Biodiversity Models: What If Unsaturation Is the Rule?
Authors: Rubén G. Mateo, Karel Mokany, Antoine Guisan
Journal: Trends in Ecology & Evolution (2017)

The majority of biodiversity modelling approaches do not explicitly address the question of saturation. Theoretical and methodological implications of saturation or unsaturation in biodiversity modelling. Addressing saturation or unsaturation is vital to produce more reliable conservation strategies. Integrative community modelling frameworks may be the way forward. Improving biodiversity predictions is essential if we are to meet the challenges posed by global change. As knowledge is key to feed models, we need to evaluate how debated theory can affect models. An important ongoing debate is whether environmental constraints limit the number of species that can coexist in a community (saturation), with recent findings suggesting that species richness in many communities might be unsaturated. Here, we propose that biodiversity models could address this issue by accounting for a duality: considering communities as unsaturated but where species composition is constrained by different scale-dependent biodiversity drivers. We identify a variety of promising advances for incorporating this duality into commonly applied biodiversity modelling approaches and improving their spatial predictions. Improving biodiversity predictions is essential if we are to meet the challenges posed by global change. As knowledge is key to feed models, we need to evaluate how debated theory can affect models. An important ongoing debate is whether environmental constraints limit the number of species that can coexist in a community (saturation), with recent findings suggesting that species richness in many communities might be unsaturated. Here, we propose that biodiversity models could address this issue by accounting for a duality: considering communities as unsaturated but where species composition is constrained by different scale-dependent biodiversity drivers. We identify a variety of promising advances for incorporating this duality into commonly applied biodiversity modelling approaches and improving their spatial predictions. A recent debate [1Harmon L.J. Harrison S. Species diversity is dynamic and unbounded at local and continental scales.Am. Nat. 2015; 185: 584-593Crossref PubMed Scopus (137) Google Scholar, 2Rabosky D.L. Hurlbert A.H. Species richness at continental scales is dominated by ecological limits.Am. Nat. 2015; 185: 572-583Crossref PubMed Scopus (169) Google Scholar] has revived an old discussion in ecology: are communities 'saturated'? Is there a limit to the number of species that a community (see Glossary) can support? Whether saturation plays a role in shaping communities remains a fundamental issue widely assessed in ecology and evolution, but no formal consensus has been reached (see Figure I in Box 1) . If saturation occurs, is it caused by ecological limits or by geographic area? In particular, are communities constrained by an 'environmental carrying capacity' (ECC) that limits the number of coexisting species?Box 1Are Communities Saturated?In the past, a large body of literature aimed at identifying the ecological limits of species richness concluded that strong control is exerted by environmental factors, especially climate conditions and environmental heterogeneity [29Currie D.J. Energy and large-scale patterns of animal-species and plant-species richness.Am. Nat. 1991; 137: 27-49Crossref Scopus (1310) Google Scholar, 61MacArthur R.H. Wilson E.O. An equilibrium theory of insular zoogeography.Evolution. 1963; 17: 373-387Crossref Google Scholar] (and references therein). In this context, a central method used has been the regression of local on regional richness [62Lawton J.H. Are there general laws in ecology?.Oikos. 1999; 84: 177-192Crossref Scopus (1060) Google Scholar]; saturating or nonlinear functions indicate that an upper limit of local diversity has been set by ecological limits [63Cornell H.V. Unsaturation and regional influences on species richness in ecological communities: a review of the evidence.Ecoscience. 1999; 6: 303-315Crossref Scopus (116) Google Scholar]. However, important methodological limitations, such as pseudoreplication, limit the use of this graphical approach [30Srivastava D.S. Using local–regional richness plots to test for species saturation: pitfalls and potentials.J. Anim. Ecol. 1999; 68: 1-16Crossref Scopus (291) Google Scholar, 64Hillebrand H. Regressions of local on regional diversity do not reflect the importance of local interactions or saturation of local diversity.Oikos. 2005; 110: 195-198Crossref Scopus (57) Google Scholar]. More recently, ecological analyses of seed addition experiments [65Myers J.A. Harms K.E. Seed arrival, ecological filters, and plant species richness: a meta-analysis.Ecol. Lett. 2009; 12: 1250-1260Crossref PubMed Scopus (259) Google Scholar, 66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar], contemporary biotic exchanges [66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar], fossil records [67Marcot J.D. et al.Late Cenozoic onset of the latitudinal diversity gradient of North American mammals.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: 7189-7194Crossref PubMed Scopus (30) Google Scholar], invasion processes [66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar, 68Stohlgren T.J. et al.The myth of plant species saturation.Ecol. Lett. 2008; 11: 313-322Crossref PubMed Scopus (124) Google Scholar, 69Sax D.F. et al.Ecological and evolutionary insights from species invasions.Trends Ecol. Evol. 2007; 22: 465-471Abstract Full Text Full Text PDF PubMed Scopus (676) Google Scholar, 70Seebens H. et al.No saturation in the accumulation of alien species worldwide.Nat. Commun. 2017; 8: 14435Crossref PubMed Scopus (1082) Google Scholar], the packing and filling of functional space considering functional trait data [71Swenson N.G. Weiser M.D. On the packing and filling of functional space in eastern North American tree assemblages.Ecography. 2014; 37: 1056-1062Crossref Scopus (64) Google Scholar] (but see [72Swenson N.G. et al.Constancy in functional space across a species richness anomaly.Am. Nat. 2016; 187: E83-E92Crossref PubMed Scopus (15) Google Scholar]), diversification models [73Lagomarsino L.P. et al.The abiotic and biotic drivers of rapid diversification in Andean bellflowers (Campanulaceae).New Phytol. 2016; 210: 1430-1442Crossref PubMed Scopus (219) Google Scholar], and molecular phylogenies [1Harmon L.J. Harrison S. Species diversity is dynamic and unbounded at local and continental scales.Am. Nat. 2015; 185: 584-593Crossref PubMed Scopus (137) Google Scholar, 39Cornell H.V. Harrison S.P. What are species pools and when are they important?.Annu. Rev. Ecol. Evol. Syst. 2014; 45: 45-67Crossref Scopus (205) Google Scholar, 74Etienne R.S. et al.Diversity-dependence brings molecular phylogenies closer to agreement with the fossil record.Proc. Biol. Sci. 2011; 279: 1300-1309Crossref PubMed Scopus (237) Google Scholar] have suggested that ecological systems might rarely be saturated and instead remain open to new species (Figure I).The relevance of saturation might therefore be relative to the scale [31Godfray H.C.J. Lawton J.H. Scale and species numbers.Trends Ecol. Evol. 2001; 16: 400-404Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar, 34Ricklefs R.E. Jenkins D.G. Biogeography and ecology: towards the integration of two disciplines.Philos. Trans. R. Soc. Lond. B. Biol. Sci. 2011; 366: 2438-2448Crossref PubMed Scopus (77) Google Scholar, 75Fridley J.D. et al.The invasion paradox: reconciling pattern and process in species invasions.Ecology. 2007; 88: 3-17Crossref PubMed Scopus (674) Google Scholar]. Thus, some authors suggest that communities might not be saturated [76Gaston K.J. Blackburn T.M. Pattern and Process in Macroecology. Blackwell, 2000Crossref Scopus (185) Google Scholar] or might only be saturated at very small scales (e.g., <1 m2 for plant communities [69Sax D.F. et al.Ecological and evolutionary insights from species invasions.Trends Ecol. Evol. 2007; 22: 465-471Abstract Full Text Full Text PDF PubMed Scopus (676) Google Scholar]). At such scales, stochasticity (e.g., disturbances such as fires, neutral processes) [7Lomolino M.V. et al.Biogeography. Sinauer Associates, 2010Google Scholar, 50Tilman D. Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly.Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 10854-10861Crossref PubMed Scopus (1076) Google Scholar] is an important element in understanding biodiversity, with purely physical parameters becoming more important, such as how many individuals can occupy the total area. At coarser resolutions, according to several authors [1Harmon L.J. Harrison S. Species diversity is dynamic and unbounded at local and continental scales.Am. Nat. 2015; 185: 584-593Crossref PubMed Scopus (137) Google Scholar, 66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar], communities are dynamic in space and time and are virtually always accessible to new species (with speciation and migration occurring more often than extinction), and community diversity is not set by ecological limits. Nevertheless, richness would remain correlated with variables that are related to productivity or energy within a geographic space [77Currie D.J. et al.Predictions and tests of climate-based hypotheses of broad-scale variation in taxonomic richness.Ecol. Lett. 2004; 7: 1121-1134Crossref Scopus (895) Google Scholar], though this correlation might be explained by alternative hypotheses [1Harmon L.J. Harrison S. Species diversity is dynamic and unbounded at local and continental scales.Am. Nat. 2015; 185: 584-593Crossref PubMed Scopus (137) Google Scholar, 38Cornell H.V. Is regional species diversity bounded or unbounded?.Biol. Rev. 2013; 88: 140-165Crossref PubMed Scopus (86) Google Scholar] (Boxes 2 and 3).Although we take into consideration that communities might be unsaturated, at local scales and for high values of richness, the invasion of a community by new species is likely to become progressively more arduous [66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar]. This resistance might result from more intense biotic interactions at a higher species richness [66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar] and the inhibitory properties of established species [50Tilman D. Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly.Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 10854-10861Crossref PubMed Scopus (1076) Google Scholar]. Therefore, at fine spatial resolution, locally dominant species, organism body size, and disturbances are important factors [7Lomolino M.V. et al.Biogeography. Sinauer Associates, 2010Google Scholar]. In the past, a large body of literature aimed at identifying the ecological limits of species richness concluded that strong control is exerted by environmental factors, especially climate conditions and environmental heterogeneity [29Currie D.J. Energy and large-scale patterns of animal-species and plant-species richness.Am. Nat. 1991; 137: 27-49Crossref Scopus (1310) Google Scholar, 61MacArthur R.H. Wilson E.O. An equilibrium theory of insular zoogeography.Evolution. 1963; 17: 373-387Crossref Google Scholar] (and references therein). In this context, a central method used has been the regression of local on regional richness [62Lawton J.H. Are there general laws in ecology?.Oikos. 1999; 84: 177-192Crossref Scopus (1060) Google Scholar]; saturating or nonlinear functions indicate that an upper limit of local diversity has been set by ecological limits [63Cornell H.V. Unsaturation and regional influences on species richness in ecological communities: a review of the evidence.Ecoscience. 1999; 6: 303-315Crossref Scopus (116) Google Scholar]. However, important methodological limitations, such as pseudoreplication, limit the use of this graphical approach [30Srivastava D.S. Using local–regional richness plots to test for species saturation: pitfalls and potentials.J. Anim. Ecol. 1999; 68: 1-16Crossref Scopus (291) Google Scholar, 64Hillebrand H. Regressions of local on regional diversity do not reflect the importance of local interactions or saturation of local diversity.Oikos. 2005; 110: 195-198Crossref Scopus (57) Google Scholar]. More recently, ecological analyses of seed addition experiments [65Myers J.A. Harms K.E. Seed arrival, ecological filters, and plant species richness: a meta-analysis.Ecol. Lett. 2009; 12: 1250-1260Crossref PubMed Scopus (259) Google Scholar, 66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar], contemporary biotic exchanges [66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar], fossil records [67Marcot J.D. et al.Late Cenozoic onset of the latitudinal diversity gradient of North American mammals.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: 7189-7194Crossref PubMed Scopus (30) Google Scholar], invasion processes [66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar, 68Stohlgren T.J. et al.The myth of plant species saturation.Ecol. Lett. 2008; 11: 313-322Crossref PubMed Scopus (124) Google Scholar, 69Sax D.F. et al.Ecological and evolutionary insights from species invasions.Trends Ecol. Evol. 2007; 22: 465-471Abstract Full Text Full Text PDF PubMed Scopus (676) Google Scholar, 70Seebens H. et al.No saturation in the accumulation of alien species worldwide.Nat. Commun. 2017; 8: 14435Crossref PubMed Scopus (1082) Google Scholar], the packing and filling of functional space considering functional trait data [71Swenson N.G. Weiser M.D. On the packing and filling of functional space in eastern North American tree assemblages.Ecography. 2014; 37: 1056-1062Crossref Scopus (64) Google Scholar] (but see [72Swenson N.G. et al.Constancy in functional space across a species richness anomaly.Am. Nat. 2016; 187: E83-E92Crossref PubMed Scopus (15) Google Scholar]), diversification models [73Lagomarsino L.P. et al.The abiotic and biotic drivers of rapid diversification in Andean bellflowers (Campanulaceae).New Phytol. 2016; 210: 1430-1442Crossref PubMed Scopus (219) Google Scholar], and molecular phylogenies [1Harmon L.J. Harrison S. Species diversity is dynamic and unbounded at local and continental scales.Am. Nat. 2015; 185: 584-593Crossref PubMed Scopus (137) Google Scholar, 39Cornell H.V. Harrison S.P. What are species pools and when are they important?.Annu. Rev. Ecol. Evol. Syst. 2014; 45: 45-67Crossref Scopus (205) Google Scholar, 74Etienne R.S. et al.Diversity-dependence brings molecular phylogenies closer to agreement with the fossil record.Proc. Biol. Sci. 2011; 279: 1300-1309Crossref PubMed Scopus (237) Google Scholar] have suggested that ecological systems might rarely be saturated and instead remain open to new species (Figure I). The relevance of saturation might therefore be relative to the scale [31Godfray H.C.J. Lawton J.H. Scale and species numbers.Trends Ecol. Evol. 2001; 16: 400-404Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar, 34Ricklefs R.E. Jenkins D.G. Biogeography and ecology: towards the integration of two disciplines.Philos. Trans. R. Soc. Lond. B. Biol. Sci. 2011; 366: 2438-2448Crossref PubMed Scopus (77) Google Scholar, 75Fridley J.D. et al.The invasion paradox: reconciling pattern and process in species invasions.Ecology. 2007; 88: 3-17Crossref PubMed Scopus (674) Google Scholar]. Thus, some authors suggest that communities might not be saturated [76Gaston K.J. Blackburn T.M. Pattern and Process in Macroecology. Blackwell, 2000Crossref Scopus (185) Google Scholar] or might only be saturated at very small scales (e.g., <1 m2 for plant communities [69Sax D.F. et al.Ecological and evolutionary insights from species invasions.Trends Ecol. Evol. 2007; 22: 465-471Abstract Full Text Full Text PDF PubMed Scopus (676) Google Scholar]). At such scales, stochasticity (e.g., disturbances such as fires, neutral processes) [7Lomolino M.V. et al.Biogeography. Sinauer Associates, 2010Google Scholar, 50Tilman D. Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly.Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 10854-10861Crossref PubMed Scopus (1076) Google Scholar] is an important element in understanding biodiversity, with purely physical parameters becoming more important, such as how many individuals can occupy the total area. At coarser resolutions, according to several authors [1Harmon L.J. Harrison S. Species diversity is dynamic and unbounded at local and continental scales.Am. Nat. 2015; 185: 584-593Crossref PubMed Scopus (137) Google Scholar, 66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar], communities are dynamic in space and time and are virtually always accessible to new species (with speciation and migration occurring more often than extinction), and community diversity is not set by ecological limits. Nevertheless, richness would remain correlated with variables that are related to productivity or energy within a geographic space [77Currie D.J. et al.Predictions and tests of climate-based hypotheses of broad-scale variation in taxonomic richness.Ecol. Lett. 2004; 7: 1121-1134Crossref Scopus (895) Google Scholar], though this correlation might be explained by alternative hypotheses [1Harmon L.J. Harrison S. Species diversity is dynamic and unbounded at local and continental scales.Am. Nat. 2015; 185: 584-593Crossref PubMed Scopus (137) Google Scholar, 38Cornell H.V. Is regional species diversity bounded or unbounded?.Biol. Rev. 2013; 88: 140-165Crossref PubMed Scopus (86) Google Scholar] (Boxes 2 and 3). Although we take into consideration that communities might be unsaturated, at local scales and for high values of richness, the invasion of a community by new species is likely to become progressively more arduous [66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar]. This resistance might result from more intense biotic interactions at a higher species richness [66Smith S.A. Shurin J.B. et al.Room for one more? Evidence for invasibility and saturation in ecological communities.in: Cadotte M.W. Conceptual Ecology and Invasion Biology: Reciprocal Approaches to Nature. Springer, 2006: 423-447Crossref Scopus (14) Google Scholar] and the inhibitory properties of established species [50Tilman D. Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly.Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 10854-10861Crossref PubMed Scopus (1076) Google Scholar]. Therefore, at fine spatial resolution, locally dominant species, organism body size, and disturbances are important factors [7Lomolino M.V. et al.Biogeography. Sinauer Associates, 2010Google Scholar]. Answering this overarching saturation question is not only crucial from a fundamental perspective, but also constitutes a major challenge for the biodiversity models used to predict current and future patterns of species composition and richness. Here, we do not intend to continue or solve the saturation debate per se, but we aim instead to assess the key implications that this question has for modelling communities (see Outstanding Questions), particularly taking a spatial and temporal perspective. Explicitly considering potential for unsaturation or saturation in biodiversity models can have drastic effects on their predictions as well as on their interpretation, yet this issue is rarely addressed. Predictions under global change or following biological invasions might differ markedly depending on the approaches applied and whether saturation is assumed or not. This is especially important if we consider that, under global change, the flux of species in and out of communities (i.e., the species turnover) is likely to increase [3Alexander J.M. et al.When climate reshuffles competitors: a call for experimental macroecology.Trends Ecol. Evol. 2016; 31: 831-841Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar]. The level to which communities are saturated is therefore likely to have a very large influence on the potential for species to shift their distributions and track changing climates, given inertia in the composition of communities, founder priority effects, and limits on space resources [4Blois J.L. et al.Modeling the climatic drivers of spatial patterns in vegetation composition since the last glacial maximum.Ecography. 2013; 36: 460-473Crossref Scopus (56) Google Scholar, 5Blonder B. et al.Linking environmental filtering and disequilibrium to biogeography with a community climate framework.Ecology. 2015; 96: 972-985Crossref PubMed Scopus (56) Google Scholar]. Hence, improving our capacity to predict species richness and community composition by considering saturation or unsaturation is key for using models to produce more reliable conservation strategies in response to global environmental change [6Nogués-Bravo D. Rahbek C. Communities under climate change.Science. 2011; 334: 1070-1071Crossref PubMed Scopus (43) Google Scholar]. Tremendous effort has been expended to explain biodiversity patterns [7Lomolino M.V. et al.Biogeography. Sinauer Associates, 2010Google Scholar], including high-profile theories [8Cabral J.S. et al.Mechanistic simulation models in macroecology and biogeography: state-of-art and prospects.Ecography. 2017; 40: 267-280Crossref Scopus (93) Google Scholar] (Box 2) and an increasing number of spatiotemporal biodiversity modelling techniques [9D'Amen M. et al.Spatial predictions at the community level: from current approaches to future frameworks.Biol. Rev. 2017; 92: 169-187Crossref PubMed Scopus (112) Google Scholar] (Table 1, Key Table). These theories and models focus on different questions, biodiversity drivers, contexts, and scales, with corresponding variety in the ways in which community saturation is considered. Here, we focus primarily on modelling approaches developed for constructing spatially explicit predictions of species richness and community composition across large regions and over changing environmental conditions. We emphasize correlative modelling approaches (and not stochastic ones, like neutral models; see [9D'Amen M. et al.Spatial predictions at the community level: from current approaches to future frameworks.Biol. Rev. 2017; 92: 169-187Crossref PubMed Scopus (112) Google Scholar] and references therein) because they are the techniques most commonly applied to address conservation issues [10Guisan A. et al.Predicting species distributions for conservation decisions.Ecol. Lett. 2013; 16: 1424-1435Crossref PubMed Scopus (1107) Google Scholar] and predict future patterns of biodiversity in the face of global change [6Nogués-Bravo D. Rahbek C. Communities under climate change.Science. 2011; 334: 1070-1071Crossref PubMed Scopus (43) Google Scholar], such as for global biodiversity assessments [11Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (2016) The Methodological Assessment Report on Scenarios and Models of Biodiversity and Ecosystem Services (Ferrier, S., et al., eds), pp. 348, Secretariat of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem ServicesGoogle Scholar].Box 2Towards a Reinterpretation of Some Macroecological Theories?The relationship between available energy and biodiversity is well known [78Wright D.H. Species-energy theory: an extension of species-area theory.Oikos. 1983; 41: 496-506Crossref Scopus (1045) Google Scholar]. This species–energy relationship was suggested by Hutchinson [79Hutchinson G.E. Homage to Santa Rosalia, or why are there so many kinds of animals?.Am. Nat. 1959; 93: 145-159Crossref Google Scholar] and has been interpreted by some authors as a limit on the number of species based on the available energy [80Hairston N.G. et al.Community structure, population control, and competition.Am. Nat. 1960; 94: 421-425Crossref Google Scholar]. Consequently, fifty years ago, many ecologists assumed that communities were saturated [34Ricklefs R.E. Jenkins D.G. Biogeography and ecology: towards the integration of two disciplines.Philos. Trans. R. Soc. Lond. B. Biol. Sci. 2011; 366: 2438-2448Crossref PubMed Scopus (77) Google Scholar]. Later, Currie [29Currie D.J. Energy and large-scale patterns of animal-species and plant-species richness.Am. Nat. 1991; 137: 27-49Crossref Scopus (1310) Google Scholar] theorized that a measure of energy availability ought to enable the species richness in a region to be predicted [2Rabosky D.L. Hurlbert A.H. Species richness at continental scales is dominated by ecological limits.Am. Nat. 2015; 185: 572-583Crossref PubMed Scopus (169) Google Scholar]. This measure is interpreted by some authors as the ECC of a unit and defines the boundaries of community saturation [81Boucher-Lalonde V. et al.Does climate limit species richness by limiting individual species' ranges?.Proc. Biol. Sci. 2014; 281: 20132695Crossref PubMed Scopus (47) Google Scholar, 82Hurlbert A.H. Stegen J.C. When should species richness be energy limited, and how would we know?.Ecol. Lett. 2014; 17: 401-413Crossref PubMed Scopus (93) Google Scholar].Nevertheless, the macroecological theories of a direct association between current climate and biodiversity, such as the 'species–energy' theory [78Wright D.H. Species-energy theory: an extension of species-area theory.Oikos. 1983; 41: 496-506Crossref Scopus (1045) Google Scholar], 'species–area' theory [83Hurlbert A.H. Jetz W. More than "more individuals": the nonequivalence of area and energy in the scaling of species richness.Am. Nat. 2010; 176: 50-65Crossref Scopus (66) Google Scholar], or 'ecological limit' hypothesis [2Rabosky D.L. Hurlbert A.H. Species richness at continental scales is dominated by ecological limits.Am. Nat. 2015; 185: 572-583Crossref PubMed Scopus (169) Google Scholar], have not led to an adequate description of all the drivers of species richness patterns [47Gotelli N.J. et al.Patterns and causes of species richness: a general simulation model for macroecology.Ecol. Lett. 2009; 12: 873-886Crossref PubMed Scopus (201) Google Scholar, 77Currie D.J. et al.Predictions and tests of climate-based hypotheses of broad-scale variation in taxonomic richness.Ecol. Lett. 2004; 7: 1121-1134Crossref Scopus (895) Google Scholar]; for example, they do not consider evolutionary history factors [47Gotelli N.J. et al.Patterns and causes of species richness: a general simulation model for macroecology.Ecol. Lett. 2009; 12: 873-886Crossref PubMed Scopus (201) Google Scholar]. At least three alternative theories [1Harmon L.J. Harrison S. Species diversity is dynamic and unbounded at local and continental scales.Am. Nat. 2015; 185: 584-593Crossref PubMed Scopus (137) Google Scholar, 7Lomolino M.V. et al.Biogeography. Sinauer Associates, 2010Google Scholar, 38Cornell H.V. Is regional species diversity bounded or unbounded?.Biol. Rev. 2013; 88: 140-165Crossref PubMed Scopus (86) Google Scholar, 39Cornell H.V. Harrison S.P. What are species pools and when are they important?.Annu. Rev. Ecol. Evol. Syst. 2014; 45: 45-67Crossref Scopus (205) Google Scholar], the 'niche conservative' [84Rangel T. et al.Species richness and evolutionary niche dynamics: a spatial pattern-oriented simulation experiment.Am. Nat. 2007; 170: 602-616Crossref PubMed Scopus (111) Google Scholar], the 'faster diversification', and the 'metabolic' theories [