Latitude, elevational climatic zonation and speciation in New World vertebrates

Many biodiversity hotspots are located in montane regions, especially in the tropics. A possible explanation for this pattern is that the narrow thermal tolerances of tropical species and greater climatic stratification of tropical mountains create more opportunities for climate-associated parapatric or allopatric speciation in the tropics relative to the temperate zone. However, it is unclear whether a general relationship exists among latitude, climatic zonation and the ecology of speciation. Recent taxon-specific studies obtained different results regarding the role of climate in speciation in tropical versus temperate areas. Here, we quantify overlap in the climatic distributions of 93 pairs of sister species of mammals, birds, amphibians and reptiles restricted to either the New World tropics or to the Northern temperate zone. We show that elevational ranges of tropical- and temperate-zone species do not differ from one another, yet the temperature range experienced by species in the temperate zone is greater than for those in the tropics. Moreover, tropical sister species tend to exhibit greater similarity in their climatic distributions than temperate sister species. This pattern suggests that evolutionary conservatism in the thermal niches of tropical taxa, coupled with the greater thermal zonation of tropical mountains, may result in increased opportunities for allopatric isolation, speciation and the accumulation of species in tropical montane regions. Our study exemplifies the power of combining phylogenetic and spatial datasets of global climatic variation to explore evolutionary (rather than purely ecological) explanations for the high biodiversity of tropical montane regions.     

LATITUDINAL VARIATION IN SPECIATION MECHANISMS IN FROGS

Speciation often has a strong geographical and environmental component, but the ecological factors that potentially underlie allopatric and parapatric speciation remain understudied. Two ecological mechanisms by which speciation may occur on geographic scales are allopatric speciation through niche conservatism and parapatric or allopatric speciation through niche divergence. A previous study on salamanders found a strong latitudinal pattern in the prevalence of these mechanisms, with niche conservatism dominating in temperate regions and niche divergence dominating in the tropics, and related this pattern to Janzen's hypothesis of greater climatic zonation between different elevations in the tropics. Here, we test for latitudinal patterns in speciation in a related but more diverse group of amphibians, the anurans. Using data from up to 79 sister-species pairs, we test for latitudinal variation in elevational and climatic overlap between sister species, and evaluate the frequency of speciation via niche conservatism versus niche divergence in relation to latitude. In contrast to salamanders, we find no tendency for greater niche divergence in the tropics or for greater niche conservatism in temperate regions. Although our results support the idea of greater climatic zonation in tropical regions, they show that this climatic pattern does not lead to straightforward relationships between speciation, latitude, and niche evolution.     

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.     

Explaining Andean megadiversity: the evolutionary and ecological causes of glassfrog elevational richness patterns

The Tropical Andes are an important global biodiversity hotspot, harbouring extraordinarily high richness and endemism. Although elevational richness and speciation have been studied independently in some Andean groups, the evolutionary and ecological processes that explain elevational richness patterns in the Andes have not been analysed together. Herein, we elucidate the processes underlying Andean richness patterns using glassfrogs (Centrolenidae) as a model system. Glassfrogs show the widespread mid‐elevation diversity peak for both local and regional richness. Remarkably, these patterns are explained by greater time (montane museum) rather than faster speciation at mid‐elevations (montane species pump), despite the recency of the major Andean uplift. We also show for the first time that rates of climatic‐niche evolution and elevational change are related, supporting the hypothesis that climatic‐niche conservatism decelerates species' shifts in elevational distributions and underlies the mid‐elevation richness peak. These results may be relevant to other Andean clades and montane systems globally.     

Patterns of speciation are similar across mountainous and lowland regions for a Neotropical plant radiation (Costaceae: Costus)

High species richness and endemism in tropical mountains are recognized as major contributors to the latitudinal diversity gradient. The processes underlying mountain speciation, however, are largely untested. The prevalence of steep ecogeographic gradients and the geographic isolation of populations by topographic features are predicted to promote speciation in mountains. We evaluate these processes in a species-rich Neotropical genus of understory herbs that range from the lowlands to montane forests and have higher species richness in topographically complex regions. We ask whether climatic niche divergence, geographic isolation, and pollination shifts differ between mountain-influenced and lowland Amazonian sister pairs inferred from a 756-gene phylogeny. Neotropical Costus ancestors diverged in Central America during a period of mountain formation in the last 3 million years with later colonization of Amazonia. Although climatic divergence, geographic isolation, and pollination shifts are prevalent in general, these factors do not differ between mountain-influenced and Amazonian sister pairs. Despite higher climatic niche and species diversity in the mountains, speciation modes in Costus appear similar across regions. Thus, greater species richness in tropical mountains may reflect differences in colonization history, diversification rates, or the prevalence of rapidly evolving plant life forms, rather than differences in speciation mode.     

Phylogenetic conservatism and biogeographic affinity influence woody plant species richness–climate relationships in eastern Eurasia

Mechanisms underlying species richness patterns remain a central yet controversial issue in biology. Climate has been regarded as a major determinant of species richness. However, the relative influences of different evolutionary processes, (i.e. niche conservatism, diversification rate and time for speciation) on species richness–climate relationships remain to be tested. Here, using newly compiled distribution maps for 11 422 woody plant species in eastern Eurasia, we estimated species richness patterns for all species and for families with tropical and temperate affinities separately, and explored the phylogenetic signals in species richness patterns of different families and their relationships with contemporary climate and climate change since the Last Glacial Maximum (LGM). We further compared the effects of niche conservatism (represented by contemporary-ancestral climatic niches differences), diversification rate and time for speciation (represented by family age) on variation in the slopes of species richness–climate relationships. We found that winter coldness was the best predictor for species richness patterns of most tropical families while Quaternary climate change was the best predictor for those of most temperate families. Species richness patterns of closely-related families were more similar than those of distantly-related families within eudicots, and significant phylogenetic signals characterized the slopes of species richness–climate relationships across all angiosperm families. Contemporary-ancestral climatic niche differences dominated variation in the relationships between family-level species richness and most climate variables. Our results indicate significant phylogenetic conservatism in family-level species richness patterns and their relationships with contemporary climate within eudicots. These findings shed light on the mechanisms underlying large-scale species richness patterns and suggest that ancestral climatic niche may influence the evolution of species richness–climate relationships in plants through niche conservatism.     

Large-scale phylogenetic analyses reveal the causes of high tropical amphibian diversity

Many groups show higher species richness in tropical regions but the underlying causes remain unclear. Despite many competing hypotheses to explain latitudinal diversity gradients, only three processes can directly change species richness across regions: speciation, extinction and dispersal. These processes can be addressed most powerfully using large-scale phylogenetic approaches, but most previous studies have focused on small groups and recent time scales, or did not separate speciation and extinction rates. We investigate the origins of high tropical diversity in amphibians, applying new phylogenetic comparative methods to a tree of 2871 species. Our results show that high tropical diversity is explained by higher speciation in the tropics, higher extinction in temperate regions and limited dispersal out of the tropics compared with colonization of the tropics from temperate regions. These patterns are strongly associated with climate-related variables such as temperature, precipitation and ecosystem energy. Results from models of diversity dependence in speciation rate suggest that temperate clades may have lower carrying capacities and may be more saturated (closer to carrying capacity) than tropical clades. Furthermore, we estimate strikingly low tropical extinction rates over geological time scales, in stark contrast to the dramatic losses of diversity occurring in tropical regions presently.     

Temperature shapes opposing latitudinal gradients of plant taxonomic and phylogenetic β diversity

Latitudinal and elevational richness gradients have received much attention from ecologists but there is little consensus on underlying causes. One possible proximate cause is increased levels of species turnover, or β diversity, in the tropics compared to temperate regions. Here, we leverage a large botanical dataset to map taxonomic and phylogenetic β diversity, as mean turnover between neighboring 100 × 100 km cells, across the Americas and determine key climatic drivers. We find taxonomic and tip‐weighted phylogenetic β diversity is higher in the tropics, but that basal‐weighted phylogenetic β diversity is highest in temperate regions. Supporting Janzen's ‘mountain passes’ hypothesis, tropical mountainous regions had higher β diversity than temperate regions for taxonomic and tip‐weighted metrics. The strongest climatic predictors of turnover were average temperature and temperature seasonality. Taken together, these results suggest β diversity is coupled to latitudinal richness gradients and that temperature is a major driver of plant community composition and change.     

DOES NICHE CONSERVATISM PROMOTE SPECIATION? A CASE STUDY IN NORTH AMERICAN SALAMANDERS

Recent speciation research has generally focused on how lineages that originate in allopatry evolve intrinsic reproductive isolation, or how ecological divergence promotes nonallopatric speciation. However, the ecological basis of allopatric isolation, which underlies the most common geographic mode of speciation, remains poorly understood and largely unstudied. Here, we explore the ecological and evolutionary factors that promote speciation in Desmognathus and Plethodon salamanders from temperate eastern North America. Based on published molecular phylogenetic estimates and the degree of geographic range overlap among extant species, we find strong evidence for a role for geographic isolation in speciation. We then examine the relationship between climatic variation and speciation in 16 sister-taxon pairs using geographic information system maps of climatic variables, new methods for modeling species' potential geographic distributions, and data on geographic patterns of genetic variation. In contrast to recent studies in tropical montane regions, we found no evidence for parapatric speciation along climatic gradients. Instead, many montane sister taxa in the Appalachian Highlands inhabit similar climatic niches and seemingly are allopatric because they are unable to tolerate the climatic conditions in the intervening lowlands. This temporal and spatial-ecological pattern suggests that niche conservatism, rather than niche divergence, plays the primary role in promoting allopatric speciation and montane endemism in this species-rich group of vertebrates. Our results demonstrate that even the relatively subtle climatic differences between montane and lowland habitats in eastern North America may play a key role in the origin of new species.     

Vertical stratification influences global patterns of biodiversity

Species distributions in terrestrial ecosystems are three‐dimensional, spanning both the horizontal landscape and the vertical space provided by the physical environment. Classical hypotheses suggest that communities become more vertically stratified with increasing species richness, owing to reduced competition or finer niche subdivision. However, this assertion remains untested in the context of the broader realm of biogeography. Here, integrating traits and distribution data for amphibians globally, we show how vertical strategies interact with the physical and climatic environments to govern global patterns of species richness and community composition. Our results reveal a marked latitudinal shift in strategies of vertical habitat use, from highly arboreal assemblages in the tropics to highly fossorial assemblages in sub‐tropical and temperate regions. Arboreality is strongly associated with precipitation, vegetation structure and climatic stability, whereas fossoriality is more common in dry environments with high diurnal temperature range and low vegetation structure. These analyses shed light on the importance of vertical stratification for species coexistence in species‐rich regions. As certain tropical habitats become drier from climate change, the rich biological diversity that is emblematic of the tropics may transition from vertically stratified to ‘flattened’, with future communities living mostly on or beneath the ground.     

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'.     

Phylogenetic history underlies elevational biodiversity patterns in tropical salamanders

Elevational variation in species richness is ubiquitous and important for conservation, but remains poorly explained. Numerous studies have documented higher species richness at mid-elevations, but none have addressed the underlying evolutionary and biogeographic processes that ultimately explain this pattern (i.e. speciation, extinction and dispersal). Here, we address the evolutionary causes of the mid-elevational diversity hump in the most species-rich clade of salamanders, the tropical bolitoglossine plethodontids. We present a new phylogeny for the group based on DNA sequences from all 13 genera and 137 species. Using this phylogeny, we find no relationship between rates of diversification of clades and their elevational distribution, and no evidence for a rapid 'species pump' in tropical montane regions. Instead, we find a strong relationship between the number of species in each elevational zone and the estimated time when each elevational band was first colonized. Mid-elevation habitats were colonized early in the phylogenetic history of bolitoglossines, and given similar rates of diversification across elevations, more species have accumulated in the elevational zones that were inhabited the longest. This pattern may be widespread and suggests that mid-elevation habitats may not only harbour more species, but may also contain more phylogenetic diversity than other habitats within a region.     

A stochastic, evolutionary model for range shifts and richness on tropical elevational gradients under Quaternary glacial cycles

Quaternary glacial-interglacial cycles repeatedly forced thermal zones up and down the slopes of mountains, at all latitudes. Although no one doubts that these temperature cycles have left their signature on contemporary patterns of geography and phylogeny, the relative roles of ecology and evolution are not well understood, especially for the tropics. To explore key mechanisms and their interactions in the context of chance events, we constructed a geographical range-based, stochastic simulation model that incorporates speciation, anagenetic evolution, niche conservatism, range shifts and extinctions under late Quaternary temperature cycles along tropical elevational gradients. In the model, elevational patterns of species richness arise from the differential survival of founder lineages, consolidated by speciation and the inheritance of thermal niche characteristics. The model yields a surprisingly rich variety of realistic patterns of phylogeny and biogeography, including close matches to a variety of contemporary elevational richness profiles from an elevational transect in Costa Rica. Mountaintop extinctions during interglacials and lowland extinctions at glacial maxima favour mid-elevation lineages, especially under the constraints of niche conservatism. Asymmetry in temperature (greater duration of glacial than of interglacial episodes) and in lateral area (greater land area at low than at high elevations) have opposing effects on lowland extinctions and the elevational pattern of species richness in the model--and perhaps in nature, as well.     

EVOLUTIONARY AND BIOGEOGRAPHIC ORIGINS OF HIGH TROPICAL DIVERSITY IN OLD WORLD FROGS (RANIDAE)

Differences in species richness between regions are ultimately explained by patterns of speciation, extinction, and biogeographic dispersal. Yet, few studies have considered the role of all three processes in generating the high biodiversity of tropical regions. A recent study of a speciose group of predominately New World frogs (Hylidae) showed that their low diversity in temperate regions was associated with relatively recent colonization of these regions, rather than latitudinal differences in diversification rates (rates of speciation–extinction). Here, we perform parallel analyses on the most species-rich group of Old World frogs (Ranidae; ∼1300 species) to determine if similar processes drive the latitudinal diversity gradient. We estimate a time-calibrated phylogeny for 390 ranid species and use this phylogeny to analyze patterns of biogeography and diversification rates. As in hylids, we find a strong relationship between the timing of colonization of each region and its current diversity, with recent colonization of temperate regions from tropical regions. Diversification rates are similar in tropical and temperate clades, suggesting that neither accelerated tropical speciation rates nor greater temperate extinction rates explain high tropical diversity in this group. Instead, these results show the importance of historical biogeography in explaining high species richness in both the New World and Old World tropics.     

Environmental harshness is positively correlated with intraspecific divergence in mammals and birds

Life on Earth is conspicuously more diverse in the tropics. Although this intriguing geographical pattern has been linked to many biotic and abiotic factors, their relative importance and potential interactions are still poorly understood. The way in which latitudinal changes in ecological conditions influence evolutionary processes is particularly controversial, as there is evidence for both a positive and a negative latitudinal gradient in speciation rates. Here, we identify and address some methodological issues (how patterns are analysed and how latitude is quantified) that could lead to such conflicting results. To address these issues, we assemble a comprehensive data set of the environmental correlates of latitude (including climate, net primary productivity and habitat heterogeneity) and combine it with biological, historical and molecular data to explore global patterns in recent divergence events (subspeciation). Surprisingly, we find that the harsher conditions that typify temperate habitats (lower primary productivity, decreased rainfall and more variable and unpredictable temperatures) are positively correlated with greater subspecies richness in terrestrial mammals and birds. Thus, our findings indicate that intraspecific divergence is greater in regions with lower biodiversity, a pattern that is robust to both sampling variation and latitudinal biases in taxonomic knowledge. We discuss possible causal mechanisms for the link between environmental harshness and subspecies richness (faster rates of evolution, greater likelihood of range discontinuities and more opportunities for divergence) and conclude that this pattern supports recent indications that latitudinal gradients of diversity are maintained by simultaneously higher potentials for both speciation and extinction in temperate than tropical regions.     

Latitudinal pattern of mountain vegetation zonation in southern and eastern Asia

A new scheme of altitudinal and latitudinal vegetation zonation is proposed for eastern Asia. The latitudinal patterns of mountain vegetation zonation show a clear boundary at ca. 20°–30° N. For the tropical mountains south of 20° N, the altitudinal series includes tropical lowland, tropical lower montane, and tropical upper montane zones. For the temperate mountains north of 30° N, the series includes temperate lowland, temperate lower montane, and temperate upper montane zones. The mountains located between 20° and 30° N show a transitional zonation pattern; the lower two zones are comparable to the lower two of the tropical zonation (tropical lowland and tropical lower montane), and the upper two zones are comparable to those of the temperate zonation (temperate lower montane and temperate upper montane). The tropical upper montane zone is not found north of 20°–30° N, while the tropical lower montane zone reaches down to sea level and constitutes the temperate lowland zone. Thus the zonation between 20° and 30° N includes tropical lowland, tropical lower montane/temperate lowland, temperate lower montane, and temperate upper montane zones. The latitudinal series of lowland rain forests follows the scheme of climatic division into tropical, subtropical/warm-temperate, cool-temperate and cold-temperate, with a shift of the respective life forms, evergreen, evergreen notophyllous, deciduous, and evergreen needle-leaved. The tropical lower montane forest can be correlated to the horizontal subtropical/ warm-temperate zone. The temperate altitudinal and latitudinal zonations above 30° N are correlated and show an inclined parallel pattern from high altitudes in the south to low altitudes down to sea level in the north.     

Macroevolutionary dynamics in environmental space and the latitudinal diversity gradient in New World birds

Correlations between species richness and climate suggest non-random occupation of environmental space and niche evolution through time. However, the evolutionary mechanisms involved remain unresolved. Here, we partition the occupation of environmental space into intra- and inter-clade components to differentiate a model based on pure conservation of ancestral niches with higher diversification rates in the tropics, and an adaptive radiation model based on shifts in adaptive peaks at the family level allowing occupation of temperate regions. We examined these mechanisms using within- and among-family skewness components based on centroids of 3560 New World bird species across four environmental variables. We found that the accumulation of species in the tropics is a result of both processes. The components of adaptive radiation have family level skewness of species' distributions strongly structured in space, but not phylogenetically, according to the integrated analyses of spatial filters and phylogenetic eigenvectors. Moreover, stronger radiation components were found for energy variables, which are often used to argue for direct climatic effects on diversity. Thus, the correspondence between diversity and climate may be due to the conservation of ancestral tropical niches coupled with repeated broad shifts in adaptive peaks during birds' evolutionary history more than by higher diversification rates driven by more energy in the tropics.     

A PHYLOGENETIC PERSPECTIVE ON ELEVATIONAL SPECIES RICHNESS PATTERNS IN MIDDLE AMERICAN TREEFROGS: WHY SO FEW SPECIES IN LOWLAND TROPICAL RAINFORESTS?

Differences in species richness at different elevations are widespread and important for conservation, but the causes of these patterns remain poorly understood. Here, we use a phylogenetic perspective to address the evolutionary and biogeographic processes that underlie elevational diversity patterns within a region. We focus on a diverse but well-studied fauna of tropical amphibians, the hylid frogs of Middle America. Middle American treefrogs show a "hump-shaped" pattern of species richness (common in many organisms and regions), with the highest regional diversity at intermediate elevations. We reconstructed phylogenetic relationships among 138 species by combining new and published sequence data from 10 genes and then used this phylogeny to infer evolutionary rates and patterns. The high species richness of intermediate elevations seems to result from two factors. First, a tendency for montane clades to have higher rates of diversification. Second, the early colonization of montane regions, leaving less time for speciation to build up species richness in lowland regions (including tropical rainforests) that have been colonized more recently. This "time-for-speciation" effect may explain many diversity patterns and has important implications for conservation. The results also imply that local-scale environmental factors alone may be insufficient to explain the high species richness of lowland tropical rainforests, and that diversification rates are lower in earth's most species-rich biome.     

Why do mountains support so many species of birds?

Although topographic complexity is often associated with high bird diversity at broad geographic scales, little is known about the relative contributions of geomorphologic heterogeneity and altitudinal climatic gradients found in mountains. We analysed the birds in the western mountains of the New World to examine the two‐fold effect of topography on species richness patterns, using two grains at the intercontinental extent and within temperate and tropical latitudes. Birds were also classified as montane or lowland, based on their overall distributions in the hemisphere. We estimated range in temperature within each cell and the standard deviation in elevation (topographic roughness) based on all pixels within each cell. We used path analysis to test for the independent effects of topographic roughness and temperature range on species richness while controlling for the collinearity between topographic variables. At the intercontinental extent, actual evapotranspiration (AET) was the primary driver of species richness patterns of all species taken together and of lowland species considered separately. In contrast, within‐cell temperature gradients strongly influenced the richness of montane species. Regional partitioning of the data also suggested that range in temperature either by itself or acting in combination with AET had the strongest “effect” on montane bird species richness everywhere. Topographic roughness had weaker “effects” on richness variation throughout, although its positive relationship with richness increased slightly in the tropics. We conclude that bird diversity gradients in mountains primarily reflect local climatic gradients. Widespread (lowland) species and narrow‐ranged (montane) species respond similarly to changes in the environment, differing only in that the richness of lowland species correlates better with broad‐scale climatic effects (AET), whereas mesoscale climatic variation accounts for richness patterns of montane species. Thus, latitudinal and altitudinal gradients in species richness can be explained through similar climatic‐based processes, as has long been argued.     

Minimal effects of latitude on present-day speciation rates in New World birds

The tropics contain far greater numbers of species than temperate regions, suggesting that rates of species formation might differ systematically between tropical and non-tropical areas. We tested this hypothesis by reconstructing the history of speciation in New World (NW) land birds using BAMM, a Bayesian framework for modelling complex evolutionary dynamics on phylogenetic trees. We estimated marginal distributions of present-day speciation rates for each of 2571 species of birds. The present-day rate of speciation varies approximately 30-fold across NW birds, but there is no difference in the rate distributions for tropical and temperate taxa. Using macroevolutionary cohort analysis, we demonstrate that clades with high tropical membership do not produce species more rapidly than temperate clades. For nearly any value of present-day speciation rate, there are far more species in the tropics than the temperate zone. Any effects of latitude on speciation rate are marginal in comparison to the dramatic variation in rates among clades.