. The second cluster concerned extant benthic and epibenthic marine megafauna: the characterization of their communities [44–46], the environmental factors that determine their composition [47–49] and their ecological properties [9,30]. In general, the terms of this cluster were linked to definitions not specifying a body-size threshold [3,32]. The third cluster covered studies on the impacts of bycatch in fisheries, mainly on marine airbreathing vertebrates [12,32,50], as well as on strategies for their conservation [51,52]. These clusters were not totally disconnected, as electronic supplementary material, figure S2 reveals several bridging terms that have the potential to link different clusters in the network [53]. For example, terrestrial and pelagic clusters were recently connected by research on the conservation of threatened vertebrates in relation to global change [54–57]. In this case, important bridging terms were impact, climate and review (electronic supplementary material, figure S2). Similarly, benthic and pelagic clusters were interlinked by research on biodiversity conservation in marine environments [58], with biodiversity, use and fish being bridging terms (electronic supplementary material, figure S2). Thus, our lexical analysis revealed a growing, albeit still weak, tendency to connect the different conceptual clusters that make up the main megafauna research network. Our findings indicate that the increasing concern about the causes and consequences of human impacts on the conservation of large animals has a promising potential to foster collaboration among researchers focusing on different ecosystems (e.g. [59]). royalsocietypublishing.org/journal/rspb historical (28; 10.1%) (137; 49.6%) (a) 1.0 megafauna-yes 0.6 0.4 0.2 invertebrates vertebrates 0 (b) 1.0 100 10 000 1 × 106 mammals birds reptiles amphibians fishes freshwater invertebrates terrestrial invertebrates marine invertebrates 0.8 megafauna-yes 10 0.6 0.4 0.2 invertebrates vertebrates 0 1 10 100 10 000 1 × 106 body mass (g) Figure 3. Relationship between species body mass and the proportion of respondents to the questionnaire that classified the showed species as megafauna, either for the whole set of species (a) or broken down by taxonomic group (b). Solid lines represent the fitted values of the model including only body mass as predictor (for (a): F1,118 = 510.3, p < 0.001; R 2 = 0.81). According to a regression tree analysis (see electronic supplementary material, appendix S4), the species included in the questionnaires with body mass greater than or equal to 61 kg (vertical dotted line) had the highest probability of being classified as megafauna ( probability greater than or equal to 0.69; horizontal dotted line). (Online version in colour.) taxonomic groups, as reflected by the significance of the interaction coefficient (F7,104 = 4.13, p < 0.001; figure 3b). Mammals, birds and reptiles had steeper slopes, fish species had intermediate values, and amphibians and invertebrates exhibited shallower slopes (figure 3b). Thus, for a given body mass, the classification of a species as megafauna depended on its taxonomy, likely reflecting a bias arising from the prominence of terrestrial vertebrate species in scientific research or the general (average) size of the species in the different groups. These patterns were consistent despite variability in respondents’ characteristics such as age and expertize (see electronic supplementary material, appendix S4 and figures S3b and S4). (b) What criteria should define megafauna? We also used the questionnaire to assess researchers’ recommendations for defining megafauna. We explicitly asked the respondents to choose among six criteria needed to define megafauna: body mass, taxonomy, ecological function, ecological context, life-history traits and extinction risk. 4. Rethinking the megafauna concept As evidenced in the literature, the term megafauna has been widely applied in ecological and palaeontological research. However, our literature review revealed that researchers have been adopting a context-dependent use of the term, most often using operational definitions with varying and largely arbitrary body-size thresholds and taxonomic groups as proxies, depending on the study system and research question. Only a few studies have explicitly emphasized the functional importance of the largest species in a given ecosystem and over a specific period [16,24,26]. In addition, our survey of researchers provided consensus that body size (e.g. body mass) is a crucial descriptor, but not necessarily sufficient, for addressing the different applications of the term megafauna. When rethinking the megafauna concept, the primary question that should arise is whether we need a threshold. As argued next, there are reasons that justify the search for non-arbitrary thresholds and that indicate that these are, in fact, achievable, at least in some cases. First, avoiding a threshold-based definition would make the use of the megafauna term largely impractical. Second, clear breakpoints in either body size or ecological features have been identified for some animal groups (see below). Thus, a follow-up agenda exploring whether corresponding thresholds do, or do not exist in different groups of organisms is needed. Below, we reconsider the megafauna concept and propose a general working scheme for its use in various ecological and evolutionary contexts. These include either natural systems (i.e. before Homo sapiens began to defaunate them [26]) or systems that have been impacted by human-mediated extinctions and introductions of wild and domestic species [60]. (a) The largest The central challenge in using a threshold concept to define megafauna—as is also the case for other popular ecological terms such as keystone, flagship or umbrella species (see Proc. R. Soc. B 287: 20192643 1 5 royalsocietypublishing.org/journal/rspb 0.8 Respondents could choose as many of them as they wanted and could also name additional criteria (see electronic supplementary material, appendix S4 for methodological details). Among the criteria provided, 92% of respondents identified body mass as the key criterion (electronic supplementary material, figure S5). However, body mass was very often (86% of respondents) chosen in combination with other criteria (mean total number ± s.d. of criteria selected by respondents: 2.9 ± 1.3). This suggests that body size alone is insufficient for defining megafauna. Extinction risk was rarely taken into account in defining megafauna, probably because respondents identified this criterion as a circular and extrinsic argument or because it cannot be applied to extinct taxa, which frequently contributed to megafauna research. The selection of criteria was again barely affected by respondents’ characteristics (see electronic supplementary material, table S6, figures S6 and S7). Only 7% of the respondents suggested alternative criteria to define megafauna. These additional suggestions (namely species’ volume, habitat requirements, ‘importance’ within the food web, ecological ‘status’, ecosystem and temporal context) were closely related to the six criteria already provided in the questionnaires. We refer to operational definitions as those using specific body size criteria but that are not based on a body size distribution, namely most definitions enumerated in the electronic supplementary material, tables S1 and S2. A prominent example is Martin’s definition of megafauna (ca 45 kg [4]), which can be seen as a human-centred perspective, partitioning animals similar or larger in size than humans from those smaller. These definitions have been the core of the megafauna scientific literature, most likely because of their obvious practical advantages. For instance, they facilitate data processing and analysis, and they may normally apply to both extant and extinct species. The main feature of operational definitions is their strong dependence on the research discipline, which makes them highly applicable to conduct comparisons within disciplines but strongly limits their trans-disciplinary use. However, some attempts have recently been made to move certain operational definitions beyond the original research context. In particular, the application or adaptation of Martin’s megafauna standard [4] to aquatic environments [14,21,22] represents a connection among terrestrial, marine pelagic and freshwater megafauna research. In addition, soil and (c) Functional definitions: looking for a new approach While some existing definitions go beyond body size (e.g. [16,26]), we largely lack a conceptual definition of megafauna that integrates the ecological function and functional traits of a species along with its size (e.g. represented by body mass; but see [24]; figure 4). In this section, we present a functionoriented framework for the use of the megafauna concept, therefore, responding to the general perception of researchers that body size alone is an incomplete descriptor of megafauna (see above). Here, unlike previous definitions, which were primarily based on body size, breakpoints are associated with biological and ecological features/qualities that vary with body size. These functional concepts can be applied to different communities and ecosystems, from terrestrial and soil to marine and freshwater systems, and are, at least a priori, not biased towards vertebrates or invertebrates. The first concept, which combines a body-size based megafauna definition with the keystone species concept [69], assumes that the largest species in an ecosystem generally have disproportionally large effects on the structure and functioning of their communities and ecosystems, both in magnitude and in the spatial and temporal heterogeneity they create [70]. In line with this concept, a disproportionate increase in energy use (e.g. represented by population biomass) in relation to body mass increases has been identified in many vertebrate [24,63] and invertebrate phylogenetic groups [64]. Accordingly, ‘keystone megafauna’ would be the subset of animals among the largest in size that have consistently strong effects on the structure or functioning of a community or an ecosystem. Smaller animals would exhibit high variation in relation to the effects that they exert on their ecosystems, from very weak to very strong (figure 4a). All species that have a strong influence on their ecosystems, in general, stronger than expected by their abundance or biomass, may be regarded as keystone species [61,66–69], but only those with relatively large body size should be termed as keystone megafauna (figure 4b). In practice, this concept of megafauna may require extensive ecological knowledge of the biotic communities and their functioning [66], which would encourage a research agenda to better understand the ecological roles of large species [61,66]. However, the 6 Proc. R. Soc. B 287: 20192643 (b) Operational definitions marine benthos megafauna research, which is concerned with communities characterized by relatively small-sized species, may be closely linked because they use similar— body length-based—definitions. However, a weak connection between terrestrial/marine pelagic/freshwater and soil/ benthos megafauna research is anticipated due to their very different conceptions of ‘mega’ (figure 1). Nevertheless, while operational definitions could seem conducive to multidisciplinary coordination and collaboration in megafauna research (e.g. to undertake biodiversity inventories and conservation status assessments), the application of operational thresholds to different disciplines relies on the unrealistic assumption that body mass (and functional traits; see below) distributions are comparable among different communities or ecosystems. Thus, operational definitions, which are inherently arbitrary, are at risk of including or ignoring species that respectively should or should not be considered as megafauna, in both intra- and cross-disciplinary approaches. royalsocietypublishing.org/journal/rspb [61])—is how to empirically establish a metric (e.g. body mass, or body length) and a corresponding value above which an animal may be effectively regarded as megafauna. This value needs to be placed within a community or an ecosystem context to make any sense. We could circumvent this threshold concept by simply defining ‘megafauna’ as the subset of largest species in a community or an ecosystem. To answer the critical question of what the threshold should be, we could follow two approaches. In its simplest form, we could refer to the single largest species. Going beyond this, a transparent definition of ‘subset’ requires exploring the frequency distributions of body size (e.g. body mass) values within the community or ecosystem under study, and determining a breakpoint in body size. Although body size data are not available for all animal species within an ecosystem, this information is often biased towards larger species [62]. Another approach would be to focus on particular clades or guilds to restrict the species pool under consideration, facilitating the identification of megafauna. Thus, ‘clade- or guild-specific megafauna’ would be the subset of largest species of a given clade or guild in a community or an ecosystem. This implies acknowledging that the megafauna within a clade or guild do not necessarily include the largest species in the ecosystem. Within phylogenetic lineages, body mass is skewed towards smaller sizes, with larger species being almost invariably rarer than smaller species [24,63,64]. For instance, greater than 90% of sub-Saharan vertebrate herbivore species weigh less than 500 kg, while only ca 5% of species has a body mass exceeding 1000 kg [24]. However, most animals, with the exceptions of birds and mammals, grow through prolonged ontogenetic stages. For instance, giant bluefin tuna (Thunnus thynnus) covers 5–6 orders of magnitude in mass from larvae to adult [65]. Whether scales of ontogenetic change cause taxa with long developmental changes in size to have a shallower slope than in cases where the break might be more obvious needs to be investigated. (a) high effect small medium size keystone and megafauna medium high medium keystone low low mean s.d. max.–min. small medium size large large Figure 4. A general, conceptual definition of megafauna based on body size and its coupling to the effect of the species population on ecosystems. (a) The largest animals exert strong, consistently high impacts on local ecosystems. By contrast, the effect of small animals on local ecosystems is highly variable, with different species having low or high effects. The empirical challenge is to identify the shape of the size–effect relationship. (b) Qualitative distribution of animal species in the two-dimensional space defined by body size and ecosystem effects. Animals exerting high effects are defined as keystone species [61,66–68], but only the largest keystone species are considered as megafauna. Note that large animals exerting low/medium effects are rare. (Online version in colour.) use of proxies for ecological effects, such as size-density relationships [63], could greatly simplify the identification of keystone megafauna within different clades or guilds, including extinct fauna. Comparing the magnitude, variability and skewness, as well as related breakpoints, of these relationships (figure 4a for a general formulation) among different animal groups seems an exciting avenue for future megafauna research. The second functional concept for megafauna is referred to as ‘functional megafauna’, which can be defined as the subset of largest species of a given clade or guild that have distinctive functional traits (sensu [71]). An important practical advantage of this concept is that the identification of megafauna could be relatively easily accomplished because it only needs a basic ecological knowledge. Ideally, studies should focus on traits with high interspecific variation, that may be easily measurable and, therefore, comparable among the members of a given animal group. For instance, within terrestrial mammals, megaherbivores differ from smaller herbivores in almost all ecological and life-history aspects (e.g. age at first conception, birth interval and gestation time [24]). Also in terrestrial mammals, there is a functional transition associated with a number of life-history traits between carnivores exceeding an average mass of 13–16 kg and those carnivores of smaller size [72]. In other, less-studied cases, the key question is, of course, to define the subset of functional traits to be explored. A feasible variant of the functional megafauna concept would be ‘apex megafauna’: animals so large that they have escaped most non-anthropogenic predation as adults. This concept is related to the megaherbivore and apex predator concepts [24,25,72] and can be applied to humans too. In Africa, herbivores larger than 150 kg are subject to reduced predation rates than smaller mammalian prey in some areas [73], but only for herbivores exceeding 1000 kg predation is a consistently negligible cause of adult mortality [24,73,74]. Within the order Carnivora, an average mass of ca 15 kg corresponds to the transition between extrinsic- and self-regulation [72]. 5. Conclusion Our comprehensive literature review and survey of researchers point to a dichotomy between the need to establish operational body-size thresholds and a more functional definition of megafauna. This confirms that the concept of megafauna is far from simple, and, probably, it should not be simplified either. However, we highlight that assessing megafauna from a functional perspective could challenge the perception that there may not be a unifying definition of megafauna that can be applied to all eco-evolutionary contexts and scientific approaches. The functional framework we present, which arises from the perception of megafauna researchers that body size is insufficient to capture the varied eco-evolutionary ramifications of megafauna, could help to reach ecological generality and to minimize the arbitrariness of operational and other non-functional definitions, which present ambiguity problems even at the withindiscipline level. This requires exploring thresholds in ecological functions and functional traits of animals pertaining to different clades, guilds, communities and ecosystems. Addressing this challenge could help to broaden out megafauna research, and provides an opportunity to increase our biological understanding of megafauna too. Interestingly, important advances have already been made in terrestrial mammalian systems, so that herbivores exceeding 1000 kg and carnivores above an average body mass of ca 15 kg could be considered as paradigmatic examples of both functional and apex megafauna. Until studies exploring other animal groups and ecosystems are available, we encourage scientists to define megafauna unambiguously and clearly present the distinct logic behind their definition in every megafaunal study. Only by being explicit and appropriately contextualizing the concept will we be able to reach the needed conceptual disambiguation. We found that cross-disciplinary investigations of megafauna are virtually non-existent (but see e.g. [59]), which may be due, in part, to the fact that most megafauna definitions in the scientific literature are strongly context-dependent. Proc. R. Soc. B 287: 20192643 keystone royalsocietypublishing.org/journal/rspb effect 7 (b) Data accessibility. Data and code to replicate analyses are available from the Dryad Digital Repository: https://dx.doi.org/10.5061/dryad. dv41ns1v3 [77]. and designed the study; M.M. undertook the literature review and collected data; M.M. and Z.M.-R. created the databases; M.M., C.G.-C. and B.M.-L. conducted the semantic and statistical analyses, with critical inputs from all co-authors; M.M. drafted the manuscript; all authors participated in discussions, contributed critically to data interpretation and manuscript reviewing and gave final approval for publication. Competing interests. We declare we have no competing interests. Funding. This article was inspired by the workshop ‘Megafauna: from human–wildlife conflicts to ecosystem services’ held in 2016 and jointly funded by the Leibniz-Institute of Freshwater Ecology and Acknowledgements. We thank the Estación Biológica de Doñana (EBDCSIC, Spain) for the facilities and support during the workshop ‘Megafauna: from human–wildlife conflicts to ecosystem services’ held in 2016. The thought-provoking revisions made by M. Authier and an anonymous reviewer greatly improved the original version. References 1. 2. 3. 4. 5. 6. 7. 8. Gunn RG, Douglas LC, Whear RL. 2011 What bird is that? Identifying a probable painting of Genyornis newtoni in western Arnhem Land. Aust. Archaeol. 73, 1–12. Wallace AR. 1876 The geographical distribution of animals. New York, NY: Harper. Grassle JF, Sanders HL, Hessler RR, Rowe GT, McLellan T. 1975 Pattern and zonation: a study of the bathyal megafauna using the research submersible Alvin. Deep-Sea Res. 22, 457–481. (doi:10.1016/0011-7471(75)90020-0) Martin PS. 1967 Prehistoric overkill. In Pleistocene extinctions: the search for a cause (eds PS Martin, HE Wright), pp. 75–120. New Haven, CT: Yale University Press. 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M.M. acknowledges financial support through the Severo Ochoa Program for Centres of Excellence in R+D+I (SEV-2012-0262) and by a research contract Ramón y Cajal from the MINECO (RYC-2015-19231). C.G.-C. is supported by a ‘Juan de la Cierva’ research contract (MINECO, FJCI-2015-25785), and Z.M.-R. by a postdoctoral contract co-funded by the Generalitat Valenciana and the European Social Fund (APOSTD/2019/016). M.G. thanks to Programa BIOTA from Fundação de Amparo à Pesquisa do Estado de São Paulo (2014/01986-0) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). F.H. acknowledges the Erasmus Mundus Joint Doctorate Program SMART (Science for MAnagement of Rivers and their Tidal systems) funded by the European Union. N.S. was supported by the National Science Center in Poland (DEC-2013/08/M/NZ9/00469). J.-C.S. considers this work a contribution to his Carlsberg Foundation Semper Ardens project MegaPast2Future (CF16-0005) and to his VILLUM Investigator project (VILLUM FONDEN, grant 16549). S.C.J. acknowledges funding from the German Federal Ministry of Education and Research (BMBF) for ‘GLANCE’ (Global change effects in river ecosystems; 01LN1320A) N.G. acknowledges financial support through the project PGC2018-093925-B-C33. royalsocietypublishing.org/journal/rspb The existence of recurrent topics among megafauna researchers concerned with different animal taxa and ecosystems, such as the conservation of threatened megafauna, compels the search for unifying tools. 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Supplementary material for: Rethinking megafauna Proc. R. Soc. B doi: 10.1098/rspb.2019.2643 Marcos Moleón, José A. Sánchez-Zapata, José A. Donázar, Eloy Revilla, Berta Martín-López, Cayetano Gutiérrez-Cánovas, Wayne M. Getz, Zebensui Morales-Reyes, Ahimsa Campos-Arceiz, Larry B. Crowder, Mauro Galetti, Manuela González-Suárez, Fengzhi He, Pedro Jordano, Rebecca Lewison, Robin Naidoo, Norman Owen-Smith, Nuria Selva, Jens-Christian Svenning, José L. Tella, Christiane Zarfl, Sonja C. Jähnig, Matt W. Hayward, Søren Faurby, Nuria García, Anthony D. Barnosky and Klement Tockner This file includes: Appendix S1. Etymology and popular definition of megafauna. Appendix S2. Species and photograph credits of Figure 1. Appendix S3. References reviewed. Appendix S4. Methods. Table S1. The disparity of megafauna definitions, according to the general criteria used to define megafauna, the ecosystem in which the definition is normally applied, and the broad taxonomy of the species included as megafauna. Table S2. Definitions of megafauna found in the reviewed scientific literature, according to the studied ecosystem. Table S3. List of species included in the questionnaires. Table S4. Description of the characteristics of the species included in the questionnaires. Table S5. Description of the variables used to characterize the questionnaires’ respondents. Table S6. Results of the GLM relating respondents’ age, expertise on mammals and ecosystem of expertise with the criteria used by respondents to classify species as megafauna and previous definitions of megafauna. Fig. S1. Number of articles on megafauna published per year, according to ecosystem, period and clusters defined by the semantic network analysis. Fig. S2. The semantic network of the most relevant terms extracted from the megafauna literature. Fig. S3. Relative importance of variables included in the selected models. Fig. S4. Relationship between the respondents characteristics and megafauna classification probability. Fig. S5. Respondents’ preferences regarding the criteria to define megafauna and three commonly used definitions. Fig. S6. Relationship between respondents’ expertise on mammals and their propensity to consider taxonomy as a criterion to be taken into account when defining megafauna. Fig. S7. Relationship between respondents’ age, ecosystem of expertise and expertise on mammals and their propensity to consider extinction risk as a criterion to be taken into account when defining megafauna. Appendix S1. Etymology and popular definition of megafauna. According to the Oxford dictionary (https://en.oxforddictionaries.com/), megafauna, a term resulting from combining “mega” (from the Greek “megalos”, which means large, or denoting a factor of 106 or, in computing grounds, 220) and “fauna” (from the ancient Rome nature-goddess Fauna), are either “the large mammals of a particular region, habitat, or geological period” or “animals that are large enough to be seen with the naked eye”. As emphasized in this review, the ambiguity and disparity of these popular definitions is also reflected in the scientific literature. In fact, vague terminology such as “large animals”, “large terrestrial/marine animals”, “large-bodied animals/mammals”, “mega-mammals/herbivores/vertebrates”, “beasts”, “big/biggest beasts” “giants”, “giant mammals” and “large-gigantic vertebrates” is common in the scientific literature. Appendix S2. Species and photograph credits of Figure 1. From left to right, top to bottom: proboscidean (extinct; N. García), Mammuthus sp. (extinct; A. Campos-Arceiz), Ursus deningeri (extinct; N. García), Megantereon whitei (extinct; E. Revilla), Loxodonta africana (W.M. Getz), Ceratotherium simum (F.D. Carmona-López), Giraffa camelopardalis (S. Justicia-Carmona), Ursus arctos (A. Wajrak), Bison bonasus (A. Wajrak), Megaptera novaeangliae (A. Wajrak), Carcharhinus amblyrhynchos (A. Ibáñez-Yuste), Chelonias mydas (A. Ibáñez-Yuste), Larus michahellis (S. Eguía), Urogymnus polylepis (Z. Hogan), Crocodylus niloticus (F.D. Carmona-López), Hippopotamus amphibius, (F.D. Carmona-López), Fromia nodosa (A. Ibáñez-Yuste), Clavelina dellavallei (A. Ibáñez-Yuste), Dardanus calidus (A. Ibáñez-Yuste), Hermocide carunculata (A. Ibáñez-Yuste), Pseudoceros ferrugineus (A. Ibáñez-Yuste), Lumbricus sp. (J.M. Barea-Azcón), Alphasida sp. (F. Sánchez-Piñero), Scolopendra sp. (J.M. Barea-Azcón), Lycosa tarantula (S. Justicia-Carmona). Appendix S3. References reviewed. 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Following guidelines provided by Haddaway et al. (2015), we used the Web of Science to search for publications (appearing prior to October 2016) with the term “megafauna” in the title, thus obtaining a first output of 374 papers. We further restricted our search to scientific papers written in English and that we could fully access, thereby obtaining 276 articles (see Appendix S3 for a complete list of reviewed references). We read each article to extract the following information. First, we divided the papers in two main groups: those that provide an explicit definition of megafauna and those that do not provide such a definition. For the former, we noted if authors supported their definition with citations, arguments or both; or if definitions were not supported by any citation or argument. Also, we recorded if definitions included these criteria: animal body size, taxonomy, region, ecological function, and life history traits. Second, we recorded the period (prehistorical and historical), ecosystem (terrestrial, marine, freshwater and soil) and taxonomic group (vertebrate and invertebrate) referred in the articles. By “prehistoric megafauna” we mean large animals that are now extinct and are known only through the fossil record (Koch and Barnosky 2006), as opposed to currently extant megafauna or historically extinct megafauna which are known through written records. Terminology associated with megafauna research. We used VOSviewer (http://www.vosviewer .com/), a freeware for creating and visualizing semantic networks or term maps of scientific literature (van Eck and Waltman 2011, van Eck et al. 2013). The software applies text mining in the abstract and title of the selected papers and clustering functions to analyze co-occurrence of terms (van Eck and Waltman 2010). In a first selection, we obtained 8,251 terms. From these terms, we focused on the terms (n=123) that co-occurred in the title or abstract of more than 10 articles. Then, we removed general terms that randomly co-occurred in articles, such as ‘data’, ‘analysis’, ‘study’ or ‘result’. We also removed the term ‘megafauna’ when it appeared alone without any adjective (e.g. benthic megafauna, marine megafauna or Pleistocene megafauna) to avoid any bias towards this term that could hide emergent patterns in megafauna related research. The final subset of terms (n=71) was used to build a co-occurrence network that revealed three major megafauna research clusters (Figs. S1 and S2). Survey of researchers Sampling procedure. We designed a questionnaire to investigate the megafauna concept among researchers. The questionnaire was divided in three parts: Part 1 was devoted to researchers’ personal and expertise data; Part 2 showed pictures of 120 species (15 mammals, 15 birds, 15 reptiles, 15 amphibians, 15 fishes, 15 terrestrial invertebrates, 15 marine invertebrates and 15 freshwater invertebrates, spanning the whole body size range of each group; habitat refers to the main habitat during the whole life, being also the main habitat for adults); and Part 3 formulated questions regarding different definitions of megafauna. We used foreground color pictures of animals without human-related scale references. All photographs avoided backlighting, represented the entire body of the animal, had less than 10% visible sky and pictured adult individuals. The complete list of species included in the questionnaire is shown in Table S4. Among the list of pictures, we asked respondents to indicate the species that fell into their concept of megafauna. Other questions included are described in Tables S5 and S6 and below. We selected the respondents in several steps following Sutherland et al. (2013) and Hays et al. (2016). First, we identified leading experts in the ecology and conservation of megafauna (particularly, human-wildlife conflicts and ecosystem services associated with megafauna), as well as in paleontology (mainly, vertebrate paleontology and paleoecology), based on their publication record and extent of work in these fields. These experts were invited to participate in a workshop organized in the Doñana Biological Station-CSIC (EBD; Seville, Spain, November 9-11 2016; http://www.ebd.csic.es/web/megafauna-workshop/home) to discuss on megafauna and their benefits and detriments to humans. Before the meeting, experts were asked to fill in the questionnaire on-line. Second, the same questionnaires were distributed to researchers who attended the first workshop session, which was open to all EBD researchers. These EBD researchers were asked to fill in the questionnaires at the beginning of the session, i.e. before hearing invited researchers and participating in discussions. Third, experts attending the workshop (n=20) were asked to contact and distribute the questionnaires to other researchers of their fields with the aim of obtaining a minimum of ten researchers belonging to the 12 broad fields resulted from all combinations of ecosystem of expertise (terrestrial, marine and freshwater), period of expertise (prehistorical and historical) and taxonomic expertise (vertebrates and invertebrates). We obtained a total of 93 questionnaires (note that one researcher can be expert in more than one combination of ecosystem, period and taxonomic expertise). Species traits associated with megafauna. To represent species traits and other relevant characteristics that may determine the probability of a species to be classified as megafauna, we compiled information on the taxonomy, biology, ecology, behavior, conservation status and popularity among the general public of the species included in the questionnaires (see Table S5 for details on the traits and the references used). Also, we recorded respondents’ characteristics that might have an influence on the questionnaires’ results (personal details, and research experience and expertise; see Table S6 for details). We used Boosted Regression Trees (BRTs; Elith et al. 2008) to rank species characteristics in relation with their capacity to predict the probability of each species to be classified as megafauna. Then, we built a Generalized Linear Model (GLM) with a Gaussian distribution error relating the probability of each species to be classified as megafauna with body mass, taxonomic group and their interaction. In this analysis, we applied a logit-transformation to the probability of each species to be classified as megafauna. Additionally, we conducted a regression tree analysis (Strobl et al. 2009) to identify the body mass threshold for which species have the highest probability of being classified as megafauna. We also checked if respondents’ features might influence their probability of classifying a species as megafauna. First, we also used BRTs and linear models to relate respondents’ features to the number of species classified as megafauna by each researcher (0 to 120). Then, we used GLMs with Gaussian error distribution to test if the most important respondent features identified by BRTs (age, expertise on mammals and ecosystem of expertise; respondent experience was discarded here and in further analysis because its close relationship with respondent age) had a significant effect on the number of species classified as megafauna. Post-hoc tests were performed using the max-t test robust against residual departures from normality and homoscedasticity (Herberich et al. 2010). Second, we used a manyGLM model (model-based analysis of multivariate data; Wang et al. 2012) with a binomial distribution error to explore if respondents’ characteristics influenced the probability of a given species to be classified as megafauna. For this, we used the most influential variables identified by BRTs as predictors. The results on species traits were in general consistent over the variation of the respondents’ characteristics tested (age, expertise in mammals, and ecosystem of expertise; Fig. S2b). However, older respondents (GLM, age slope=-0.388, P=0.023, R2=0.06) and experts in mammals (ANOVA test, F1,79=4.27, P=0.042, R2=0.05) were slightly more conservative; i.e. they classified a lower percentage of species as megafauna (Fig. S4). The ecosystem of expertise also had a significant effect on the percentage of species classified as megafauna (ANOVA test, F3,77=2.86, P=0.042, R2=0.10), but post-hoc analyses showed no significant differences among categories. In addition, the many-GLM analysis showed that these respondents’ characteristics (age, expertise on mammals and ecosystem of expertise) did not influence the probability of an individual species to be classified as megafauna (many-GLM test statistic=18.6, P=0.152). What criteria should define megafauna? We asked whether respondents would include any of the following criteria (binary response, no/yes) within a definition of megafauna: body mass (i.e., megafauna species should be those above a given body mass threshold), taxonomy (i.e., the “megafauna threshold” should be defined within each taxonomic group, irrespective of the threshold defined for other groups), ecological function (i.e., the “megafauna threshold” should be defined within each functional group, irrespective of the threshold defined for other groups), ecological context (i.e., the definition should be context-dependent, according to the structure and species richness of the local natural community; thus, the same species could be defined as megafauna in one region of the world but not in other), life history traits (i.e., the “megafauna threshold” should be defined according to key life history traits such as lifespan, reproductive strategy and fecundity), and extinction risk (i.e., the “megafauna threshold” should be defined according to the risk of extinction; obviously, this criterion does not apply for extinct taxa). We discarded two questionnaires (2.2% of total questionnaires) because this part was incomplete or wrong. In addition, we included one open-ended question to identify other criteria that could be taking into account to define megafauna. By means of GLMs (binomial distribution errors), we analyzed the influence of respondents’ characteristics on their preferences for the given criteria to classify a species as megafauna. We focused on the three characteristics that were more important in determining the propensity to classify a given species as megafauna (see previous point and Fig. S2b): age, ecosystem of expertise and expertise on mammals. References Elith J., Leathwick J.R., Hastie T. (2008). A working guide to boosted regression trees. Journal of Animal Ecology 77, 802–813. Haddaway, N.R., Woodcock, P., Macura, B., Collins, A. (2015) Making literature reviews more reliable through application of lessons from systematic reviews. Conserv. Biol. 29, 1596-1605. Hays, G.C. et al. 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(2010) Text mining and visualization using VOSviewer. ISSI Newsletter 7, 50-54. van Eck, N.J., Waltman, L. (2011) Software survey: VOSviewer, a computer program for bibliometric mapping. Scientometrics 84, 523-538. van Eck, N.J., Waltman, L., van Raan, A.F.J., Klautz, R.J.M., Peul, W.C. (2013) Citation analysis may severely underestimate the impact of clinical research as compared to basic research. PLoS ONE 8, e62395. Wang, Y., Naumann, U., Wright, S.T., Warton, D.I. (2012) mvabund — an R package for model-based analysis of multivariate abundance data. Methods in Ecology and Evolution 3, 471–474.  Table S1. The disparity of megafauna definitions, according to the general criteria used to define megafauna, the ecosystem in which the definition is normally applied, and the broad taxonomy of the species included as megafauna. Criterion Definition Ecosystem Taxonomy Size (mass) Species over 100 pounds (c. 45 kg; e.g. Martin 1967) Terrestrial Vertebrates Species over 1,000 kg in adult body mass (Doughty et al. 2016), derived from the megaherbivore concept of Owen-Smith (Owen-Smith 1988, 2013) Terrestrial Vertebrates Species over 45 kg (Estes et al. 2016) Marine (pelagic) Vertebrates and invertebrates Species over 30 kg (He et al. 2017) Freshwater Vertebrates Size (length) Species over 20 mm in length that exert strong influences on gross soil structure (Coleman and Crossley 2004) Soil Vertebrates and invertebrates Species visible on seabed photographs (normally over c. 1 cm) or caught by trawl nets (e.g. Grassle et al. 1975, Smith and Hamilton 1983) Marine (benthic and epibenthic) Vertebrates and invertebrates Size (implicit) Particular taxonomic groups, such as marine mammals, sea turtles and seabirds (termed “air-breathing marine megafauna”; e.g. Lewison et al. 2014), as well as sharks, rays, other predatory fish (e.g. Sleeman et al. 2007) and even polar bears and cephalopods (Hooker and Gerber 2004) Marine (pelagic) Mainly vertebrates Particular taxonomic groups, such as decapods and fish (Cartes et al. 2010, Papiol et al. 2013) Marine (benthic and epibenthic) Vertebrates and invertebrates Particular taxonomic groups, such as crustaceans, amphibians and fish (e.g. Vilella et al. 2004) Freshwater Vertebrates and invertebrates Particular functional groups, such as higher/apex marine predators (McClellan et al. 2014, Hooker and Gerber 2004) Marine (pelagic) Mainly vertebrates Note: With the exception of Estes et al. (2016) and Coleman and Crossley (2004), all definitions come from papers obtained during our literature review. The complete references are shown in Appendix S3.  Table S2. Definitions of megafauna found in the reviewed scientific literature, according to the studied ecosystem (terrestrial, marine and freshwater). Definition Based on citation by Arguments Reference Terrestrial Land animals weighing more than 45 kg, along with a few smaller species, constituted the “megafauna” (none) no Roberts et al. 2001 Large (>5 kg) mammals (none) no Price and Webb 2007 We decided to restrict this analysis to five orders of (mostly) large bodied taxa (Artiodactyla, Carnivora, Primates, Perissodactyla and Proboscidea) (none) no Louys et al. 2007 Megafaunal (>45 kg) carnivores and herbivores (none) no Fox-Dobbs et al. 2008 Large mammals from distinct orders with body mass ≥1000 kg (none) no Guimarães et al. 2008 Megafauna are defined here as extinct species with body mass estimates of >30 kg or attaining estimates of ≥30% greater body mass than their closest living relatives (none) no Ayliffe et al. 2008 Megafauna (animals >40 kg) (none) no Turney et al. 2008 Weighing at least 44 kg (none) no Barnosky 2008 Body size limit of approximately 7 kg (none) no Pushkina and Raia 2008 The weight definition of megafauna is a little heavier (50 kg) than Martin’s (1984) >44 kg standard (none) no Webb 2008 Medium- and large-bodied mammals (> 1 kg, termed here ‘megafauna’) (none) no Louys and Meijaard 2010 Megafauna (defined as animals >44 kg) Barnosky et al. 2004 no Doughty et al. 2010 The concept of megafauna employed in this paper considers as belonging to this set, large animals over 44 kg Martin and Klein 1984, Barnosky et al. 2004, Barnosky 2008 no Ghilardi et al. 2011 Megafaunal species (terrestrial mammals weighing >44 kg) Barnosky et al. 2004 no Mann et al. 2013 The term megafauna refers to an arbitrary compilation of relatively large mammalian, reptilian, and avian taxa, ranging in size from ∼10 kg or less up to >2,000 kg Horton 1984, Wroe et al. 2004a, b, Fry et al. 2009 no Wroe et al. 2013b Body masses >44 kg Martin and Klein 1984, MacPhee 1999, Barnosky et al. 2004 no Turvey et al. 2013 Megafauna (prey over 150 kg) (none) no Bird et al. 2013 Large mammals (more than or equal to 10 kg) (none) no Sandom et al. 2014 Large-bodied mammals (>44 kg) (none) no Boulanger and Lyman 2014 Megamammals (over 1000 kg) and most large mammals (over 44 kg) (none) no Prado et al. 2015 Megafauna genera (animals weighing >45 kg) Barnosky et al. 2004 no Mann et al. 2015 Megafaunal (>45 kg) mammals (none) no Feranec et al. 2016 ‘Megafauna’ (that is, species > 44 kg) Koch and Barnosky 2006 no Johnson et al. 2016a Terrestrial megafaunal species (average body weight exceeding 44 kg) Koch and Barnosky 2006 no Villavicencio et al. 2016 Mammal megafauna (≥ 44 kg body mass) Sandom et al. 2014 no Doughty et al. 2016a Megaherbivores (herbivores ≥ 1 ton in body weight) Owen-Smith 2013 no Doughty et al. 2016b Large herbivores (≥45 kg in body weight) […] megaherbivores (≥1,000 kg) (none) no Bakker et al. 2016 Megafauna (animals with more than 44 kg body weight) Doughty et al. 2013, Wolf et al. 2013 no Gross 2016 ‘Megafauna’ genera (that is, large vertebrates with mature individuals >40 kg) Koch and Barnosky 2006 no Saltre et al. 2016 Terrestrial/marine We selected the largest species (>10 kg) (none) no McClenachan et al. 2016 Terrestrial/freshwater Megafauna (>50 kg) (none) no Webb 2009 Marine Megafauna (operationally defined as organisms readily visible in photographs) (none) yes Grassle et al. 1975 Megafauna were operationally defined as any organism large enough to be identified in a photograph. Animals less than 2 cm in diameter were visible, but could not be identified (none) yes Schneider et al. 1987 Operationally, megafauna may be defined as organisms large enough to be recognized in photographs (typically ≥1-2 cm) Grassle et al. 1975, Rex 1981, Ohta 1983, Smith and Hamilton 1983 yes Kaufmann et al. 1989 All organisms visible in each frame of film (none) yes Arquit 1990 Megafauna were operationally defined as organisms large enough to be identified. Animals less than 20 mm in diameter were visible, but could not be identified (none) yes Schneider and Haedrich 1991 Megafauna (i.e. cetaceans and other large organisms) (none) no Viale and Frontier 1994 The community fraction encompassing the benthic organisms which are large enough to be seen in bottom images and/or to be caught by trawls Gage and Tyler 1991 yes Piepenburg and Schmid 1996 Megafauna, in this context, are large marine vertebrates that can be surveyed from the air (none) yes Preen et al. 1997 Megafauna are animals […] large enough to be visible in photographs of the sea floor (> ca 1 cm) Grassle et al. 1975, Rice et al. 1982, Smith and Hamilton 1983, Smith et al. 1993, Lauerman et al. 1996 yes Kaufmann and Smith 1997 Megafauna (animals living on the sediment surface and large enough to be visible in photographs) (none) yes Lauerman et al. 1997 Megafauna (> 1 cm) (none) no Arango and Solano 1999 Megafauna are defined as animals visible in bottom photographs or ‘trawl-caught’ organisms more than 3 cm stretch-mesh Rowe 1983 and refs. therein yes Rodrigues et al. 2001 The grouping of higher marine predators describes ocean megafauna, including a variety of taxa: cetaceans, pinnipeds, sea otters, polar bears, seabirds, sharks, cephalopods, and predatory fish (none) no Hooker and Gerber 2004 Sensu Gage & Tyler 1991 Gage and Tyler 1991 no Echeverria et al. 2005 The usefulness of photogram metric methods in deep-sea research has led to megafauna being defined as those organisms large enough (typically > 1 cm) to be identified in photographs (none) yes Ruhl 2007 Visible megafauna (none) yes Braby et al. 2007 Megafauna (whales, dolphins, sharks, turtles, manta rays, dugongs) (none) no Sleeman et al. 2007 The megafauna’s component (for this study, animals retained in the OTMS’ codend: mesh size = 12 mm) (none) yes Ramírez-Llodra et al. 2008 All discernible organisms (none) yes Gonzalez-Mirelis et al. 2009 Megafauna, defined as large animals visible in bottom photographs or caught in trawl samples (none) yes Gooday et al. 2009 >4 mm (none) no Bluhm et al. 2009 Visible megafauna using a camera system (none) yes MacDonald et al. 2010 Decapod crustaceans (none) no Cartes et al. 2010 Megafauna, defined as individuals >10 mm Collie et al. 1997 no Ragnarsson and Burgos 2012 Megafauna (>1 cm) (none) no Sonnewald and Türkay 2012 Megafauna includes those organisms over 1 cm that inhabit the sediment-water interface Grassle et al. 1975 no Gates and Jones 2012 Megafauna (fishes and decapods) (none) no Papiol et al. 2013 (Megafauna), that are defined as those typically large enough to be viewed in photographs or caught with trawls Gage and Tyler 1991 yes Valentine and Benfield 2013 Megafauna, i.e. the group of organisms inhabiting the sediment-water interface and ≥1 cm Grassle et al. 1975, Rex 1981 no Beazley et al. 2013 Megafauna are defined as those organisms >1 cm which inhabit the sediment–water interface, or are arbitrarily delineated as any organism which is visible with a camera Bergmann et al. 2011 yes Meyer et al. 2013 Visible megafauna (none) yes Rybakova et al. 2013 A selection of megafauna (Lophelia pertusa, Madrepora oculata, Paragorgia arborea, Primnoa resedaeformis, Mycale lingua, Geodia baretti, Acesta excavata and fish) (none) no Purser et al. 2013a Megafauna (organisms of >1 cm) (none) no Würzberg et al. 2014 Seabirds, marine mammals, and sea turtles, collectively termed air-breathing marine megafauna Baum et al. 2003, Rivalan et al. 2010, Crowder and Heppell 2011, Wallace et al. 2010 no Lewison et al. 2014 Apex predators such as dolphins, whales, sharks, seals, seabirds and marine turtles, together known as marine megafauna (none) no McClellan et al. 2014 Epibenthic megafauna (>1 cm) (none) no Yesson et al. 2015 Megafaunal organisms were defined as those being larger than a few centimeters and were visible on the sediment surface (none) yes Kita et al. 2015 Deep-sea epibenthic megafauna are animals (usually >1 cm) that occupy the surface layer of seabed sediment and are visible in photographs Grassle et al. 1975, Smith et al. 1993 yes Dunlop et al. 2015 The marine megafauna we focus on in this study include small cetaceans (dolphins and porpoises), dugongs, and turtles (none) no Teh et al. 2015 Freshwater Megafauna (crustaceans, amphibians and fish) (none) no Vilella et al. 2004 Note: Only articles with the term “megafauna” in the title were considered for this purpose. The complete references are shown in Appendix S3.  Table S3. List of species included in the questionnaires, according to their taxonomic group. Group Species Group Species Group Species Group Species Mammals Acerodon jubatus Reptiles Blanus cinereus Fishes Acipenser sturio Marine invertebrates Aplysia fasciata Alces alces Chamaeleo jacksonii Amphiprion ocellaris Architeuthis dux Physeter macrocephalus Chelonoidis hoodensis Cetoscarus bicolor Cestum veneris Delphinus capensis Crocodylus niloticus Pterois antennata Eurythenes gryllus Homo sapiens Dermochelys coriacea Hippocampus histrix Gorgonia flabellum Hydrochoerus hydrochaeris Eunectes murinus Manta birostris Physalia physalis Loxodonta africana Hydrophis belcheri Mola mola Homarus gammarus Odobenus rosmarus Iguana delicatissima Neoceratodus forsteri Macrocheira kaempferi Ornithorhynchus anatinus Naja naja Oncorhynchus nerka Octopus vulgaris Orycteropus afer Tarentola mauritanica Silurus glanis Ophidiaster ophidianus Panthera tigris Testudo graeca Petromyzon marinus Rhizostoma pulmo Pongo pygmaeus Trachemys scripta Rhincodon typus Riftia pachyptila Procyon lotor Varanus albigularis Salmo trutta Salpa maxima Suncus etruscus Varanus komodoensis Sphyrna mokarran Tridacna gigas Trichechus inunguis Crotalus mitchellii Xiphias gladius Xestospongia muta Birds Anodorhynchus hyacinthinus Amphibians Agalychnis callidryas Terrestrial invertebrates Achatina fulica Freshwater invertebrates Anax imperator Anser anser Amphiuma means Actias isabellae Austropotamobius pallipes Aptenodytes forsteri Andrias japonicus Aedes albopictus Daphnia magna Aquila chrysaetos Bufo bufo Attacus atlas Ditiscus marginalis Campephilus melanoleucos Caecilia thompsoni Vespa vetulina Argyroneta aquatica Ciconia ciconia Megophrys nasuta Iberus gualterianus Dugesia tigrina Diomedea exulans Phyllobates terribilis Lucanus cervus Ephemera danica Gymnogyps californianus Pipa pipa Mantis religiosa Hirudo medicinalis Falco peregrinus Proteus anguinus Morpho peleides Hydra viridis Heliomaster longirostris Pyxicephalus adspersus Pandinus imperator Lethocerus indicus Larus canus Rhacophorus nigropalmatus Phobaeticus serratipes Leuctra bidula Otis tarda Salamandra salamandra Mastotermes darwiniensis Nepa cinerea Pelecanus conspicillatus Scaphiophryne gottlebei Scolopendra cingulata Pomacea canaliculata Struthio camelus Siphonops annulatus Brachypelma smithi Potamon fluviatile   Tyto alba   Triturus marmoratus   Scarabaeus sacer   Unio tumidiformis  Table S4. Description of the characteristics of the species included in the questionnaires. Variable Label Type Levels Details Taxonomy (coarse) Taxonomy Binary vertebrates, invertebrates Taxonomy (group) Group Categorical mammals, birds, reptiles, amphibians, fishes, terrestrial invertebrates, marine invertebrates, freshwater invertebrates Ecosystem System Categorical terrestrial, marine, freshwater, mixed Ecological function Eco_function Categorical predation, scavenging, parasitism, herbivory, pollination, seed dispersal, other, mixed for adults Diet Diet Categorical carnivorous, herbivorous, omnivorous main diet of adults Body mass Body_mass Numeric 0.50 – 4.25 x 107 mean adult – female for dimorphic species – weight, in g (decimal log transformed). When this information was unavailable, weight was estimated according to orders of magnitude Relative body mass Rel_body_mass Numeric 6.27 x 10-7 – 13.54 species body mass, in g / mean group body mass, in g ratio (decimal log transformed) Reproductive system Reprod_system Categorical viviparous, oviparous, ovoviviparous, asexual, other, mixed Offspring Offspring Binary altricial, precocial Lifespan Lifespan Numeric 0.1 – 2,300 maximum species lifespan in the wild, in years (log-transformed). When this information was unavailable, lifespan was obtained from captive individuals or from similar species Color Color Binary contrasted-colorful, cryptic adult body color pattern Activity Activity Categorical diurnal, nocturnal, both main daily activity of adults Social system Social_system Categorical eusocial/colonial, congregatory, other, mixed for adults Locomotion Locomotion Categorical aquatic, terrestrial, aerial, passive, sessile, mixed for adults Island dwelling Island Binary yes, no main distribution restricted to islands Migratory Migratory Binary yes, no capacity to perform long-distance migrations Invasive Invasive Binary yes, no IUCN status IUCN Ordinal IUCN Red List Category: NE/DD (Not Evaluated/Data Deficient), LC (Least Concern), NT (Near Threatened), VU (Vulnerable), EN (Endangered), CR (Critically Endangered) Popularity Popularity Ordinal lower, higher according to searching interest in Google Trends of Diceros bicornis in 2016 References Amaral, R.S., da Silva, V.M. and Rosas, F.C., 2010. Body weight/length relationship and mass estimation using morphometric measurements in Amazonian manatees Trichechus inunguis (Mammalia: Sirenia). Marine Biodiversity Records, 3, p.e105. AmphibiaWeb 2017 http://amphibiaweb.org/ (accessed on 20 February 2017) AnAge: The Animal Ageing and Longevity Database 2017 http://genomics.senescence.info/species/ (accessed on 20 February 2017) Barnett, M., Imre, I., Wagner, C.M., Di Rocco, R.T., Johnson, N.S. and Brown, G.E., 2016. Evaluating potential artefacts of photo-reversal on behavioural studies with nocturnal invasive sea lamprey (Petromyzon marinus). Canadian Journal of Zoology, 94(6), pp.405-410. Bennett, C., 2007. A seven year study of the life cycle of the mayfly Ephemera danica. In Freshwater Forum (Vol. 27, pp. 3-14). Bishop, J.M., Leslie, A.J., Bourquin, S.L. and O’Ryan, C., 2009. Reduced effective population size in an overexploited population of the Nile crocodile (Crocodylus niloticus). Biological Conservation, 142(10), pp.2335-2341. Corbet, P.S., 1957. The life-history of the emperor dragonfly Anax imperator Leach (Odonata: Aeshnidae). The Journal of Animal Ecology, pp.1-69. Enciclopedia virtual de los vertebrados españoles 2017 http://www.vertebradosibericos.org/ (accessed on 20 February 2017) Faurby S, Svenning JC 2016 Resurrection of the island rule – human-driven extinctions have obscured a basic evolutionary pattern. American Naturalist 187: 812-820. Ferguson-Lees, J., Christie, D.A. 2010 Raptors of the World. Bloomsbury Publishing, London. Guerra, A. 1992 Mollusca, Cephalopoda. In: Fauna Ibérica, Vol. 10 (Ramos, M.A. et al., Eds.). MNCN-CSIC, Madrid. 705 pp. Hockey, P.A.R., Dean, W.R.J., Ryan, P.G. (Eds.) 2005 Roberts - Birds of Southern Africa, VIIth ed. The Trustees of the John Voelcker Bird Book Fund, Cape Town. IUCN Red List 2017 http://www.iucnredlist.org/ (accessed on 20 February 2017) Jones, K.E., Bielby, J., Cardillo, M., Fritz, S.A., O'Dell, J., Orme, C.D.L., Safi, K., Sechrest, W., Boakes, E.H., Carbone, C., Connolly, C., Cutts, M.J., Foster, J.K., Grenyer, R., Habib, M., Plaster, C.A., Price, S.A., Rigby, E.A., Rist, J., Teacher, A., Bininda-Emonds, O.R.P., Gittleman, J.L., Mace, G.M., Purvis, A. 2009 PanTHERIA: a species-level database of life history, ecology, and geography of extant and recently extinct mammals. Ecology 90, 2648. Kaufman, G.A., Gibbons, J.W. 1975 Weight-lenght relationships in thirteen species of snakes in the Southeastern United States. Herpetologica 31, 31-37. Kemp, A., 1995. Threatened fishes of the world: Neoceratodus forsteri (Kreft, 1870)(Neoceratodontidae). Environmental Biology of Fishes, 43(3), pp.310-310. Kingdon, J. 2001. The Kingdon Field Guide to African Mammals. Academic Press, London. Lepage, M., Taverny, C., Piefort, S., Dumont, P., Rochard, E. and Brosse, L., 2005. Juvenile sturgeon (Acipenser sturio) habitat utilization in the Gironde estuary as determined by acoustic telemetry. In Fifth Conference on Fish Telemetry held in Europe. Ustica, Italy: FAO/COISPA (pp. 169-177). Lopes‐Lima, M., Sousa, R., Geist, J., Aldridge, D.C., Araujo, R., Bergengren, J., Bespalaya, Y., Bódis, E., Burlakova, L., Van Damme, D. and Douda, K., 2016. Conservation status of freshwater mussels in Europe: state of the art and future challenges. Biological Reviews. Miller, D.L., Radi, Z.A., Stiver, S.L. and Thornhill, T.D., 2004. Cutaneous and pulmonary mycosis in green anacondas (Eunectes murinus). Journal of Zoo and Wildlife Medicine, 35, pp.557-561. Myers, P., Espinosa, R., Parr, C.S., Jones, T., Hammond, G.S., Dewey, T.A. 2016 The Animal Diversity Web. http://animaldiversity.org/ (accessed on February 2017) Myhrvold, N.P., Baldridge, E., Chan, B., Sivam, D., Freeman, D.L., Ernest, S.K.M. 2015 An amniote life-history database to perform comparative analyses with birds, mammals, and reptiles. Ecology, 96, 3109. Reis, J. and Araujo, R., 2009. Redescription of Unio tumidiformis Castro, 1885 (Bivalvia, Unionidae), an endemism from the south-western Iberian Peninsula. Journal of Natural History, 43, 1929-1945. Salvador, A. (Coord.) 1997 Reptiles. En: Fauna Ibérica, Vol. 10 (Ramos, M.A. et al., Eds.). MNCN-CSIC, Madrid. 705 pp. Skinner, J.D., Chimimba, C.T. (Revs.) 2005 The Mammals of the Southern African Region. Third Edition. Cambridge University Press, Cambridge. Takeuchi, I., Watanabe, K. 1998 Respiration rate and swimming speed of the necrophagous amphipod Eurythenes gryllus from Antarctic deep waters. Marine Ecology Progress Series 163, 285-288. The Reptile Database 2017. http://reptile-database.reptarium.cz/ (accessed on 20 February 2017) Wilson, D.E., Mittermeier, R.A. (Eds.) 2009 Handbook of the Mammals of the World. Vol. 1. Carnivores. Lynx Edicions, Barcelona. Wilson, D.E., Mittermeier, R.A. (Eds.) 2011 Handbook of the Mammals of the World. Vol. 2. Hoofed Mammals. Lynx Edicions, Barcelona. Woods, C.M.C. 2005 Growth of cultured seahorses (Hippocampus abdominalis) in relation to feed ration. Aquaculture International 13, 305-314.  Table S5. Description of the variables used to characterize the respondents to the questionnaire. Variable Label Type Levels Age Age Numeric 21-74 Gender Gender Binary female, male Experience as a researcher Experience Ordinal pre-researcher (i.e. University students that aim to start a scientific career), PhD student, post-doc researcher, senior researcher Megafauna workshop attendant Workshop Binary yes, no Expertise on plants Plant Binary yes, no Expertise on mammals Mammal Binary yes, no Expertise on birds Bird Binary yes, no Expertise on reptiles Reptile Binary yes, no Expertise on amphibians Amphibian Binary yes, no Expertise on fish Fish Binary yes, no Expertise on invertebrates Invertebrate Binary yes, no Number of taxonomic groups of expertise Tax_expertise Ordinal 0-6 Ecosystem of expertise System Categorical terrestrial, marine, freshwater, mixed Period of expertise Period Categorical prehistorical, historical, both  Table S6. Results of the GLM relating respondents’ age, expertise on mammals and ecosystem of expertise with the criteria used by respondents to classify species as megafauna. Variable (Age, Expertise on mammals, Ecosystem of expertise) significance, goodness of fit (R2) and sample size (n) are shown. Significant results (α<0.05) are given in bold. Criteria Age Expertise on mammals Ecosystem of expertise R2 n Body mass 0.773 0.522 0.228 0.09 86 Taxonomy 0.888 0.018 0.636 0.05 86 Ecological function 0.204 0.346 0.328 0.04 86 Ecological context 0.344 0.968 0.056 0.07 85 Traits 0.134 0.895 0.442 0.06 86 Risk 0.011 0.024 0.034 0.27 86  Fig. S1. Number of articles on megafauna published per year, according to ecosystem (t: terrestrial, f: freshwater, m: marine), period (p: prehistorical, h: historical) and clusters defined by the semantic network analysis (1: terrestrial megafauna, 2: marine ecology of benthic and epibenthic megafauna, 3: biodiversity conservation of marine air-breathing large vertebrates and other large pelagic species; see main text and Fig. S2). Only articles with the term “megafauna” in the title were considered.   Fig. S2. The semantic network of the most relevant terms extracted from the megafauna literature. This semantic network indicates three clusters: terrestrial megafauna (red cluster), marine ecology of benthic and epibenthic megafauna (green cluster), and biodiversity conservation of marine air-breathing large vertebrates and other large pelagic species (blue cluster). Node size denotes the relative frequency with which the labeled term occurs in the title and abstract of the surveyed publications.   Fig. S3. Results of Boosted Regression Trees (BRTs) showing the relative importance of species traits (a) and respondents’ characteristics (b) to predict the probability of a species to be classified as megafauna. See the variable full names in Tables S5 and S6.  Fig. S4. Relationships between the respondents’ characteristics and the number of species they classified as megafauna (range: 0-120). Non: pre-researcher (i.e. University students that aim to start a scientific career); Pre: predoctoral researcher; Post: postdoctoral researcher; Sen: senior researcher. Ecosystem labels are T: terrestrial; F: freshwater; M: marine; Mixed. Period labels are: Pre: prehistorical, His: historical; Both. Variable full names in Table S6.  Fig. S5. Respondents’ preferences regarding the criteria to define megafauna. Panels show the proportion of respondents that selected each criteria (black: criterion selected in combination with other criteria; grey: criterion selected alone). See Appendix S2 for a definition of the different criteria.  Fig. S6. Relationship between respondents’ expertise on mammals and their propensity to consider taxonomy as a criterion to be taken into account when defining megafauna.   Fig. S7. Relationship between respondents’ age, expertise on mammals and ecosystem of expertise and their propensity to consider extinction risk as a criterion to be taken into account when defining megafauna. M: marine; T: terrestrial; FW: freshwater. Fitted values are shown in a red solid line. 24