Morphological variability in time and space: an example of patterns within buchiid bivalves (Bivalvia,Buchiidae)

Abstract: We studied morphological variation of the bivalve Buchia over its geographical and temporal range. Buchia was widely distributed during the Late Jurassic and Early Cretaceous, and, while previous quantitative studies have shown that species are characterized by large amounts of variation, there have been no prior attempts to measure how morphology varies geographically. We employed traditional morphometric techniques using nine linear/angular measurements on 1855 buchiid shells from eight localities taken from widely separated, but mostly coeval, sections across its range. Principal component and canonical variate analyses indicate that geographical morphospace of buchiids varied significantly, but we did not find evidence of a latitudinal gradient in shell shape. The amount of variation between localities was similar to the amount of variation between species, indicating the importance of geographical effects on morphology. Disparity (morphological diversity within a taxon, calculated by the sum of variances) and diversity (number of species) were calculated for each location and time period (age). Disparity and diversity reached ultimate lows just before the genus’ extinction in the Hauterivian, and is suggestive that extinction was morphologically selective. We did not find significant trends for either metric, but there were discordances throughout its temporal range. Latitudinal trends of disparity and diversity within Buchia are not apparent. This research adds to the growing body of work on geographical variation and is a preliminary step to understanding the nature and variation of buchiid species and of biodiversity in general.     

Into and out of the tropics: global diversification patterns in a hyperdiverse clade of ectomycorrhizal fungi

Ectomycorrhizal (ECM) fungi, symbiotic mutualists of many dominant tree and shrub species, exhibit a biogeographic pattern counter to the established latitudinal diversity gradient of most macroflora and fauna. However, an evolutionary basis for this pattern has not been explicitly tested in a diverse lineage. In this study, we reconstructed a mega‐phylogeny of a cosmopolitan and hyperdiverse genus of ECM fungi, Russula, sampling from annotated collections and utilizing publically available sequences deposited in GenBank. Metadata from molecular operational taxonomic unit cluster sets were examined to infer the distribution and plant association of the genus. This allowed us to test for differences in patterns of diversification between tropical and extratropical taxa, as well as how their associations with different plant lineages may be a driver of diversification. Results show that Russula is most species‐rich at temperate latitudes and ancestral state reconstruction shows that the genus initially diversified in temperate areas. Migration into and out of the tropics characterizes the early evolution of the genus, and these transitions have been frequent since this time. We propose the ‘generalized diversification rate’ hypothesis to explain the reversed latitudinal diversity gradient pattern in Russula as we detect a higher net diversification rate in extratropical lineages. Patterns of diversification with plant associates support host switching and host expansion as driving diversification, with a higher diversification rate in lineages associated with Pinaceae and frequent transitions to association with angiosperms.     

Continental and regional ranges of North American mammals: Rapoport's rule in real and null worlds

Aim To assess the relationship between species richness and distribution within regions arranged along a latitudinal gradient we use the North American mammalian fauna as a study case for testing theoretical models. Location North America. Methods We propose a conceptual framework based on a fully stochastic mid‐domain model to explore geographical patterns of range size and species richness that emerge when the size and position of species ranges along a one‐dimensional latitudinal gradient are randomly generated. We also analyse patterns for the mammal fauna of North America by comparing empirical results from a biogeographical data base with predictions based on randomization null models. Results We confirmed the validity of Rapoport's rule for the mammals of North America by documenting gradients in the size of the continental ranges of species. Additionally, we demonstrated gradients of mean regional range size that parallel those of continental range. Our data also demonstrated that mean range size, measured both as a continental or a regional variable, is significantly correlated with the geographical pattern in species richness. All these patterns deviated sharply from null models. Main conclusions Rapoport's statement of an areographic relationship between species distribution and richness is highly relevant in modern discussions about ecological patterns at the geographical scale.     

Phylogeny,niche conservatism and the latitudinal diversity gradient in mammals

Biologists have long searched for mechanisms responsible for the increase in species richness with decreasing latitude. The strong correlation between species richness and climate is frequently interpreted as reflecting a causal link via processes linked to energy or evolutionary rates. Here, we investigate how the aggregation of clades, as dictated by phylogeny, can give rise to significant climate–richness gradients without gradients in diversification or environmental carrying capacity. The relationship between climate and species richness varies considerably between clades, regions and time periods in a global-scale phylogenetically informed analysis of all terrestrial mammal species. Many young clades show negative richness–temperature slopes (more species at cooler temperatures), with the ages of these clades coinciding with the expansion of temperate climate zones in the late Eocene. In carnivores, we find steeply positive richness–temperature slopes in clades with restricted distributions and tropical origins (e.g. cat clade), whereas widespread, temperate clades exhibit shallow, negative slopes (e.g. dog–bear clade). We show that the slope of the global climate–richness gradient in mammals is driven by aggregating Chiroptera (bats) with their Eutherian sister group. Our findings indicate that the evolutionary history should be accounted for as part of any search for causal links between environment and species richness.     

The tangled link between β‐ and γ‐diversity: a Narcissus effect weakens statistical inferences in null model analyses of diversity patterns

Understanding the structure of and spatial variability in the species composition of ecological communities is at the heart of biogeography. In particular, there has been recent controversy about possible latitudinal trends in compositional heterogeneity across localities (β‐diversity). A gradient in the size of the regional species pool alone can be expected to impose a parallel gradient on β‐diversity, but whether β‐diversity also varies independently of the size of the species pool remains unclear. A recently suggested methodological approach to correct latitudinal β‐diversity gradients for the species pool effect is based on randomization null models that remove the effects of gradients in α‐ and γ‐diversity on β‐diversity. However, the randomization process imposes constraints on the variability of α‐diversity, which in turn force γ‐ and β‐diversity to become interdependent, such that any change in one is mirrored in the other. We argue that simple null model approaches are inadequate to discern whether correlations between α‐, β‐ and γ‐diversity reflect processes of ecological interest or merely differences in the size of the species pool among localities. We demonstrate that this kind of Narcissus effect may also apply to other metrics of spatial or phylogenetic species distribution. We highlight that Narcissus effects may lead to artificially high rejection rates for the focal pattern (Type II errors) and caution that these errors have not received sufficient attention in the ecological literature.     

The macroevolution of climatic niches and its role in ant diversification

1. Investigating how climatic niches change over evolutionary timescales is a necessary step to understanding the current distribution of lineages, yet few studies have addressed this issue using comprehensive datasets. In this study, the evolution of ant climatic niches is investigated at a global scale based on bioclimatic data associated with 163 481 ant occurrence records. The resulting dataset was subjected to principal component analysis, and the scores obtained were used to characterise the main axes of ant climatic niche evolution. 2. Principal component axis 1 (PC1) reflected variation in average temperature and seasonality – consistent with typical tropical/temperate gradients – whereas PC2 was associated with varying levels of aridity. Evolution along these two niche axes was markedly different: differences in the amount of explained variance between PC1 (65%) and PC2 (19%) suggest that climatic niche evolution was nearly three times more pronounced along a tropical–temperate climate axis. 3. There was statistically significant phylogenetic signal on PC1, with genera occupying more tropical conditions diversifying at a faster rate, yet neither of these results is significant on PC2. In addition, most of the ancient ant lineages are associated with conditions of low seasonality and high temperatures. 4. These results provide partial support for the tropical conservatism hypothesis as an explanation for geographical patterns of ant species richness.     

Geographic patterns and environmental correlates of taxonomic and phylogenetic beta diversity for large-scale angiosperm assemblages in China

A full understanding of the origin and maintenance of β-diversity patterns in a region requires exploring the relationships of both taxonomic and phylogenetic β-diversity (TBD and PBD, respectively), and their respective turnover and nestedness components, with geographic and environmental distances. Here, we simultaneously investigated all these aspects of β-diversity for angiosperms in China. Specifically, we evaluated the relative importance of environmental filtering vs dispersal limitation processes in shaping β-diversity patterns. We found that TBD and PBD as quantified using a moving window approach decreased towards higher latitudes across the whole of China, and their turnover components were correlated with latitude more strongly than their nestedness components. When quantifying β-diversity as pairwise distances, geographic and climatic distances across China together explained 60 and 53% of the variation in TBD and PBD, respectively. After the variation in β-diversity explained by climatic distance was accounted for, geographic distance independently explained about 23 and 12% of the variation in TBD and PBD, respectively, across China. Overall, our results suggest that environmental filtering based on climatic tolerance conserved across lineages is the main force shaping β-diversity patterns for angiosperms in China.     

Diversification rates and the latitudinal gradient of diversity in mammals

The latitudinal gradient of species richness has frequently been attributed to higher diversification rates of tropical groups. In order to test this hypothesis for mammals, we used a set of 232 genera taken from a mammalian supertree and, additionally, we reconstructed dated Bayesian phylogenetic trees of 100 genera. For each genus, diversification rate was estimated taking incomplete species sampling into account and latitude was assigned considering the heterogeneity in species distribution ranges. For both datasets, we found that the average diversification rate was similar among all latitudinal bands. Furthermore, when we used phylogenetically independent contrasts, we did not find any significant correlation between latitude and diversification parameters, including different estimates of speciation and extinction rates. Thus, other factors, such as the dynamics of dispersal through time, may be required to explain the latitudinal gradient of diversity in mammals.     

The latitudinal gradient in dispersal constraints: ecological specialisation drives diversification in tropical birds

Physiological and behavioural constraints arising from ecological specialisation are proposed to limit gene flow and promote diversification in tropical lineages. In this study, we use phylogenetic analyses to test this idea in 739 Amazonian bird species. We show that patterns of species and subspecies richness are best predicted by a suite of avian specialisms common in tropical avifaunas but rare in the temperate zone. However, this only applied to niche traits associated with dispersal limitation rather than vagility. These findings are consistent with the view that diversity is promoted by more finely partitioned niches, although not simply by coevolutionary adaptation and niche packing as is often assumed. Instead, they suggest that diversification is driven by dispersal constraints, and that niches characterised by these constraints are biased towards tropical systems. We conclude that specialised tropical niches reduce the likelihood of dispersal across barriers, thereby increasing allopatric diversification and contributing to the latitudinal diversity gradient.     

Geographic patterns in the distribution of social systems in terrestrial arthropods

The role of ecology in the evolution and maintenance of arthropod sociality has received increasing research attention in recent years. In some organisms, such as halictine bees, polistine wasps, and social spiders, researchers are investigating the environmental factors that may contribute to high levels of variation in the degree of sociality exhibited both among and within species. Within lineages that include only eusocial members, such as ants and termites, studies focus more on identifying extrinsic factors that may contribute to the dramatic variation in colony size, number of queens, and division of labour that is evident across these species. In this review, I propose a comparative approach that seeks to identify environmental factors that may have a common influence across such divergent social arthropod groups. I suggest that seeking common biogeographic patterns in the distribution of social systems or key social traits may help us to identify ecological factors that play a common role in shaping the evolution of sociality across different organisms. I first review previous studies of social gradients that form along latitudinal and altitudinal axes. Within families and within species, many organisms show an increasing degree of sociality at lower latitudes and altitudes. In a smaller number of cases, organisms form larger groups or found nests cooperatively at higher latitudes and altitudes. I then describe several environmental factors that vary consistently along such gradients, including climate variables and abundance of predators, and outline their proposed role in the social systems of terrestrial arthropods. Finally, I map distributions of a social trait against several climatic factors in five case studies to demonstrate how future comparative studies could inform empirical research.     

The role of spatial scale and the perception of large-scale species-richness patterns

Despite two centuries of exploration, our understanding of factors determining the distribution of life on Earth is in many ways still in its infancy. Much of the disagreement about governing processes of variation in species richness may be the result of differences in our perception of species‐richness patterns. Until recently, most studies of large‐scale species‐richness patterns assumed implicitly that patterns and mechanisms were scale invariant. Illustrated with examples and a quantitative analysis of published data on altitudinal gradients of species richness (n = 204), this review discusses how scale effects (extent and grain size) can influence our perception of patterns and processes. For example, a hump‐shaped altitudinal species‐richness pattern is the most typical (c. 50%), with a monotonic decreasing pattern (c. 25%) also frequently reported, but the relative distribution of patterns changes readily with spatial grain and extent. If we are to attribute relative impact to various factors influencing species richness and distribution and to decide at which point along a spatial and temporal continuum they act, we should not ask only how results vary as a function of scale but also search for consistent patterns in these scale effects. The review concludes with suggestions of potential routes for future analytical exploration of species‐richness patterns.     

Exploring the phylogenetic history of mammal species richness

Aim At broad geographical scales, species richness is a product of three basic processes: speciation, extinction and migration. However, determining which of these processes predominates is a major challenge. Whilst palaeontological studies can provide information on speciation and extinction rates, data are frequently lacking. Here we use a recent dated phylogenetic tree of mammals to explore the relative importance of these three processes in structuring present‐day richness gradients. Location The global terrestrial biosphere. Methods We combine macroecological data with phylogenetic methods more typically used in community ecology to describe the phylogenetic history of regional faunas. Using simulations, we explore two simple phylogenetic metrics, the mean and variance in the pairwise distances between taxa, and describe their relationship to phylogenetic tree topology. We then use these two metrics to characterize the evolutionary relationships among mammal species assemblages across the terrestrial biome. Results We show that the mean and variance in the pairwise distances describe phylogenetic tree topology well, but are less sensitive to phylogenetic uncertainty than more direct measures of tree shape. We find the phylogeny for South American mammals is imbalanced and ‘stemmy’ (long branches towards the root), consistent with recent diversification within evolutionarily disparate lineages. In contrast, the phylogeny for African mammals is balanced and ‘tippy’ (long branches towards the tips), more consistent with the slow accumulation of diversity over long times, reflecting the Old World origin of many mammal clades. Main conclusions We show that phylogeny can accurately capture biogeographical processes operating at broad spatial scales and over long time periods. Our results support inferences from the fossil record – that the New World tropics are a diversity cradle whereas the Old World tropics are a museum of old diversity.     

Origins of marine patterns of biodiversity: some correlates and applications

Abstract: Marine shelf diversity patterns correlate with macroecological features of basic importance that may play causal roles in macroevolution. We have investigated the global diversity pattern of living Bivalvia, which is dominated by the latitudinal diversity gradient (LDG), maintained by high tropical origination rates. Generic‐level lineages expand poleward, chiefly through speciation, so that species richness within provinces and globally is positively correlated with generic geographical ranges. A gradient in diversity accommodation progressively lowers both immigration and speciation rates in higher latitudes. The LDG correlates with seasonality of trophic resources but not with area; tropical provinces are not diverse because they are large but because they are tropical. A similar dynamic evidently underlays Jurassic and Carboniferous LDGs. Larval developmental modes correlate with the LDG and thus with resource seasonality, with tropical dominance of planktotrophs offset by increasing nonplanktotrophy to poleward. The acquisition of planktotrophy in several early Palaeozoic clades indicates a change in macroecological relationships during Cambrian and Ordovician radiations.     

Species-genus ratios reflect a global history of diversification and range expansion in marine bivalves

The distribution of marine bivalve species among genera and higher taxa takes the form of the classic hollow curve, wherein few lineages are species rich and many are species poor. The distribution of species among genera (S/G ratio) varies with latitude, with temperate S/G's falling within the null expectation, and tropical and polar S/G's exceeding it. Here, we test several hypotheses for this polar overdominance in the species richness of small numbers of genera. We find a significant positive correlation between the latitudinal range of a genus and its species richness, both globally and within regions. Genus age and species richness are also positively related, but this relationship breaks down when the analysis is limited to genera endemic to climate zones or with narrow latitudinal ranges. The data suggest a link between speciation and range-expansion, with genera expanding out of the tropical latitudinal bins tending to speciate more prolifically, both globally and regionally. These genera contain more species within climate zones than taxa endemic to that zone. Range expansion thus appears to be fundamentally coupled with speciation, producing the skewed distribution of species among genera, both globally and regionally, whereas clade longevity is achieved through extinction -- resistance conferred by broad geographical ranges.     

Global patterns of diversity and community structure in marine bacterioplankton

Because of their small size, great abundance and easy dispersal, it is often assumed that marine planktonic microorganisms have a ubiquitous distribution that prevents any structured assembly into local communities. To challenge this view, marine bacterioplankton communities from coastal waters at nine locations distributed world-wide were examined through the use of comprehensive clone libraries of 16S ribosomal RNA genes, used as operational taxonomic units (OTU). Our survey and analyses show that there were marked differences in the composition and richness of OTUs between locations. Remarkably, the global marine bacterioplankton community showed a high degree of endemism, and conversely included few cosmopolitan OTUs. Our data were consistent with a latitudinal gradient of OTU richness. We observed a positive relationship between the relative OTU abundances and their range of occupation, i.e. cosmopolitans had the largest population sizes. Although OTU richness differed among locations, the distributions of the major taxonomic groups represented in the communities were analogous, and all local communities were similarly structured and dominated by a few OTUs showing variable taxonomic affiliations. The observed patterns of OTU richness indicate that similar evolutionary and ecological processes structured the communities. We conclude that marine bacterioplankton share many of the biogeographical and macroecological features of macroscopic organisms. The general processes behind those patterns are likely to be comparable across taxa and major global biomes.     

Bats, clocks, and rocks: diversification patterns in Chiroptera

Identifying nonrandom clade diversification is a critical first step toward understanding the evolutionary processes underlying any radiation and how best to preserve future phylogenetic diversity. However, differences in diversification rates have not been quantitatively assessed for the majority of groups because of the lack of necessary analytical tools (e.g., complete species-level phylogenies, estimates of divergence times, and robust statistics which incorporate phylogenetic uncertainty and test appropriate null models of clade growth). Here, for the first time, we investigate diversification rate heterogeneity in one of the largest groups studied thus far, the bats (Mammalia: Chiroptera). We use a recent, robust statistical approach (whole-tree likelihood-based relative rate tests) on complete dated species-level supertree phylogenies. As has been demonstrated previously for most other groups, among-lineage diversification rate within bats has not been constant. However, we show that bat diversification is more heterogeneous than in other mammalian clades thus far studied. The whole-tree likelihood-based relative rates tests suggest that clades within the families Phyllostomidae and Molossidae underwent a number of significant changes in relative diversification rate. There is also some evidence for rate shifts within Pteropodidae, Emballonuridae, Rhinolophidae, Hipposideridae, and Vespertilionidae, but the significance of these shifts depends on polytomy resolution within each family. Diversification rate in bats has also not been constant, with the largest diversification rate shifts occurring 30-50 million years ago, a time overlapping with the greatest number of shifts in flowering plant diversification rates.     

Large-scale biogeographic patterns of vascular plant richness in North America: an analysis at the generic level

The richness pattern of native vascular plants in North America (north of Mexico) was studied at the generic level. North America was divided into thirteen geographical regions, which were latitudinally grouped into four horizontal zones (northern, north-middle, south-middle, and southern zones); and longitudinally grouped into three vertical zones (eastern, central, and western zones). The native vascular flora of North America consisted of 1904 genera in 235 families and eighty-three orders. Along the latitudinal gradient, generic richness (in terms of the number of genera) showed a striking increase with decreasing latitude. The southern zone had more than four times as many genera as did the northern zone (with a difference of 1436 genera). 93.3% of genera in the northern zone also occurred in the southern zone. Along the longitudinal gradient, the central zone had the highest generic richness and the eastern zone had the lowest, but the difference in generic richness between the two zones was only sixty-one genera. The western and eastern zones shared 60% or more of their genera. Generic richness of vascular plants in North America was highly correlated ( r =0.965) to available environmental energy (expressed by annual potential evapotranspiration).     

生态学代谢理论: 基于个体新陈代谢过程解释物种多样性的地理格局

新陈代谢是生物的基本生理过程。生态学代谢理论(metabolic theory of ecology)基于生物个体大小和环境温度对个体新陈代谢过程的影响, 使用尺度推移(scaling)的方法来解释多种生态学格局和过程。James Borwn等将这一理论用于解释物种多样性的大尺度格局, 并从机制上解释了物种多样性与环境温度的关系。这一理论提出了两个明确的预测: (1)物种多样性的对数与绝对温度的倒数之间呈线性关系; (2) 这一线性关系的斜率为–0.6至–0.7。这一理论自提出以来, 饱受争议, 经过了正反两方面经验数据的检验, 目前仍未形成一致的结论。虽然这一理论仍面临着一些有待解决的问题, 但它以崭新的思路和方法有别于以往的基于统计学方法的研究。人们过去对该理论的实证检验忽略了两个重要的约束条件, 即除温度以外的环境条件处于理想状态和群落处于平衡状态, 而这两个约束条件是理解该理论的基础。本文对生态学代谢理论的理论框架、预测和含义, 以及以往的检验结果进行阐述, 在此基础上提出了作者对该理论的若干认识和未来研究中应考虑的若干问题。