Thermal niches of planktonic foraminifera are static throughout glacial–interglacial climate change

Abiotic niche lability reduces extinction risk by allowing species to adapt to changing environmental conditions in situ. In contrast, species with static niches must keep pace with the velocity of climate change as they track suitable habitat. The rate and frequency of niche lability have been studied on human timescales (months to decades) and geological timescales (millions of years), but lability on intermediate timescales (millennia) remains largely uninvestigated. Here, we quantified abiotic niche lability at 8-ka resolution across the last 700 ka of glacial–interglacial climate fluctuations, using the exceptionally well-known fossil record of planktonic foraminifera coupled with Atmosphere–Ocean Global Climate Model reconstructions of paleoclimate. We tracked foraminiferal niches through time along the univariate axis of mean annual temperature, measured both at the sea surface and at species’ depth habitats. Species’ temperature preferences were uncoupled from the global temperature regime, undermining a hypothesis of local adaptation to changing environmental conditions. Furthermore, intraspecific niches were equally similar through time, regardless of climate change magnitude on short timescales (8 ka) and across contrasts of glacial and interglacial extremes. Evolutionary trait models fitted to time series of occupied temperature values supported widespread niche stasis above randomly wandering or directional change. Ecotype explained little variation in species-level differences in niche lability after accounting for evolutionary relatedness. Together, these results suggest that warming and ocean acidification over the next hundreds to thousands of years could redistribute and reduce populations of foraminifera and other calcifying plankton, which are primary components of marine food webs and biogeochemical cycles.

Abiotic niche dynamics determine patterns of community composition over space and regulate trajectories of diversity over time (1). Both niche lability (2, 3) and conservatism (1, 4) have been proposed to spur speciation, and abiotic niche lability has been associated with ecological invasions (5–7) and with reduced risk of extinction during times of climate change (8). Thus, a deeper understanding of species’ propensity for niche stasis versus lability could improve predictions of biodiversity restructuring in response to anthropogenic climate change (9).Stasis in species’ abiotic niches through time has been documented in empirical research, but most such studies have been limited to ecological niche modeling on decadal scales (reviewed in ref. 10) or paleoecological examination on 10⁶ to 10⁷ y scales (5, 11, 12). Since empirical rates of niche change are scarce and difficult to acquire, many studies merely assume that niche evolution occurs at a constant rate along branches of a phylogeny (2, 3, 6, 7). Niche dynamics at intermediate timescales of centuries to millennia are particularly poorly documented (10), and studies at this meso scale have been restricted to terrestrial systems (e.g., refs. 13–15) or to comparisons between the present day and the single historical time step of the Last Glacial Maximum, ∼21 ka (16–20). Quantifying the rate and relative frequency of niche change in marine species over timescales of 10² to 10⁵ years is important, however, because species will adapt or go extinct in response to anthropogenic ocean changes over this timescale (21).Here, we investigated climatic niche lability from the rich sedimentary archive of global planktonic foraminifera across the last 700 ka of glacial–interglacial cycles at 8-ka resolution. Planktonic foraminifera (Protista) construct “shells” (tests) of calcite, thereby sequestering carbon and recording an isotopic signature of past ocean conditions. Tests readily accumulate over large expanses of the seafloor. Consequently, the fossil record of foraminifera—arguably “the best fossil record on Earth” (22)—affords an exceptionally high-resolution view into past species distributions. This detailed record fuels studies of biostratigraphy, paleoclimatology, and paleoecology (20, 22–25). Moreover, the complete species diversity of planktonic foraminifera has been described for the Plio–Pleistocene, with good agreement between morphological and molecular phylogenies (22, 25–27). Although some have speculated that foraminifera competitively exclude each other (24), recent work found that planktonic foraminifera species seldom restrict each other’s distributions (28). Presumably, therefore, species occupy the full envelope of existing environmental conditions within their tolerance limits, and geographic distributions are determined almost entirely by physical ocean conditions.We developed five analyses to investigate the degree of abiotic niche lability in foraminifera. All methods examined the univariate niche axis of temperature, which is the single most important explanatory variable in regard to geographic distributions of foraminifera (20, 29–32) and is a climate-related stressor and extinction driver for diverse marine fauna across timescales (33, 34). The adaptive potential of thermal niches has been taken as a key determinant of global community structure and genetic connectance in plankton (35). Primary productivity and other environmental variables, however, may also structure abiotic niches of plankton (36). Our suite of analyses quantified whether and by how much planktonic foraminiferal niches shifted along a temperature axis. First, we correlated time series of species’ thermal optima with global temperature to determine whether species tracked suitable habitat or experienced environmental fluctuations in situ. We then quantified species’ niche dissimilarity between pairs of time bins—either tracking niches across bin boundaries or contrasting niches at climatic extremes of glacial maxima and interglacial thermal peaks. To characterize niche change we applied trait evolution models to time series of temperatures at occupied sites. Lastly, we explored variation in intraspecific niche lability among ecotypes while accounting for phylogenetic relatedness. SI Appendix, Table S1 lists the response variable and sample size for each analysis.     

Geomorphic controls on elevational gradients of species richness

Elevational gradients of biodiversity have been widely investigated, and yet a clear interpretation of the biotic and abiotic factors that determine how species richness varies with elevation is still elusive. In mountainous landscapes, habitats at different elevations are characterized by different areal extent and connectivity properties, key drivers of biodiversity, as predicted by metacommunity theory. However, most previous studies directly correlated species richness to elevational gradients of potential drivers, thus neglecting the interplay between such gradients and the environmental matrix. Here, we investigate the role of geomorphology in shaping patterns of species richness. We develop a spatially explicit zero-sum metacommunity model where species have an elevation-dependent fitness and otherwise neutral traits. Results show that ecological dynamics over complex terrains lead to the null expectation of a hump-shaped elevational gradient of species richness, a pattern widely observed empirically. Local species richness is found to be related to the landscape elevational connectivity, as quantified by a newly proposed metric that applies tools of complex network theory to measure the closeness of a site to others with similar habitat. Our theoretical results suggest clear geomorphic controls on elevational gradients of species richness and support the use of the landscape elevational connectivity as a null model for the analysis of the distribution of biodiversity.The search for the mechanisms determining the distribution of life on Earth has long been, and still is, a challenge of great importance for ecologists and biogeographers. Indeed, developing conservation strategies demands knowledge of ex ante and ex post biodiversity patterns through their linkage with ecological processes. As a common approach, general patterns in species richness are sought to understand the underlying processes (1–3). One outstanding example is the study of elevational gradients of species richness, the subject of much attention because strong elevational gradients can be observed in any mountainous landscape (4, 5). Among possible drivers, temperature directly controls biological productivity of the community, which, in turn, has been linked to diversity (6). A simplistic association of elevational gradients with temperature gradients in mountainous ecosystems suggests a decline of species richness with increasing elevation (1, 2, 7, 8), echoing the latitudinal decline from the equator to the poles (9). However, such an expectation is clearly inconsistent with empirical observations that often show a hump-shaped rather than a monotonically decreasing pattern (4, 8, 10–12).A possible explanation is that both productivity versus elevation and species richness versus productivity may be described by nonmonotonic relations (10, 13, 14). At low elevations, in particular, human disturbance may play a major role in reducing biodiversity (15). Whereas several factors [such as temperature, habitat capacity, precipitation, anthropogenic pressure and geometric constraints (1, 5, 15)] change (somewhat) predictably with elevation, other relevant factors (such as moisture, clear-sky turbidity and cloudiness, sunshine exposure and aspect, wind strength, season length, and exposed lithology) are not elevation-specific (16). Thus, empirical results may hardly sort out general rules unambiguously. Given the multitude of possible confounding factors, theoretical analyses are key to understand elevational gradients of diversity and how biota respond to geophysical drivers and controls (5, 17, 18) [e.g., the foreseen upward shift in plant species optimum elevation (19)].Here, we identify and analyze three distinctive geomorphic features characteristic of mountainous landscapes that can systematically affect the distribution of species and result in hump-shaped patterns of biodiversity along elevational gradients: (i) finiteness of the landscape elevational range; (ii) frequency distribution of areal extent at different elevation; and (iii) differential elevational connectivity.Geometrically constrained landscapes are subject to the so-called middomain effect, according to which, if the species’ ranges are randomly distributed over a bounded geographic domain free of environmental gradients, ranges would increasingly overlap over the center of the domain (20, 21). Applying the same principle to a finite landscape elevational range would support hump-shaped patterns of local species richness along elevational gradients (e.g., refs. 22–24).The frequency distribution of elevation in real-life landscapes is distinctly hump-shaped, with the majority of land situated at midelevations (Fig. 1 and ref. 25). This pattern is ubiquitous in landscapes shaped by fluvial erosion when a sufficiently large region rather than a single slope or mountain is considered, and the pattern is altered only if large areas outside runoff-producing zones (e.g., large plains) are included in the domain (Supporting Information). This pattern is often overlooked (e.g., refs. 1 and 4) because the mountain-cone analogy suggests a monotonically decreasing distribution of elevation. However, mountains are not cones (26) but complex fractal structures. The area of available habitat within a given elevational band may have a direct effect on the diversity of the regional community it hosts [γ-diversity (8, 13, 22, 27, 28)], as predicted by the species–area relationship (29). The area of available habitat may also have an indirect effect on the local species richness [diversity of equal-area plots (i.e., α-diversity)] because local communities can be assembled from a more diverse regional pool of species that are fit to live at similar elevation (28).Open in a separate windowFig. 1.Comparison between a real-life elevation field (a fluvial landscape in the Swiss Alps, 50? × ?50 km²) (A) and an oversimplified, 1D elevation field (B). (C) Frequency distributions of elevation of the two landscapes. Fig. S1 reports other examples.Finally, a feature potentially capable of shaping elevational diversity patterns is the inherent elevational connectivity of fluvial landscapes. When mapping the fitness, assumed to be elevation-dependent, of three hypothetical species with the same niche width but different niche position [the elevation at which fitness is maximum (30)] over a real mountainous landscape, we find that suitable habitat patches for different species feature very different connectivity (Fig. 2). Valleys (low-elevation sites) and mountain tops (high-elevation sites) form fragmented patches nearly isolated from each other, whereas midelevation sites are both more abundant and more interconnected. Habitat size and connectivity are key determinants of extinction and immigration rates, and thus of diversity, as first predicted by the classic theory of island biogeography (31) and later confirmed by many experimental and theoretical studies (e.g., refs. 32–42). It is thus expected that communities at low (high) elevation, being more isolated, exhibit lower species richness than those at midelevation. This effect has already been discussed for mountain tops (4, 31), and yet isolation of valleys has been so far overlooked, and a comprehensive framework to quantify this effect is missing.Open in a separate windowFig. 2.Habitat maps as a function of elevation. (A) A real fluvial landscape (same of Fig. 1A). (B) Fitness of three different species as a function of elevation. (C–E) Fitness maps of the three species shown in B. Darker pixels indicate higher fitness.Another limitation of previous studies (e.g., refs. 4, 10, 11, and 13) is that the environmental matrix and the elevational gradients were considered disconnected, and movement and dispersal of organisms across space ignored. Indeed, deriving species distribution patterns directly from elevational gradients of potential drivers implicitly requires the assumption of a 1D landscape (Fig. 1B), where all sites at the same elevation share the same characteristics. Such a landscape model is in stark contrast to the complexity of a typical real-life mountainous region (Fig. 1). The hypothesis that the very structure of landscapes can lead to nontrivial diversity patterns, even in the absence of species’ preferential elevation or gradients of productivity and habitat capacity, is tested here. To that end, we simulate ecological dynamics in 3D landscapes using a zero-sum metacommunity model (43, 44) (Materials and Methods). The model has been formalized by invoking the minimum set of assumptions principle. Specifically, the following set of rules has been implemented: (i) individuals of each species have a fitness (i.e., a competitive ability in this context) that depends on elevation, with all other vital rates being the same; (ii) different species have different niche positions but the same niche width (Fig. 2B); (iii) niche positions are uniformly distributed along the elevational range of the domain, so that there is no preferential elevation at the metacommunity scale; (iv) dispersal is isotropic (toward the four nearest-neighbor communities in a regular 2D lattice); and (v) the size of local communities is constant over the entire domain (i.e., constant habitat capacity). The above assumptions could be straightforwardly relaxed to mimic more realistic metacommunities. However, this set of assumptions is specifically designed to provide a null model to single out the effect of geomorphic controls of landscape structure on elevational diversity, while deliberately excluding other possible confounding factors.In addition to real-life landscapes, we run the zero-sum model over synthetic elevation fields derived from optimal channel networks (OCNs) (Supporting Information), which are topological structures that minimize a functional describing the total energy dissipated along drainage directions by landscape-forming discharges that hierarchically accumulate toward the outlet of the basin. OCNs are known to systematically reproduce all mutually connected scaling exponents of topological and metric landscape features (25, 45) and are exact steady-state solutions to the landscape evolution equation in the small gradient approximation (46). The use of OCNs has a twofold advantage. First, the use of synthetic elevation fields allows generating consistent replicas of fluvial landscapes in the same domain as the minimization process produces dynamically accessible, yet different, stable states endowed with the same statistical features. Second, OCNs allow producing periodic elevation fields and simulating ecological dynamics over a pseudoinfinite domain, thus avoiding edge effects.     

Adult and larval traits as determinants of geographic range size among tropical reef fishes

Most marine organisms disperse via ocean currents as larvae, so it is often assumed that larval-stage duration is the primary determinant of geographic range size. However, empirical tests of this relationship have yielded mixed results, and alternative hypotheses have rarely been considered. Here we assess the relative influence of adult and larval-traits on geographic range size using a global dataset encompassing 590 species of tropical reef fishes in 47 families, the largest compilation of such data to date for any marine group. We analyze this database using linear mixed-effect models to control for phylogeny and geographical limits on range size. Our analysis indicates that three adult traits likely to affect the capacity of new colonizers to survive and establish reproductive populations (body size, schooling behavior, and nocturnal activity) are equal or better predictors of geographic range size than pelagic larval duration. We conclude that adult life-history traits that affect the postdispersal persistence of new populations are primary determinants of successful range extension and, consequently, of geographic range size among tropical reef fishes.Geographic range size is a fundamental biogeographic variable that, among other effects (1, 2), strongly influences a species susceptibility to extinction (3, 4). Because most marine organisms disperse as larval propagules transported by ocean currents, it is often assumed that the duration of the larval stage is the fundamental determinant of their dispersal ability, and hence their range size (5, 6). Tropical reef fishes have geographic ranges that vary greatly in size, from a few square kilometers around tiny isolated islands to entire ocean basins (7–9). Given that pelagic larval duration (PLD) also varies greatly among such fishes, from only a few days to many months, the effects of PLD on dispersal potential became an early focus of investigation on general determinants of range size among those fishes and other near-shore marine species (10–12). However, although it has become evident that PLD is unlikely to be a primary determinant of geographic range size (13–16), alternative hypotheses have only recently begun to be considered (9).To expand its geographic range, a species must successfully colonize new areas following the dispersal of its propagules (17). Consequently, attributes other than pelagic dispersal capacity may largely determine how widely reef fishes are distributed geographically (9). Here we assess the relative importance of seven adult and larval traits in influencing geographic range sizes of tropical reef fishes at the global scale. We do so using data from 590 species of tropical reef fishes in 47 families, the largest compilation of such data currently available for any marine group (Dataset S1). Traits directly linked to larval dispersal potential include PLD and spawning mode. Adult traits include maximum body size, schooling behavior, nocturnal activity, use of multiple habitat types, and adult depth range. The adult-biology traits chosen are not directly related to larval dispersal potential, but may influence the propensity for range expansion by affecting the establishment and persistence of new populations, as suggested by a recent study on Atlantic reef fishes (9). For example, schooling (18, 19) and nocturnal activity (20) reduce predation risk and thereby increase the chance of postsettlement survival. Broad habitat use and depth range indicate ecological generality, which is thought to influence establishment success in new environments (21). Finally, body size is linked to both predation risk and ecological generality (22).Evaluation of these hypotheses is challenging because species traits are phylogenetically nonindependent (23) and unevenly distributed among families. Previous studies of dispersal–range-size relationships have controlled for effects of phylogeny, and limits on range-size imposed by ocean-basin size, by separately analyzing subsets of data (7, 16). However, this approach reduces statistical power (23, 24) and the ability to assess the generality of the effects of different factors. Our analysis controls for the nonindependence of shared traits among related species by using linear mixed-effects modeling (LMM) treating family and genus as nested random effects (9, 23). Our analysis includes species from three different regions that vary greatly in maximum (longitudinal or latitudinal) extent: the Indo-Central Pacific (ICP; ∼22,000 km), the tropical Atlantic (TA; ∼12,000 km), and the tropical eastern Pacific (TEP; ∼5,000 km). To control for this variation, we include region and its interactions with other variables as fixed effects in our models. Modeling the data in this way, we are unique in being able to assess the relative importance of various adult and larval traits as determinants of range size among tropical reef-fish, as a group, at the global scale.     

Persistent predator-prey dynamics revealed by mass extinction

Predator-prey interactions are thought by many researchers to define both modern ecosystems and past macroevolutionary events. In modern ecosystems, experimental removal or addition of taxa is often used to determine trophic relationships and predator identity. Both characteristics are notoriously difficult to infer in the fossil record, where evidence of predation is usually limited to damage from failed attacks, individual stomach contents, one-sided escalation, or modern analogs. As a result, the role of predation in macroevolution is often dismissed in favor of competition and abiotic factors. Here we show that the end-Devonian Hangenberg event (359 Mya) was a natural experiment in which vertebrate predators were both removed and added to an otherwise stable prey fauna, revealing specific and persistent trophic interactions. Despite apparently favorable environmental conditions, crinoids diversified only after removal of their vertebrate consumers, exhibiting predatory release on a geological time scale. In contrast, later Mississippian (359-318 Mya) camerate crinoids declined precipitously in the face of increasing predation pressure from new durophagous fishes. Camerate failure is linked to the retention of obsolete defenses or "legacy adaptations" that prevented coevolutionary escalation. Our results suggest that major crinoid evolutionary phenomena, including rapid diversification, faunal turnover, and species selection, might be linked to vertebrate predation. Thus, interactions observed in small ecosystems, such as Lotka-Volterra cycles and trophic cascades, could operate at geologic time scales and higher taxonomic ranks. Both trophic knock-on effects and retention of obsolete traits might be common in the aftermath of predator extinction.     

Abrupt climate change and collapse of deep-sea ecosystems

We investigated the deep-sea fossil record of benthic ostracodes during periods of rapid climate and oceanographic change over the past 20,000 years in a core from intermediate depth in the northwestern Atlantic. Results show that deep-sea benthic community “collapses” occur with faunal turnover of up to 50% during major climatically driven oceanographic changes. Species diversity as measured by the Shannon–Wiener index falls from 3 to as low as 1.6 during these events. Major disruptions in the benthic communities commenced with Heinrich Event 1, the Inter-Allerød Cold Period (IACP: 13.1 ka), the Younger Dryas (YD: 12.9–11.5 ka), and several Holocene Bond events when changes in deep-water circulation occurred. The largest collapse is associated with the YD/IACP and is characterized by an abrupt two-step decrease in both the upper North Atlantic Deep Water assemblage and species diversity at 13.1 ka and at 12.2 ka. The ostracode fauna at this site did not fully recover until ≈8 ka, with the establishment of Labrador Sea Water ventilation. Ecologically opportunistic slope species prospered during this community collapse. Other abrupt community collapses during the past 20 ka generally correspond to millennial climate events. These results indicate that deep-sea ecosystems are not immune to the effects of rapid climate changes occurring over centuries or less.     

Trait similarity in reef fish faunas across the world’s oceans

Species’ traits, rather than taxonomic identities, determine community assembly and ecosystem functioning, yet biogeographic patterns have been far less studied for traits. While both environmental conditions and evolutionary history shape trait biogeography, their relative contributions are largely unknown for most organisms. Here, we explore the global biogeography of reef fish traits for 2,786 species from 89 ecoregions spanning eight marine realms with contrasting environmental conditions and evolutionary histories. Across realms, we found a common structure in the distribution of species traits despite a 10-fold gradient in species richness, with a defined “backbone” of 21 trait combinations shared by all realms globally, both temperate and tropical. Across ecoregions, assemblages under similar environmental conditions had similar trait compositions despite hosting drastically different species pools from separate evolutionary lineages. Thus, despite being separated by thousands of kilometers and millions of years of evolution, similar environments host similar trait compositions in reef fish assemblages worldwide. Our findings suggest that similar trait-based management strategies can be applied among regions with distinct species pools, potentially improving conservation outcomes across diverse jurisdictions.

Biogeographic patterns reflect how past and current environmental conditions along with evolutionary history have shaped biodiversity, ecosystem functioning, and the ecosystem services human societies depend on. While biogeography has historically focused on species composition (1), species traits (morphological, physiological, or behavioral features of organisms) are increasingly recognized as the main drivers of community assembly and ecosystem functioning (2–5). Trait composition has been shown to mediate species’ interactions (6), shape ecological niches (7), determine species’ responses to environmental fluctuations (8), and govern species’ influences on key ecological processes like nutrient cycling (9, 10) and biomass production (11).Large-scale variation in trait composition is shaped by biotic (e.g., trophic interactions) and abiotic constraints (e.g., environmental filtering) under the influence of evolutionary and biogeographic processes (i.e., speciation, extinction, and immigration), yet global patterns and processes in trait biogeography are poorly known for most organisms. In marine ecosystems, how environmental constraints and evolutionary history have shaped species diversity has been broadly debated (12, 13), and recent studies suggest that both adaptation to regional climate (14) and geographical expansion of clades (15) have influenced global patterns in marine biodiversity. On shallow rocky and coral reefs, large-scale environmental gradients and evolutionary history have led to ocean basins with vastly different species richness and composition (13, 16), yet the distribution and drivers of associated trait composition are still unclear.Here, using data collected through the Reef Life Survey (RLS) (17) (http://www.reeflifesurvey.com/) from 89 ecoregions spanning eight marine realms in both temperate and tropical oceans, we examined global biogeographic patterns in reef fish trait composition and evaluated whether trait composition is shaped primarily by the environment, taxonomic relatedness, or evolutionary history. Specifically, we examined patterns of species distribution in trait space relative to environmental conditions and taxonomic and phylogenetic composition.We explicitly considered two spatial scales—marine realms and marine ecoregions—for which we examined distinct questions. Marine realms are large areas of ocean basins for which “biotas are internally coherent at higher taxonomic levels, as a result of a shared and unique evolutionary history” (18) and are shaped by major differences in environmental regimes and historical or broadscale isolation. Given their distinct biotas, environmental regimes, species richness, and evolutionary histories, we began by examining how realms differed in trait composition and redundancy. For instance, we examined whether temperate and tropical oceans contained similar trait diversity and trait proportions and whether certain trait combinations existed in all realms. We next examined ecoregion-scale patterns to assess regional variation in trait composition and redundancy and whether trait composition varied predictably within realms. Ecoregions are smaller areas of “relatively homogeneous species composition,” (18) which are shaped by distinct oceanographic and environmental conditions. We hypothesized that regional species assemblages would show similar trait composition under similar environmental conditions worldwide regardless of species composition or shared evolutionary history.     

Ecological convergence in a rocky intertidal shore metacommunity despite high spatial variability in recruitment regimes

In open ecological systems, community structure can be determined by physically modulated processes such as the arrival of individuals from a regional pool and by local biological interactions. There is debate centering on whether niche differentiation and local interactions among species are necessary to explain macroscopic community patterns or whether the patterns can be generated by the neutral interplay of dispersal and stochastic demography among ecologically identical species. Here we evaluate how much of the observed spatial variation within a rocky intertidal metacommunity along 800 km of coastline can be explained by drift in the structure of recruits across 15 local sites. Our results show that large spatial changes in recruitment do not explain the observed spatial variation in adult local structure and that, in comparison with the large drift in structure of recruits, local adult communities converged to a common, although not unique, structure across the region. Although there is no unique adult community structure in the entire region, the observed variation represents only a small subset of the possible structures that would be expected from passive recruitment drift. Thus, in this diverse system our results do not support the idea that rocky intertidal metacommunities are structured by neutral mechanisms.     

Global environmental predictors of benthic marine biogeographic structure

Analyses of how environmental factors influence the biogeographic structure of biotas are essential for understanding the processes underlying global diversity patterns and for predicting large-scale biotic responses to global change. Here we show that the large-scale geographic structure of shallow-marine benthic faunas, defined by existing biogeographic schemes, can be predicted with 89-100% accuracy by a few readily available oceanographic variables; temperature alone can predict 53-99% of the present-day structure along coastlines. The same set of variables is also strongly correlated with spatial changes in species compositions of bivalves, a major component of the benthic marine biota, at the 1° grid-cell resolution. These analyses demonstrate the central role of coastal oceanography in structuring benthic marine biogeography and suggest that a few environmental variables may be sufficient to model the response of marine biogeographic structure to past and future changes in climate.     

Applying a regional community concept to forest birds of eastern North America

The regional community concept embraces the idea that species interactions across large areas shape both the geographic/ecological distributions and the local abundances of populations. Within this framework, I analyzed the distribution and abundance of 79 species of land birds across 142 ca. 10-ha census plots from standardized breeding bird censuses in deciduous and mixed forests of eastern North America. To characterize the regional ecological space, plots were ordinated on the basis of species abundances. Within the regional community defined by these synthetic axes, the distribution and abundance of individual species did not appear to be shaped by competition or to reflect the adaptations of individuals: (i) local abundance and population extent across the ordination axes were unrelated, (ii) pairwise correlation coefficients of species abundances were centered on 0, (iii) average species distribution and abundance were independent of the number of close relatives, and (iv) distribution and abundance exhibited no evolutionary (phylogenetic) conservatism. To explain these seemingly random patterns, I speculate that species are approximately evenly matched competitors over much of the region and that their distributions and relative abundances are determined by the labile coevolutionary outcomes of interactions with specialized pathogens. Thus, despite the appearance that random processes determine patterns in the distribution and abundance of populations in the regional community, it is plausible that species-specific deterministic interactions are responsible. Although competition is a dominant force in ecological communities, variation in the distribution and abundance of individual species might instead reflect the outcome of interactions with specialized antagonists, including pathogens.