Hot,dry and different: Australian lizard richness is unlike that of mammals,amphibians and birds

Aims (1) To map the species richness of Australian lizards and describe patterns of range size and species turnover that underlie them. (2) To assess the congruence in the species richness of lizards and other vertebrate groups. (3) To search for commonalities in the drivers of species richness in Australian vertebrates. Location Australia. Methods We digitized lizard distribution data to generate gridded maps of species richness and β‐diversity. Using similar maps for amphibians, mammals and birds, we explored the relationship between species richness and temperature, actual evapotranspiration, elevation and local elevation range. We used spatial eigenvector filtering and geographically weighted regression to explore geographical patterns and take spatial autocorrelation into account. We explored congruence between the species richness of vertebrate groups whilst controlling for environmental effects. Results Lizard richness peaks in the central deserts (where β‐diversity is low) and tropical north‐east (where β‐diversity is high). The intervening lowlands have low species richness and β‐diversity. Generally, lizard richness is uncorrelated with that of other vertebrates but this low congruence is strongly spatially structured. Environmental models for all groups also show strong spatial heterogeneity. Lizard richness is predicted by different environmental factors from other vertebrates, being highest in dry and hot regions. Accounting for environmental drivers, lizard richness is weakly positively related to richness of other vertebrates, both at global and local scales. Main conclusions Lizard species richness differs from that of other vertebrates. This difference is probably caused by differential responses to environmental gradients and different centres of diversification; there is little evidence for inter‐taxon competition limiting lizard richness. Local variation in habitat diversity or evolutionary radiations may explain weak associations between taxa, after controlling for environmental variables. We strongly recommend that studies of variation in species richness examine and account for non‐stationarity.     

Patterns of functional diversity across an extensive environmental gradient: vertebrate consumers, hidden treatments and latitudinal trends

Over the last two decades, although much has been learned regarding the multifaceted nature of biodiversity, relatively little is known regarding spatial variation in constituents other than species richness. This is particularly true along extensive environmental gradients such as latitude. Herein, we describe latitudinal gradients in the functional diversity of New World bat communities. Bat species from each of 32 communities were assigned to one of seven functional groups. Latitudinal gradients existed for the richness, diversity and scaled‐dominance of functional groups. No significant patterns were observed for evenness of functional groups. Measures of functional diversity were different in magnitude and increased towards the equator at a faster rate than expected given the underlying spatial variation in species richness. Thus, latitudinal gradient in species richness alone do not cause the latitudinal gradient in functional diversity. When variation in species composition of the regional fauna of each community was incorporated into analyses, many differences between observed and simulated patterns of functional diversity were not significant. This suggests that those processes that determine the composition of regional faunas strongly influence the latitudinal gradient in functional diversity at the local level. Nonetheless, functional diversity was lower than expected across observed sites. Community‐wide responses to variation in the quantity and quality of resources at the local level probably contribute to differences in functional diversity at local and regional scales and enhance beta diversity.     

Functional dependence underlies a positive plant-grasshopper richness relationship

A central focus of ecology is identifying the factors that shape spatial patterns of species diversity and this is particularly relevant in an era of global change. Positive relationships between plant and consumer diversity are common, but could be driven by direct responses of each trophic level to underlying environmental gradients, or indirectly where changes in environmental conditions propagate through food webs. Here we use structural equation modeling to examine the relative importance of soil resource availability and disturbance (fire) in mediating relationships between plant and grasshopper richness in insular grasslands. We found a positive relationship between plant and consumer richness that became stronger after accounting for disturbance, despite unique responses of plants and consumers to the two environmental gradients. Plant richness responded to an underlying gradient in soil resource availability. Time since the last fire had a direct positive effect on grasshopper richness but had no effect on plant richness. This work supports that plant and consumer richness are functionally linked, rather than having similar responses to environmental gradients. By disentangling the direct and indirect processes underlying a positive relationship between plant and consumer diversity in a natural system that spans multiple environmental gradients, we demonstrate the importance of investigating biodiversity through explicit multivariate models.     

Macroecological explanations for differences in species richness gradients: a canonical analysis of South American birds

Aim To evaluate how spatial variation of species richness in different bird orders responds to environmental gradients and determine which order level trait best predicts these relationships. Location South America. Methods A canonical correlation analysis was performed between the species richness in each of 17 bird orders and eight environmental variables in 374, 220 × 220 km cells. Loadings associated with the first two canonical variables were regressed against six order‐level predictors, including diversification level (number of species in each order), body size, median geographical range size and characteristics included in the model to control Type I error rates (the phylogenetic relationship among orders and levels of local‐scale spatial autocorrelation). Results Richness patterns of 14 bird orders were highly correlated with the first canonical axis, indicating that most orders respond similarly to energy‐water gradients (primarily actual evapotranspiration, minimum temperature and potential evapotranspiration). In contrast, species richness within Trochiliformes, Apodiformes and Galliformes were also correlated with the second canonical variable, representing measures of mesoscale climatic variation (range in elevation within cells, minimum temperature, and the interaction term between them) and landcover (habitat diversity). We also found that total diversification within orders was the best predictor of the loadings associated with the first canonical axis, whereas body size of each order best predicted loadings on the second axis. Conclusion Our results broadly support climatic‐related hypotheses as explanations for spatial variation in species richness of different orders. However, both historical (order‐specific variation in speciation rates) and ecological (dispersal of species that evolved by independent processes into areas amenable to birds) processes can explain the relationship between order level traits, such as body size and diversification level, and magnitude of response to current environment, furnishing then guidelines for a further and deeper understanding of broad‐scale diversity gradients.     

Environmental and geometric drivers of small mammal diversity along elevational gradients in Utah

The mechanisms shaping patterns of biodiversity along spatial gradients remain poorly known and controversial. Hypotheses have emphasized the importance of both environmental and spatial factors. Much of the uncertainty about the relative role of these processes can be attributed to the limited number of comparative studies that evaluate multiple potential mechanisms. This study examines the relative importance of six variables: temperature, precipitation, productivity, habitat heterogeneity, area, and the mid-domain effect on patterns of species richness for non-volant small mammals along four neighboring mountain ranges in central Utah. Along each of these elevational gradients, a hump-shaped relationship of richness with elevation is evident. This study evaluates whether the processes shaping this common pattern are also common to all gradients. Model selection indicates that no one factor or set of factors best explains patterns of species richness across all gradients, and drivers of diversity may vary seasonally. These findings suggest that commonality in the pattern of species richness, even among elevational gradients with a similar biogeographic history and fauna, cannot be attributed to a simple universal explanation.     

Disentangling the Role of Climate,Topography and Vegetation in Species Richness Gradients

Environmental gradients (EG) related to climate, topography and vegetation are among the most important drivers of broad scale patterns of species richness. However, these different EG do not necessarily drive species richness in similar ways, potentially presenting synergistic associations when driving species richness. Understanding the synergism among EG allows us to address key questions arising from the effects of global climate and land use changes on biodiversity. Herein, we use variation partitioning (also know as commonality analysis) to disentangle unique and shared contributions of different EG in explaining species richness of Neotropical vertebrates. We use three broad sets of predictors to represent the environmental variability in (i) climate (annual mean temperature, temperature annual range, annual precipitation and precipitation range), (ii) topography (mean elevation, range and coefficient of variation of elevation), and (iii) vegetation (land cover diversity, standard deviation and range of forest canopy height). The shared contribution between two types of EG is used to quantify synergistic processes operating among EG, offering new perspectives on the causal relationships driving species richness. To account for spatially structured processes, we use Spatial EigenVector Mapping models. We perform analyses across groups with distinct dispersal abilities (amphibians, non-volant mammals, bats and birds) and discuss the influence of vagility on the partitioning results. Our findings indicate that broad scale patterns of vertebrate richness are mainly affected by the synergism between climate and vegetation, followed by the unique contribution of climate. Climatic factors were relatively more important in explaining species richness of good dispersers. Most of the variation in vegetation that explains vertebrate richness is climatically structured, supporting the productivity hypothesis. Further, the weak synergism between topography and vegetation urges caution when using topographic complexity as a surrogate of habitat (vegetation) heterogeneity.     

Environmental correlates of leguminosae species richness in Mexico: Quantifying the contributions of energy and environmental seasonality

Explaining species richness patterns is a central issue in ecology, but a general explanation remains elusive. Environmental conditions have been proposed to be important drivers of these patterns, but we still need to better understand the relative contribution of environmental factors. Here, we aim at testing two environmental hypotheses for richness gradients: energy availability and environmental seasonality using diversity patterns of the family Leguminosae across Mexico. We compiled a data base of 502 species and 32,962 records. After dividing Mexico into 100 × 100 km grid cells, we constructed a map of variation in species richness that accounts for heterogeneity in sampling effort. We found the cells with the highest species richness of legumes are in the Neotropical region of Pacific coastal and southern Mexico, where the legume family dominates the tropical rain forests and seasonally dry tropical forests. Regression models show that energy and seasonality predictors can explain 25% and 49% of the variation in richness, respectively. Spatial autocorrelation analysis showed that richness has a strong spatial structure, but that most of this structure disappears when both energy and seasonality are used to account for richness gradient. Our study demonstrates multiple environmental conditions contribute complementarily to explain diversity gradients. Moreover, it shows that in some regions, environmental seasonality can be more important than energy availability, contradicting studies at coarser spatial scales. More basic taxonomic and floristic work is needed to help describe patterns of diversity for many groups to allow for testing the underlying mechanisms responsible for diversity gradients. Abstract in Spanish is available with online material.     

Diversity and distribution of small mammals in the South American Dry Andes

The Andean mountain range has played an important role in the evolution of South American biota. However, there is little understanding of the patterns of species diversity across latitudinal and altitudinal gradients. In this paper, we examine the diversity of small mammals along the South Central Dry Andes (SCDA) within the framework of two contrasting hypotheses: (a) species richness decreases with increasing elevation and latitude; and (b) species richness peaks at altitudinal midpoints (mid‐domain). We explore the composition of the species pool, the impact of species–area relationships and the Rapoport effect (i.e. size of geographic ranges) along latitudinal and elevational gradients. First, we constructed a database of SCDA small mammals. Then, species richness patterns were analysed through generalized models, and species–area relationships were assessed by log–log regressions; the curvilinear method (c = S/Az) was use to compute richness corrected by area size. Lastly, the Rapoport effect was evaluated using the midpoint method. Our results show: (1) a richness of 67 small mammals along the SCDA, of which 36 are endemic; (2) a hump‐shaped pattern in species richness along elevation and latitudinal gradients; (3) a species–area relationship for both gradients; (4) endemic species corrected by area present a strong and positive relationship with elevation; (5) a Rapoport effect for the latitudinal ranges, but no effect across the elevational gradient; and (6) a major species turnover between 28° and 30° south latitude. This is the first study quantifying the diversity of small mammals encompassing the central Andean region. Overall, our macrogeographic analysis supports the previously postulated role of the Andes in the diversification of small mammals (i.e. in situ cladogenesis) and highlights some basic attributes (i.e. anatomy of geographic ranges; species–area relationships) when considering the consequences of climate change on biodiversity conservation of mountain ecosystems.     

Different evolutionary histories underlie congruent species richness gradients of birds and mammals

Aim The global species richness patterns of birds and mammals are strongly congruent. This could reflect similar evolutionary responses to the Earth’s history, shared responses to current climatic conditions, or both. We compare the geographical and phylogenetic structures of both richness gradients to evaluate these possibilities. Location Global. Methods Gridded bird and mammal distribution databases were used to compare their species richness gradients with the current environment. Phylogenetic trees (resolved to family for birds and to species for mammals) were used to examine underlying phylogenetic structures. Our first prediction is that both groups have responded to the same climatic gradients. Our phylogenetic predictions include: (1) that both groups have similar geographical patterns of mean root distance, a measure of the level of the evolutionary development of faunas, and, more directly, (2) that richness patterns of basal and derived clades will differ, with richness peaking in the tropics for basal clades and in the extra‐tropics for derived clades, and that this difference will hold for both birds and mammals. We also explore whether alternative taxonomic treatments for mammals can generate patterns matching those of birds. Results Both richness gradients are associated with the same current environmental gradients. In contrast, neither of our evolutionary predictions is met: the gradients have different phylogenetic structures, and the richness of birds in the lowland tropics is dominated by many basal species from many basal groups, whereas mammal richness is attributable to many species from both few basal groups and many derived groups. Phylogenetic incongruence is robust to taxonomic delineations for mammals. Main conclusions Contemporary climate can force multiple groups into similar diversity patterns even when evolutionary trajectories differ. Thus, as widely appreciated, our understanding of biodiversity must consider responses to both past and present climates, and our results are consistent with predictions that future climate change will cause major, correlated changes in patterns of diversity across multiple groups irrespective of their evolutionary histories.     

Small mammal species richness and turnover along elevational gradient in Yulong Mountain,Yunnan, Southwest China

Understanding the species diversity patterns along elevational gradients is critical for biodiversity conservation in mountainous regions. We examined the elevational patterns of species richness and turnover, and evaluated the effects of spatial and environmental factors on nonvolant small mammals (hereafter “small mammal”) predicted a priori by alternative hypotheses (mid‐domain effect [MDE], species–area relationship [SAR], energy, environmental stability, and habitat complexity]) proposed to explain the variation of diversity. We designed a standardized sampling scheme to trap small mammals at ten elevational bands across the entire elevational gradient on Yulong Mountain, southwest China. A total of 1,808 small mammals representing 23 species were trapped. We observed the hump‐shaped distribution pattern of the overall species richness along elevational gradient. Insectivores, rodents, large‐ranged species, and endemic species richness showed the general hump‐shaped pattern but peaked at different elevations, whereas the small‐ranged species and endemic species favored the decreasing richness pattern. The MDE and the energy hypothesis were supported, whereas little support was found for the SAR, the environmental stability hypothesis, and the habitat complexity. However, the primary driver(s) for richness patterns differed among the partitioning groups, with NDVI (the normalized difference vegetation index) and MDE being the most important variables for the total richness pattern. Species turnover for all small mammal groups increased with elevation, and it supported a decrease in community similarity with elevational distance. Our results emphasized for increased conservation efforts in the higher elevation regions of the Yulong Mountain.     

Elevational gradients in ant species richness: area, geometry, and Rapoport's rule

Studying the distributions of plants and animals along environmental gradients can illuminate the factors governing and maintaining species diversity. There are two general predictions of how species richness and elevation are related: either species richness decreases monotonically with increasing elevation or richness peaks at mid-elevations. Several processes might contribute to this pattern. In this paper, I examine patterns in ant species richness along elevational gradients in three states in the western US: Colorado, Nevada, and Utah. I test for the effects of available area and the geometric constraints model on species richness patterns. I also test Rapoport's rescue hypothesis, which relates the extent of species' elevational ranges to patterns in species richness. In each state, species richness peaked at mid-elevations. Area explained more variation in species richness than the geometric constraints model in Colorado and Utah, but not in Nevada. Area and geometric constraints together explained 90%, 99%, and 57% of the variation in species richness in Colorado, Nevada, and Utah, respectively. Even though there were peaks at mid-elevations, I still found a strong Rapoport effect. This work suggests that the influences of area and geometric constraints cannot be overlooked when examining patterns in species richness along environmental gradients.     

Vegetation heterogeneity and diversity in flat and mountain landscapes of Patagonia (Argentina)

Abstract. We studied floristic and diversity patterns and their environmental controls in two landscapes of contrasting topography in the Patagonian steppe. The analyses were focused on the effects of water availability gradients and landscape configuration on plant species distribution and coexistence. Floristic variation was investigated using Correspondence Analysis. The relationship between floristic and environmental variation was analyzed using Canonical Correspondence Analysis and correlation tests. We explored diversity patterns by relating spatial distance to floristic dissimilarities. The floristic gradient was determined by shrub and grass species and was related to precipitation in the flat area, and to precipitation, elevation and potential radiation in the mountain area. Site species richness increased with water availability in both areas. Mean site species richness and species turnover in space was higher in the mountain than in the flat area. Landscape species richness and floristic gradients were more concentrated in the mountain than in the flat area. In contrast to shrubs and grasses, forb species distributions were uncoordinated and probably independent of any environmental gradient. Our results suggest (1) that landscape configuration affects species composition and diversity through its direct effect on abiotic environmental heterogeneity, and (2) that the environmental controls of the community composition vary depending on the plant functional type considered.     

Ecological biogeography of Mexican bats: the relative contributions of habitat heterogeneity,beta diversity,and environmental gradients to species richness and composition patterns

Mexico has higher mammalian diversity than expected for its size and geographic position. High environmental hetero geneity throughout Mexico is hypothesized to promote high turnover rates (β‐diversity), thus contributing more to observed species richness and composition than within‐habitat (α) diversity. This is true if species are strongly associated with their environments, such that changes in environmental attributes will result in changes in species composition. Also, greater heterogeneity in an area will result in greater species richness. This hypothesis has been deemed false for bats, as their ability to fly would reduce opportunities for habitat specialization. If so, we would expect no significant relationships between 1) species composition and environmental variables, 2) species richness and environmental heterogeneity, 3) β‐diversity and environmental heterogeneity. We tested these predictions using 31 bat assemblages distributed across Mexico. Using variance partitioning we evaluated the relative contribution of vegetation, climate, elevation, horizontal heterogeneity (a variate including vegetation, climate, and elevational heterogeneity), spatial variation (lat‐long), and vertical hetero geneity (of vegetation strata) to variation in bat species composition and richness. Variation in vegetation explained 92% of the variation in species composition and was correlated with all other variables examined, indicating that bats respond directly to habitat composition and structure. Beta‐diversity and vegetational heterogeneity were significantly correlated. Bat species richness was significantly correlated with vertical, but not horizontal, heterogeneity. Nonetheless, neither horizontal nor vertical heterogeneity were random; both were related to latitude and to elevation. Variation in bat community composition and richness in Mexico were primarily explained by local landscape heterogeneity and environmental factors. Significant relationships between β‐diversity and environmental variation reveal differences in habitat specialization by bats, and explain their high diversity in Mexico. Understanding mechanisms acting along environmental or geographic gradients is as important for understanding spatial variation in community composition as studying mechanisms that operate at local scales.     

Relative roles of ecological and energetic constraints,diversification rates and region history on global species richness gradients

Regions worldwide differ markedly in species richness. Here, for birds and mammals worldwide, we directly compare four sets of hypotheses regarding geographical richness gradients: (1) evolutionary, emphasising heterogeneity in diversification rates, (2) historical, related to differences in region ages and sizes, (3) energetic, associated with variation in productive or ambient energy and (4) ecological, reflecting differences in ecological niche diversity. Among highly independent regions, or ‘evolutionary arenas’, we find that richness is weakly influenced by richness‐standardised ecological niche diversity, questioning the significance of ecological constraints for producing large‐scale diversity gradients. In contrast, we find strong evidence for the importance of region area and its changes over time, together with a role for temperature. These predictors affect richness predominately directly without concomitant positive effects on diversification rates. This suggests that regional richness is governed by historical and evolutionary processes, which promote region‐specific accumulation of diversity through time or following asymmetrical dispersal.     

Environmental determinants of amphibian and reptile species richness in China

Understanding the factors that regulate geographical variation in species richness has been one of the fundamental questions in ecology for decades, but our knowledge of the cause of geographical variation in species richness remains poor. This is particularly true for herpetofaunas (including amphibians and reptiles). Here, using correlation and regression analyses, we examine the relationship of herpetofaunal species richness in 245 localities across China with 30 environmental factors, which include nearly all major environmental factors that are considered to explain broad-scale species richness gradients in such theories as ambient energy, water–energy dynamics, productivity, habitat heterogeneity, and climatic stability. We found that the species richness of amphibians and reptiles is moderately to strongly correlated with most of the environmental variables examined, and that the best fit models, which include explanatory variables of temperature, precipitation, net primary productivity, minimum elevation, and range in elevation, explain ca 70% the variance in species richness for both amphibians and reptiles after accounting for sample area. Although water and temperature are important explanatory variables to both amphibians and reptiles, water variables explain more variance in amphibian species richness than in reptile species richness whereas temperature variables explain more variance in reptile species richness than in amphibian species richness, which is consistent with different physiological requirements of the two groups of organisms.     

Effects of neutrality and productivity on mammal richness and evolutionary history in Australia

Explaining how heterogeneous spatial patterns of species diversity emerge is one of the most fascinating questions of biogeography. One of the great challenges is revealing the mechanistic effect of environmental variables on diversity. Correlative analyses indicate that productivity is associated with taxonomic, phylogenetic, and functional diversity of communities. Surprisingly, no unifying body of theory have been developed to understand the mechanism by which spatial variation of productivity affects the fundamental processes of biodiversity. Based on widely discussed verbal models in ecology about the effect of productivity on species diversity, we developed a spatially explicit neutral model that incorporates the effect of primary productivity on community size and confronted our model's predictions with observed patterns of species richness and evolutionary history of Australian terrestrial mammals. The imposed restrictions on community size create larger populations in areas of high productivity, which increases community turnover and local speciation, and reduces extinction. The effect of productivity on community size modeled in our study causes higher accumulation of species diversity in productive regions even in the absence of niche‐based processes. However, such a simple model is not capable of reproducing spatial patterns of mammal evolutionary history in Australia, implying that more complex evolutionary mechanisms are involved. Our study demonstrates that the overall patterns of species richness can be directly explained by changes in community sizes along productivity gradients, supporting a major role of processes associated with energetic constraints in shaping diversity patterns.     

Dark diversity reveals importance of biotic resources and competition for plant diversity across habitats

Species richness is the most commonly used metric to quantify biodiversity. However, examining dark diversity, the group of missing species which can potentially inhabit a site, can provide a more thorough understanding of the processes influencing observed biodiversity and help evaluate the restoration potential of local habitats. So far, dark diversity has mainly been studied for specific habitats or large‐scale landscapes, while less attention has been given to variation across broad environmental gradients or as a result of local conditions and biotic interactions. In this study, we investigate the importance of local environmental conditions in determining dark diversity and observed richness in plant communities across broad environmental gradients. Using the ecospace concept, we investigate how these biodiversity measures relate to abiotic gradients (defined as position), availability of biotic resources (defined as expansion), spatiotemporal extent of habitats (defined as continuity), and species interactions through competition. Position variables were important for both observed diversity and dark diversity, some with quadratic relationships, for example, plant richness showing a unimodal response to soil fertility corresponding to the intermediate productivity hypothesis. Interspecific competition represented by community mean Grime C had a negative effect on plant species richness. Besides position‐related variables, organic carbon was the most important variable for dark diversity, indicating that in late‐succession habitats such as forests and shrubs, dark diversity is generally low. The importance of highly competitive species indicates that intermediate disturbance, such as grazing, may facilitate higher species richness and lower dark diversity.     

Searching for biodiversity indicators in running waters: do bryophytes, macroinvertebrates, and fish show congruent diversity patterns?

The degree to which different taxonomic groups show congruence in diversity patterns has attracted increased attention, yet such studies on stream biota are lacking. We examined environmental correlates of and congruence in the species richness patterns of bryophytes, macroinvertebrates, and fish in 101 boreal streams in Finland. Congruence in species richness among the taxonomic groups was generally low, mainly because of their differing responses to major environmental gradients. Bryophytes and macroinvertebrates showed the strongest degree of congruence, but even this relationship had a relatively weak predictive power. Bryophyte diversity showed the strongest relationship with water colour, followed by habitat stability, and stream size. Macroinvertebrate diversity increased with stream size, and further variation was accounted for by water colour and acidity. Fish species richness showed a weak and complex relationship with geographical location, stream size, and in-stream habitat characteristics. The regression models explained 23, 45, and 26% of the variation in species richness of bryophytes, macroinvertebrates, and fish, respectively. Our results suggest that indicator taxa may be of limited value in stream biodiversity inventories. Habitat-based approaches are suggested as an alternative surrogate measure in the conservation evaluation of lotic biodiversity.     

Can stochastic geographical evolution re‐create macroecological richness–environment correlations?

Aim Richness gradients are frequently correlated with environmental characteristics at broad geographic scales. In particular, richness is often associated with energy and climate, while environmental heterogeneity is rarely its best correlate. These correlations have been interpreted as evidence in favour of environmental determinants of diversity gradients, particularly energy and climate. This interpretation assumes that the expected‐by‐random correlation between richness and environment is zero, and that this is equally true for all environmental characteristics. However, these expectations might be unrealistic. We investigated to what degree basic evolutionary/biogeographical processes occurring independently of environment could lead to richness gradients that correlate with environmental characteristics by chance alone. Location Africa, Australia, Eurasia and the New World. Methods We produced artificial richness gradients based on a stochastic simulation model of geographic diversification of clades. In these simulations, species speciate, go extinct and expand or shift their distributions independently of any environmental characteristic. One thousand two hundred repetitions of this model were run, and the resulting stochastic richness gradients were regressed against real‐world environmental variables. Stochastic species–environment relationships were then compared among continents and among three environmental characteristics: energy, environmental heterogeneity and climate seasonality. Results Simulations suggested that a significant degree of correlation between richness gradients and environment is expected even when clades diversify and species distribute stochastically. These correlations vary considerably in strength; but in the best cases, environment can spuriously account for almost 80% of variation in stochastic richness. Additionally, expected‐by‐chance relationships were different among continents and environmental characteristics, producing stronger spurious relationships with energy and climate than with heterogeneity. Main conclusions We conclude that some features of empirical species–environment relationships can be reproduced just by chance when taking into account evolutionary/biogeographical processes underlying the construction of species richness gradients. Future tests of environmental effects on richness should consider structure in richness–environment correlations that can be produced by simple evolutionary null models. Research should move away from the naive non‐biological null hypotheses that are implicit in traditional statistical tests.     

Untangling latitudinal richness gradients at higher taxonomic levels: familial perspectives on the diversity of New World bat communities

Aims (i) To describe at the level of local communities latitudinal gradients in the species richness of different families of New World bats and to explore the generality of such gradients. (ii) To characterize the relative effects of changes in the richness of each family to the richness of entire communities. (iii) To determine differences in the rate and direction of latitudinal gradients in species richness within families. (iv) To evaluate how differences among families regarding latitudinal gradients in species richness influence the latitudinal gradient in species richness of entire communities. Location Continental New World ranging from the northern continental United States (Iowa, 42° N) to eastern Paraguay (Canindeyú, 24° S). Methods Data on the species composition of communities came from 32 intensively sampled sites. Analyses focused on species richness of five of nine New World bat families. Multivariate analysis of variance and discriminant function analysis determined and described differences among temperate, subtropical, and tropical climatic zones regarding the species richness of bat families. Simple linear regression described latitudinal gradients in species richness of families. Path analysis was used to describe: (i) the direct effect of latitude on species richness of communities, (ii) the indirect effects of latitude on the species richness of communities through its effect on the species richness of each family, (iii) the relative effects of latitude on the species richness of bat families, and (iv) the relative contribution of each family to variation in the species richness of communities. Results Highly significant differences among climatic zones existed primarily because of a difference between the temperate zone and the tropical and subtropical zones combined. This difference was associated with the high number of vespertilionids in the temperate zone and the high number of phyllostomids in the tropical and subtropical zones. Latitudinal gradients in species richness were contingent on phylogeny. Although only three of the five families exhibited significant gradients, all families except for the Vespertilionidae exhibited indistinguishable increases in species richness with decreases in latitude. The Emballonuridae, Phyllostomidae and Vespertilionidae exhibited significant latitudinal gradients whereby the former two families exhibited the classical increase in species richness with decreasing latitude and the latter family exhibited the opposite pattern. Variation in species richness of all families contributed significantly to variation in the species richness of entire communities. Nonetheless, the Phyllostomidae made a significantly stronger contribution to changes in species richness of communities than did all other families. Much of the latitudinal gradient in species richness of communities could be accounted for by the effects of latitude on the species richness of constituent families. Main conclusions Ecological and evolutionary differences among higher taxonomic units, particularly those differences involving life‐history traits, predispose taxa to exhibit different patterns of diversity along environmental gradients. This may be particularly true along extensive gradients such as latitude. Nonetheless, species rich taxa, by virtue of their greater absolute rates of change, can dominate and therefore define the pattern of diversity at a higher taxonomic level and eclipse differences among less represented taxa in their response to environmental gradients. This is true not only with respect to how bats drive the latitudinal gradient in species richness for all mammals, but also for how the Phyllostomidae drives the latitudinal gradient for all bats in the New World. Better understanding of the mechanistic basis of latitudinal gradients of diversity may come from comparing and contrasting patterns across lower taxonomic levels of a higher taxon and by identifying key ecological and evolutionary traits that are associated with such differences.