Can the tropical conservatism hypothesis explain temperate species richness patterns? An inverse latitudinal biodiversity gradient in the New World snake tribe Lampropeltini

Aim  A latitudinal gradient in species richness, defined as a decrease in biodiversity away from the equator, is one of the oldest known patterns in ecology and evolutionary biology. However, there are also many known cases of increasing poleward diversity, forming inverse latitudinal biodiversity gradients. As only three processes (speciation, extinction and dispersal) can directly affect species richness in areas, similar factors may be responsible for both classical (high tropical diversity) and inverse (high temperate diversity) gradients. Thus, a modified explanation for differential species richness which accounts for both patterns would be preferable to one which only explains high tropical biodiversity.
Location  The New World.
Methods  We test several proposed ecological, temporal, evolutionary and spatial explanations for latitudinal diversity gradients in the New World snake tribe Lampropeltini, which exhibits its highest biodiversity in temperate regions.
Results  We find that an extratropical peak in species richness is not explained by latitudinal variation in diversification rate, the mid-domain effect, or Rapoport's rule. Rather, earlier colonization and longer duration in the temperate zones allowing more time for speciation to increase biodiversity, phylogenetic niche conservatism limiting tropical dispersal and the expansion of the temperate zones in the Tertiary better explain inverse diversity gradients in this group.
Main conclusions  Our conclusions are the inverse of the predictions made by the tropical conservatism hypothesis to explain higher biodiversity near the equator. Therefore, we suggest that the processes invoked are not intrinsic to the tropics but are dependent on historical biogeography to determine the distribution of species richness, which we refer to as the 'biogeographical conservatism hypothesis'.     

Time and environment explain the current richness distribution of non‐marine turtles worldwide

Ecological, historical, and evolutionary hypotheses are important to explain geographical diversity gradients in many clades, but few studies have combined them into a single analysis allowing a comparison of their relative importance. This study aimed to evaluate the relative importance of ecological, historical, and evolutionary hypotheses in explaining the current global distribution of non‐marine turtles, a group whose distribution patterns are still poorly explored. We used data from distribution range maps of 336 species of non‐marine turtles, environmental layers, and phylogeny to obtain richness estimates of these animals in 2° × 2° cells and predictors related to ecological, evolutionary and historical hypotheses driving richness patterns. Then we used a path analysis to evaluate direct and indirect effects of the predictors on turtle richness. Ancestral area reconstruction was also performed in order to evaluate the influence of time‐for‐speciation in the current diversity of the group. We found that environmental variables had the highest direct effects on non‐marine turtle richness, whereas diversification rates and area available in the last 55 million yr minimally influenced turtle distributions. We found evidence for the time‐for‐speciation effect, since regions colonized early were generally richer than recently colonized regions. In addition, regions with a high number of colonization events had a higher number of turtle species. Our results suggested that ecological processes may influence non‐marine turtle richness independent of diversification rates, but they are probably related to dispersal abilities. However, colonization time was also an important component that must be taken into account. Finally, our study provided additional support for the importance of ecological (climate and productivity) and historical (time‐for‐speciation and dispersal) processes in shaping current biodiversity patterns.     

Biogeographic anomalies in the species richness of Chilean forests: Incorporating evolution into a climatic – historic scenario

Broad‐scale richness gradients are closely associated with temperature and water availability. However, historical and evolutionary processes have also contributed to shape current diversity patterns. In this paper we focus on the potential influences of Pleistocene glaciation and phylogenetic niche conservatism (the tendency for traits to be maintained during diversification) on the tree diversity gradient in Chile, and we quantify its primary climatic correlates. Tree species richness is greatest at mid latitudes, particularly in the Andes and Coastal ranges, and decreases abruptly to the south and north. Regression tree analysis identified annual precipitation and annual temperature as the primary probable drivers of this gradient. Ice cover during the Last Glacial Maximum was also identified as an ‘important’ variable, but the contemporary and historical predictors are strongly collinear. Geographically weighted regression indicated that the relationships between richness and environmental variables vary regionally: the relationship between tree richness and precipitation is stronger in north‐central Chile, whereas tree richness and temperature are most strongly associated in south‐central Chile. By assigning each species the age of the family to which it belongs and averaging all species in each geographical unit, we also found that species from the oldest families are distributed mainly in mid to high latitudes and species from younger families are distributed mainly at lower latitudes. This pattern is closely associated with annual precipitation. Thus, the ecological component of tree richness follows contemporary climatic gradients of both energy and water, but the aridification of the Atacama Desert was an important driver over evolutionary time. The influence of recent Pleistocene glaciation remains unresolved but it cannot be discounted.     

Trait evolution rates shape continental patterns of species richness in North America's most diverse angiosperm genus (Carex,Cyperaceae)

Ecological opportunity has been associated with increases in diversification rates across the tree of life. Under an ecological diversification model, the emergence of novel environments is hypothesized to promote morpho- and ecospace evolution. Whether this model holds at the clade level within the most species-rich angiosperm genus found in North America (Carex, Cyperaceae) is yet to be tested. Recent works demonstrate a temporal coupling of climate cooling and widespread colonization of Carex in North America, implicating ecological diversification. In addition, research has consistently found asymmetric patterns of lineage-level diversification in the genus. Why does variation in clade sizes exist in the genus? Is ecological diversification involved? In this study, we tested whether rates of morphological and ecological trait evolution are correlated with clade-level species richness in Carex of North America north of Mexico. We constructed a phylogeny of 477 species—an almost complete regional sample. We estimated rates of evolution of morphological traits, habitat, and climatic niche and assessed whether differences in rates of evolution correlate with species richness differences in replicate non-nested sister clades. Our work demonstrates significant positive correlations between climatic niche rates, habitat and reproductive morphological evolution, and species richness. This coupling of trait and niche evolution and species richness in a diverse, continental clade sample strongly suggests that the ability of clades to explore niche and functional space has shaped disparities in richness and functional diversity across the North American flora region. Our findings highlight the importance of the evolutionary history of trait and niche evolution in shaping continental and regional floras.     

Climatic niche evolution in Smilacaceae (Liliales) drives patterns of species diversification and richness between the Old and New World

Geographical variation in species richness in plant groups is determined by the interplay between historical, evolutionary, and ecological processes. However, the processes underlying the striking disparity in species richness between Asia and the Americas remain poorly understood. Here, we synthesize global phylogenetic and macroecological data on the diversification of Smilacaceae, deciphering potential drivers underlying the species diversity pattern biased toward Asia. We compiled global distributions of all Smilacaceae species, and reconstructed the biogeographic history and niche evolution using a new time-calibrated phylogeny (eight genes, 135 species). Integrating these data sets, we estimated evolutionary histories and diversification rates for each region, and tested correlations among species diversification, niche evolution, and niche divergence. Smilacaceae probably originated during the Late Cretaceous/Early Palaeocene and began to diversify in middle to low latitudes in Central America and Eurasia during the Late Eocene. Both the Old and New World clades exhibited a steady, albeit slight, increase of species diversification from the Late Eocene to Early Miocene. However, the Old World clade experienced an abrupt increase in net diversification during the Late Miocene. Our findings also revealed that species diversification rates were positively correlated with ecological niche evolution and niche divergence. Niche shifts and climatic niche evolution since the Middle Miocene played crucial roles in species diversification dynamics within Smilacaceae. The high plant richness in Asia may be explained by greater diversification in this region, potentially promoted by heterogeneous environments.     

Global gradients in vertebrate diversity predicted by historical area-productivity dynamics and contemporary environment

Broad-scale geographic gradients in species richness have now been extensively documented, but their historical underpinning is still not well understood. While the importance of productivity, temperature, and a scale dependence of the determinants of diversity is broadly acknowledged, we argue here that limitation to a single analysis scale and data pseudo-replication have impeded an integrated evolutionary and ecological understanding of diversity gradients. We develop and apply a hierarchical analysis framework for global diversity gradients that incorporates an explicit accounting of past environmental variation and provides an appropriate measurement of richness. Due to environmental niche conservatism, organisms generally reside in climatically defined bioregions, or "evolutionary arenas," characterized by in situ speciation and extinction. These bioregions differ in age and their total productivity and have varied over time in area and energy available for diversification. We show that, consistently across the four major terrestrial vertebrate groups, current-day species richness of the world's main 32 bioregions is best explained by a model that integrates area and productivity over geological time together with temperature. Adding finer scale variation in energy availability as an ecological predictor of within-bioregional patterns of richness explains much of the remaining global variation in richness at the 110 km grain. These results highlight the separate evolutionary and ecological effects of energy availability and provide a first conceptual and empirical integration of the key drivers of broad-scale richness gradients. Avoiding the pseudo-replication that hampers the evolutionary interpretation of non-hierarchical macroecological analyses, our findings integrate evolutionary and ecological mechanisms at their most relevant scales and offer a new synthesis regarding global diversity gradients.     

Diversification and biogeographic patterns in four island radiations of passerine birds

Declining diversification rates over time are a well-established evolutionary pattern, often interpreted as indicating initial rapid radiation with filling of ecological niche space. Here, we test the hypothesis that island radiations may show constant net diversification rates over time, due to continued expansion into new niche space in highly dispersive taxa. We investigate diversification patterns of four passerine bird families originating from the Indo-Pacific archipelagos, and link these to biogeographic patterns to provide independent indications of niche filling. We find a declining diversification rate for only one family, the Paradisaeidae (41 species). These are almost completely restricted to New Guinea, and have on average smaller species ranges and higher levels of species richness within grid cells than the other three families. In contrast, we cannot reject constant diversification rates for Campephagidae (93 species), Oriolidae (35 species), and Pachycephalidae (53 species), groups that have independently colonized neighboring archipelagos and continents. We propose that Paradisaeidae have reached the diversity limit imposed by their restricted distribution, whereas high dispersal and colonization success across the geologically dynamic Indo-Pacific archipelagos may have sustained high speciation rates for the other three families. Alternatively, increasing extinction rates may have obscured declining speciation rates in those three phylogenies.     

Diversity dynamics of Miocene mammals in relation to the history of tectonism and climate

Continental biodiversity gradients result not only from ecological processes, but also from evolutionary and geohistorical processes involving biotic turnover in landscape and climatic history over millions of years. Here, we investigate the evolutionary and historical contributions to the gradient of increasing species richness with topographic complexity. We analysed a dataset of 418 fossil rodent species from western North America spanning 25 to 5 Ma. We compared diversification histories between tectonically active (Intermontane West) and quiescent (Great Plains) regions. Although diversification histories differed between the two regions, species richness, origination rate and extinction rate per million years were not systematically different over the 20 Myr interval. In the tectonically active region, the greatest increase in originations coincided with a Middle Miocene episode of intensified tectonic activity and global warming. During subsequent global cooling, species richness declined in the montane region and increased on the Great Plains. These results suggest that interactions between tectonic activity and climate change stimulate diversification in mammals. The elevational diversity gradient characteristic of modern mammalian faunas was not a persistent feature over geologic time. Rather, the Miocene rodent record suggests that the elevational diversity gradient is a transient feature arising during particular episodes of Earth''s history.     

Evolutionary and ecological causes of the latitudinal diversity gradient in hylid frogs: treefrog trees unearth the roots of high tropical diversity

Why are there more species in the tropics than in temperate regions? In recent years, this long-standing question has been addressed primarily by seeking environmental correlates of diversity. But to understand the ultimate causes of diversity patterns, we must also examine the evolutionary and biogeographic processes that directly change species numbers (i.e., speciation, extinction, and dispersal). With this perspective, we dissect the latitudinal diversity gradient in hylid frogs. We reconstruct a phylogeny for 124 hylid species, estimate divergence times and diversification rates for major clades, reconstruct biogeographic changes, and use ecological niche modeling to identify climatic variables that potentially limit dispersal. We find that hylids originated in tropical South America and spread to temperate regions only recently (leaving limited time for speciation). There is a strong relationship between the species richness of each region and when that region was colonized but not between the latitudinal positions of clades and their rates of diversification. Temperature seasonality seemingly limits dispersal of many tropical clades into temperate regions and shows significant phylogenetic conservatism. Overall, our study illustrates how two general principles (niche conservatism and the time-for-speciation effect) may help explain the latitudinal diversity gradient as well as many other diversity patterns across taxa and regions.     

Historical vs. ecological factors influencing global patterns of scarabaeine dung beetle diversity

Abstract. There has been much debate concerning the relative influence on biodiversity of historical vs. current ecological factors. Although both are important, we suggest that historical influences might be greater at higher taxonomic level, since one is looking further back into evolutionary history than at lower taxonomic level. Although we are unable to separate ecological from historical effects in the present global study on scarabaeine dung beetles, we are able to demonstrate differences in correlations between major environmental influences (climatic area, numbers of dung types) and major components of diversity (taxon richness, taxon diversity, functional composition) at different taxonomic levels (tribe, genus, species). Current global variation in taxon richness is correlated strongly to current biogeographical variation in the area of suitable climate at all three taxonomic levels. However, generic and species richness is correlated most strongly to climatic combinations which include tropical and warm summer rainfall climate types (I, II). In contrast, tribal richness is correlated most strongly to climatic combinations which include both warm summer rainfall and temperate climate types (II, VI, X). Regional variation in the number of available dung types shows a strong positive correlation to regional variation in taxon richness at higher tribal level but not at lower generic and species levels. Similarly, biogeographical differences in the number of available dung types show a strong negative correlation to dominance indices for taxon diversity at tribal level (distribution of generic numbers between tribes) but none at generic level (species numbers per genus). As functional diversification is linked closely to taxonomic diversification at tribal level, proportions of both ball‐rolling genera and ball‐rolling species also show strong negative correlations to the number of dung types available in each region. In conclusion, the presence of dung type correlations only at higher taxonomic level may reflect historical effects on scarabaeine taxon diversification, whereas differences in correlations to climate type with taxonomic level may reflect both current ecological and historical effects.     

Shifts in Climate Foster Exceptional Opportunities for Species Radiation: The Case of South African Geraniums

Climate change is often assumed to be a major driver of biodiversity loss. However, it can also set the stage for novel diversification in lineages with the evolutionary ability to colonize new environments. Here we tested if the extraordinary evolutionary success of the genus Pelargonium was related to the ability of its species to capitalize on the climate niche variation produced by the historical changes in southern Africa. We evaluated the relationship between rates of climate niche evolution and diversification rates in the main Pelargonium lineages and disentangled the roles of deep and recent historical events in the modification of species niches. Pelargonium clades exhibiting higher ecological differentiation along summer precipitation (SPP) gradients also experienced higher diversification rates. Faster rates of niche differentiation in spatially structured variables, along with lower levels of niche overlap among closely related species, suggest recent modification in species niches (e.g. dispersal or range shift) and niche lability. We suggest that highly structured SPP gradients established during the aridification process within southern Africa, in concert with niche lability and low niche overlap, contributed to species divergence. These factors are likely to be responsible for the extensive diversification of other lineages in this diversity hot spot.     

An inverse latitudinal gradient of diversity of peracarid crustaceans along the Pacific Coast of South America: out of the deep south

Aim To understand the ecological and historical/evolutionary processes underlying an inverse latitudinal gradient of richness (LGR) using crustacean peracarid species as a model group. Location The Pacific coast of South America, along the Chilean coast between 18° S and 56° S. Methods The LGR was evaluated using a dataset including 320 marine peracarid species reported for the coasts of Chile. Five ecological hypotheses invoking a relationship between species richness and present‐day conditions were tested: species–energy, species–area, Rapoport rescue effect, mid‐domain geometric constraint and niche breadth. Historical/evolutionary hypotheses (i.e. biogeographic conservatism, and diversification rates) were indirectly tested by analysing the latitudinal variation in the taxonomic distinctness, the taxonomic conservatism of the midpoint of the latitudinal range and the degree of nestedness at different taxonomic levels. Results Richness increased poleward, varying approximately eightfold, following an inverse LGR coupled with an increase in bathymetric distribution. Overall this inverse LGR seems robust to uncertainties in the completeness of the species inventory. We found support for only two of the five ecological hypotheses tested: species–area and Rapoport rescue effect. Historical/evolutionary hypotheses seemed important in structuring the richness pattern, as indicated by the higher taxonomic distinctness in the southern region, the strong taxonomic inertia in the mean range size and the high degree of nestedness of assemblages at different taxonomic levels. Conclusions When combined, these results underscore the importance of long‐term processes and historical/evolutionary explanations for the inverse LGR, conceptualized in what we term the ‘out of the deep south’ hypothesis that involves the effects of both biogeographic niche conservatism and evolutionary rates. We propose that the southern region may be a source of evolutionary novelties and/or exhibit higher diversification rates (i.e. higher speciation/lower extinction rates). Furthermore, phylogenetic conservatism of latitudinal range may limit the geographic expansion of these new taxa towards the depauperated northern region.     

What explains patterns of species richness? The relative importance of climatic‐niche evolution,morphological evolution,and ecological limits in salamanders

A major goal of evolutionary biology and ecology is to understand why species richness varies among clades. Previous studies have suggested that variation in richness among clades might be related to variation in rates of morphological evolution among clades (e.g., body size and shape). Other studies have suggested that richness patterns might be related to variation in rates of climatic‐niche evolution. However, few studies, if any, have tested the relative importance of these variables in explaining patterns of richness among clades. Here, we test their relative importance among major clades of Plethodontidae, the most species‐rich family of salamanders. Earlier studies have suggested that climatic‐niche evolution explains patterns of diversification among plethodontid clades, whereas rates of morphological evolution do not. A subsequent study stated that rates of morphological evolution instead explained patterns of species richness among plethodontid clades (along with “ecological limits” on richness of clades, leading to saturation of clades with species, given limited resources). However, they did not consider climatic‐niche evolution. Using phylogenetic multiple regression, we show that rates of climatic‐niche evolution explain most variation in richness among plethodontid clades, whereas rates of morphological evolution do not. We find little evidence that ecological limits explain patterns of richness among plethodontid clades. We also test whether rates of morphological and climatic‐niche evolution are correlated, and find that they are not. Overall, our results help explain richness patterns in a major amphibian group and provide possibly the first test of the relative importance of climatic niches and morphological evolution in explaining diversity patterns.     

Climate and history explain the species richness peak at mid-elevation for Schizothorax fishes (Cypriniformes: Cyprinidae) distributed in the Tibetan Plateau and its adjacent regions

Aim  We studied elevational species richness patterns of Schizothorax fishes and identified the roles of ecological and evolutionary factors in shaping the patterns of elevational diversity.
Location  The Tibetan Plateau and its adjacent regions.
Methods  We assembled distribution and altitude data for all Schizothorax species using the literature. We merged ecological and evolutionary approaches to test the relationships between species richness and ecological factors (climate, area, the mid-domain effect) or evolutionary factors (diversification rates and time of colonization).
Results  We found that species richness of Schizothorax fishes peaked at mid-elevations. Rainfall, area, the mid-domain effect and diversification rate were weak predictors of the richness pattern. Temperature showed a nonlinear relationship with species richness. Temperature and time of colonization were the most important variables in explaining the elevational diversity pattern.
Main conclusion  Our findings indicate that the time-for-speciation effect and niche conservatism play important roles in variation of species richness.     

Eco-Evolutionary Community Dynamics: Covariation between Diversity and Invasibility across Temperature Gradients *

Abstract Understanding biodiversity gradients is a long-standing challenge, and progress requires theory unifying ecology and evolution. Here, we unify concepts related to the speed of evolution, the influence of species richness on diversification, and niche-based coexistence. We focus on the dynamics, through evolutionary time, of community invasibility and species richness across a broad thermal gradient. In our framework, the evolution of body size influences the ecological structure and dynamics of a trophic network, and organismal metabolism ties temperature to eco-evolutionary processes. The framework distinguishes ecological invasibility (governed by ecological interactions) from evolutionary invasibility (governed by local ecology and constraints imposed by small phenotypic effects of mutation). The model yields four primary predictions: (1) ecological invasibility declines through time and with increasing temperature; (2) average evolutionary invasibility across communities increases and then decreases through time as the richness-temperature gradient flattens; (3) in the early stages of diversification, richness and evolutionary invasibility both increase with increasing temperature; and (4) at equilibrium, richness does not vary with temperature, yet evolutionary invasibility decreases with increasing temperature. These predictions emerge from the "evolutionary-speed" hypothesis, which attempts to account for latitudinal species richness gradients by invoking faster biological rates in warmer, tropical regions. The model contrasts with predictions from other richness-gradient hypotheses, such as "niche conservatism" and "species energy." Empirically testing our model's predictions should help distinguish among these hypotheses.     

Advancing the metabolic theory of biodiversity

A component of metabolic scaling theory has worked towards understanding the influence of metabolism over the generation and maintenance of biodiversity. Specific models within this ‘metabolic theory of biodiversity’ (MTB) have addressed temperature gradients in speciation rate and species richness, but the scope of MTB has been questioned because of empirical departures from model predictions. In this study, we first show that a generalized MTB is not inconsistent with empirical patterns and subsequently implement an eco‐evolutionary MTB which has thus far only been discussed qualitatively. More specifically, we combine a functional trait (body mass) approach and an environmental gradient (temperature) with a dynamic eco‐evolutionary model that builds on the current MTB. Our approach uniquely accounts for feedbacks between ecological interactions (size‐dependent competition and predation) and evolutionary rates (speciation and extinction). We investigate a simple example in which temperature influences mutation rate, and show that this single effect leads to dynamic temperature gradients in macroevolutionary rates and community structure. Early in community evolution, temperature strongly influences speciation and both speciation and extinction strongly influence species richness. Through time, niche structure evolves, speciation and extinction rates fall, and species richness becomes increasingly independent of temperature. However, significant temperature‐richness gradients may persist within emergent functional (trophic) groups, especially when niche breadths are wide. Thus, there is a strong signal of both history and ecological interactions on patterns of species richness across temperature gradients. More generally, the successful implementation of an eco‐evolutionary MTB opens the perspective that a process‐based MTB can continue to emerge through further development of metabolic models that are explicit in terms of functional traits and environmental gradients.     

What causes latitudinal gradients in species diversity? Evolutionary processes and ecological constraints on swallowtail biodiversity

The latitudinal diversity gradient (LDG) is one of the most striking ecological patterns on our planet. Determining the evolutionary causes of this pattern remains a challenging task. To address this issue, previous LDG studies have usually relied on correlations between environmental variables and species richness, only considering evolutionary processes indirectly. Instead, we use a phylogenetically integrated approach to investigate the ecological and evolutionary processes responsible for the global LDG observed in swallowtail butterflies (Papilionidae). We find evidence for the 'diversification rate hypothesis' with different diversification rates between two similarly aged tropical and temperate clades. We conclude that the LDG is caused by (1) climatically driven changes in both clades based on evidence of responses to cooling and warming events, and (2) distinct biogeographical histories constrained by tropical niche conservatism and niche evolution. This multidisciplinary approach provides new findings that allow better understanding of the factors that shape LDGs.     

How does resource supply affect evolutionary diversification?

The availability of different resources in the environment can affect the outcomes of evolutionary diversification. A unimodal distribution of diversity with resource supply has been widely observed and explained previously in the context of selection acting in a spatially heterogeneous environment. Here, we propose an alternative mechanism to explain the relationship between resource supply and diversification that is based on selection for exploitation of different resources. To test this mechanism, we conducted a selection experiment using the bacterium Pseudomonas fluorescens in spatially homogeneous environments over a wide range of resource supply rates. Our results show that niche diversification peaks at intermediate levels of resource availability. We suggest that this unimodal relationship is due to evolutionary diversification that is driven by competition for resources but constrained by the ecological opportunity represented by different resource types. These processes may underlie some general patterns of diversity, including latitudinal gradients in species richness and the effects of anthropogenic enrichment of the environment.     

Tectonics,topography, and mammalian diversity

Terrestrial vertebrates show striking changes in species richness across topographic gradients. For mammals, nearly twice as many species per unit area occur in topographically complex regions as in adjacent lowlands. The geological context of this pervasive biogeographic pattern suggests that tectonic processes have a first‐order impact on regional diversity. I evaluate ecological, evolutionary, and historical influences of tectonics and topography on the regional diversity of terrestrial mammals, focusing on the hypothesis that diversification rates are higher in active versus passive tectonic settings. Ten predictions follow from this hypothesis. 1) The timing of peaks in speciation should be congruent with the timescale for tectonic episodes. 2) The rates of speciation and genetic differentiation of populations should be greater for species inhabiting topographically complex regions than spatially continuous landscapes. 3) If topographic complexity per se promotes diversification, then a cluster of young divergences should occur for montane species compared to lowland relatives. 4) Endemism in tectonically active regions should reflect origination within the region rather than range reduction from larger areas. 5) Extinction rates should differ for lineages in tectonically active regions compared to adjacent lowlands. 6) The relationship between local and regional species richness should differ between topographic settings because of higher beta diversity in topographically complex regions. 7) Species originating in topographically complex regions should colonize adjacent lowlands more often than the reverse pattern. 8) North‐south mountain ranges should have higher regional species richness than east‐west mountain ranges. 9) Areas with multiple mountain ranges should have higher regional species richness than comparable areas with single mountain ranges. 10) Global climate changes should affect diversification in tectonically active regions. Research addressing these topics places elevational diversity gradients into a geohistorical context and integrates data from modern biotas and the fossil record.     

The causes of species richness patterns across space, time, and clades and the role of "ecological limits"

A major goal of research in ecology and evolution is to explain why species richness varies across habitats, regions, and clades. Recent reviews have argued that species richness patterns among regions and clades may be explained by "ecological limits" on diversity over time, which are said to offer an alternative explanation to those invoking speciation and extinction (diversification) and time. Further, it has been proposed that this hypothesis is best supported by failure to find a positive relationship between time (e.g., clade age) and species richness. Here, I critically review the evidence for these claims, and propose how we might better study the ecological and evolutionary origins of species richness patterns. In fact, ecological limits can only influence species richness in clades by influencing speciation and extinction, and so this new "alternative paradigm" is simply one facet of the traditional idea that ecology influences diversification. The only direct evidence for strict ecological limits on richness (i.e., constant diversity over time) is from the fossil record, but many studies cited as supporting this pattern do not, and there is evidence for increasing richness over time. Negative evidence for a relationship between clade age and richness among extant clades is not positive evidence for constant diversity over time, and many recent analyses finding no age-diversity relationship were biased to reach this conclusion. More comprehensive analyses strongly support a positive age-richness relationship. There is abundant evidence that both time and ecological influences on diversification rates are important drivers of both large-scale and small-scale species richness patterns. The major challenge for future studies is to understand the ecological and evolutionary mechanisms underpinning the relationships between time, dispersal, diversification, and species richness patterns.