A 450 million years long latitudinal gradient in age‐dependent extinction

Species-rich tropical communities are expected to be more specialized than their temperate counterparts. Several studies have reported increasing biotic specialization toward the tropics, whereas others have not found latitudinal trends once accounting for sampling bias or differences in plant diversity. Thus, the direction of the latitudinal specialization gradient remains contentious. With an unprecedented global data set, we investigated how biotic specialization between plants and animal pollinators or seed dispersers is associated with latitude, past and contemporary climate, and plant diversity. We show that in contrast to expectation, biotic specialization of mutualistic networks is significantly lower at tropical than at temperate latitudes. Specialization was more closely related to contemporary climate than to past climate stability, suggesting that current conditions have a stronger effect on biotic specialization than historical community stability. Biotic specialization decreased with increasing local and regional plant diversity. This suggests that high specialization of mutualistic interactions is a response of pollinators and seed dispersers to low plant diversity. This could explain why the latitudinal specialization gradient is reversed relative to the latitudinal diversity gradient. Low mutualistic network specialization in the tropics suggests higher tolerance against extinctions in tropical than in temperate communities.

"Latitudinal Gradients in Species Diversity": Reflections on Pianka's 1966 Article and a Look Forward.

A decline in species richness moving from equatorial regions to polar regions is a common, but not universal, macroecological pattern. Many studies have focused on this pattern, but few have focused on how the vital rates responsible for species richness patterns, local rates of species extinction and turnover, vary with latitude. We examine patterns of richness, turnover and extinction in North American avian communities inhabiting three ecoregions, using methods that account for failure to detect all species present. We use breeding bird point count data from > 1000 routes in the Breeding Bird Survey collected from 1982 to 2001 to estimate richness, extinction probability and turnover rates. Our analyses differ from others in 1) the use of annual estimates derived at specific locations rather than index data accumulated over numbers of years, 2) the use of estimators that incorporated detection probabilities and 3) a focus on dynamical processes (colonization, extinction) in addition to static patterns (species richness). We find average species richness estimates (48 to 135 species) increasing with latitude for all three regions, contradicting predictions based on the latitudinal diversity gradient. The estimated rates of extinction and turnover declined with latitude across the three ecoregions. We speculate that higher richness might be linked to periods of superabundant food supply in northern areas that support greater numbers of resident and migrant species. Our primary ecological conclusions are that the latitudinal gradient in species richness is reversed for North American birds in the studied ecoregions, and that both local extinction and turnover decrease from southern to northern latitudes. Thus, the vital rates that determine richness show evidence of greater stability and reduced dynamics in northern areas of higher richness. We recommend additional studies examining patterns of colonization, extinction and turnover in communities, that use clearly defined estimators that deal with detection probability.

Summary1. Although a latitudinal gradient in species diversity has been observed for various taxa, the factors generating the latitudinal gradient at broad spatial scales are difficult to identify because several candidate factors change simultaneously with latitude. We investigated latitudinal gradients in stream invertebrate assemblages in 30 headwater streams in Hokkaido Island, Japan, focusing on the regional scale to discount historical factors and to extract the effects of environmental factors on latitudinal gradients in diversity.2. Taxon diversity (Shannon index) and taxon richness (number of taxa per unit area) increased with latitude. Abundance showed a similar latitudinal gradient, whereas evenness (Δ1) did not. Hence, we conclude that the observed latitudinal gradient in taxon richness was generated by directional variation in abundance (passive accumulation), leading to that in taxon diversity.3. Precipitation, which is strongly related to flood disturbances, decreased with latitude and was an important factor explaining variation in taxon diversity, taxon richness and abundance. The probability of a taxon being present tended to increase from south to north, suggesting that the higher taxon richness observed in northern sites may be because of the presence of rare species. These findings indicate that flood disturbance varying with latitude may influence abundance and local extinction rates of rare species, consequently affecting taxon richness and taxon diversity.4. By detecting the effects of an environmental factor (precipitation) on the latitudinal gradients in taxon diversity and taxon richness without interference by historical factors, this study demonstrates processes that can produce latitudinal gradients in the diversity of stream invertebrate assemblages.

The topic of this thesis is the latitudinal diversity gradient: the general increase in numbers of species from high latitudes towards the equator. Although it is a well-documented and very general pattern, there is little consensus among biologists as to the causes of high tropical species richness. In Chapter 1 I give an overview of the latitudinal diversity gradient and hypotheses proposed to explain it, and outline a framework for developing and testing hypotheses. The emphasis in this thesis is on non-equilibrium hypotheses, and in particular on the idea that rates of species diversification are higher at low latitudes. In Chapter 2 I directly test the hypothesis that rates of species diversification increase towards the equator, using the phylogenetic method of sister-group comparisons. For birds and butterflies, 1 show that there does indeed appear to be an increase in rates of diversification towards lower latitudes. One possible explanation for this pattern links climatic conditions at low latitudes to faster speciation, via a causal chain which includes a higher rate of molecular evolution. In Chapter 3 I test the prediction that rates of molecular evolution are faster at low latitudes, using phylogenies reconstructed from DNA sequence data for birds. Although the results provide no evidence for a latitudinal effect on rates of cytb and ND2 evolution in birds, this chapter demonstrates a way in which further tests on a wider range of genes and taxa can be carried out as more sequence data becomes available. If rates of diversification are higher in the tropics, this may result from latitudinal gradients in life history traits which could influence rates of speciation or extinction. However, previous attempts to test for such patterns have rarely controlled for phylogenetic relationships among species, and have never controlled for geographic range overlap. In Chapter 4 I present a method for analyzing latitudinal variation in life history traits which takes both of these problems into account. For birds, this method indicates strong latitudinal variation in geographic range size and clutch size, less strong variation in body size and niche width, and no variation in the strength of sexual selection. While this does not prove that life history plays a part in generating the latitudinal diversity gradient, a robust demonstration of latitudinal variation in life history traits is an important prerequisite for any hypotheses which link life history with high tropical species richness. Speciation and extinction rates are believed to vary with species' life history and ecology. If speciation or extinction rates also vary with latitude, we should expect species with certain traits to show stronger latitudinal diversity gradients than others. We should also expect to see latitudinal variation in the macroecological structure of species assemblages. Chapters 5 and 6 confirm these expectations. In Chapter 5 I show that smaller-bodied bird species have steeper latitudinal diversity gradients than larger species. This is reflected in systematic latitudinal variation in the shape of body size frequency distributions, usually thought to be relatively consistent across regions. In Chapter 6 I show that body size - abundance relationships in the Australian marsupial fauna differ considerably between temperate and tropical subsets of the fauna. Whereas the temperate species display the typical negative relationship, the tropical species show no significant relationship. This pattern is consistent with the explanation that extinction rates vary with respect to body size, abundance and the latitude in which a species occurs. Together, the results of the analyses presented in this thesis provide evidence in favour of the non-equilibrium view that there are more species in the tropics because rates of species diversification are higher. It seems likely that this is due to latitudinal variation in environmental conditions, leading to latitudinal variation in species' life histories and the structure of species assemblages, and to latitudinal variation in rates of speciation or extinction. However, it is still difficult to judge whether it is variation in speciation rate (cradle model) or extinction rate (museum model) which is of primary importance, or if both are equally important. Further work on this question, applying the large-scale comparative methodology used here, will be necessary to progress towards a full understanding of the high species richness of the tropics.

Identifying where high genetic diversity is located across our planet and what factors affect the geographical patterns of genetic variation not only provides important insights into distributions of biodiversity but is also crucial for human health, animal and plant breeding and biodiversity conservation. Recent studies show that genetic diversity at nearly neutral genes decreases from the tropics to the poles in different taxa, mirroring the oldest recognized latitudinal gradient of species richness, yet functional genetic diversity has received little attention at such broad spatial or taxonomic scales. Here, we investigated latitudinal gradients in genetic diversity at a polymorphic exon 2 (MHC II DRB gene, 1515 sequences) of the major histocompatibility complex that confers resistance to parasites in 93 terrestrial mammal species at the global scale. We also estimated the effects of species traits, positive selection and anthropogenic biomes on genetic diversity. We found clear latitudinal gradients in genetic diversity and the presence of positive selection at exon 2. Absolute latitude, the presence of positive selection and body mass are important predictors of within‐species genetic diversity. A higher occurrence of positive selection, faster evolutionary speed or slower drift at low latitudes may shape the latitudinal gradient in the within‐species genetic diversity of the gene. Our results contribute to a greater understanding of how species traits, selection and drift geographically shape functional genetic diversity, broadening the generality of latitudinal biodiversity gradient. The results highlight the importance of conservation at low latitudes.

Ecology's oldest pattern?

Ecology's oldest pattern?

Penguins have been receiving a lot of bad press lately. They are considered somehow counter, spare, strange. Unlike most plant and animal groups, they do not show a peak of species richness towards the equator and a decline towards the poles. This more conventional spatial pattern is conveniently known as the latitudinal diversity gradient because of the strong covariance of richness and other measures of biodiversity that it describes. It is one of the most venerable, well-documented, and controversial large-scale patterns in macroecology (Willig et al. 2003). Equatorial peaks in species richness have characterised the planet since the Devonian (408–362 million years ago) (Crame 2001) and are typical of a wide range of both terrestrial and marine plants and animals (Gaston 1996; Willig et al. 2003). Despite the fact that this pattern has been documented since the late 1700s, sustained interest in both the regularity of the pattern and its likely underlying mechanisms is relatively modern. The realisation that human activity is posing substantial threats to biodiversity has quickened the pace of this interest (Willig et al. 2003). Where the peaks in richness lie (biodiversity hotspots), how these peaks relate to centres of endemism (areas that support large numbers of species that occur nowhere else), and how these patterns are likely to change through time, especially in the face of major environmental change, are major concerns. Without such knowledge, conservation is unlikely to succeed. Although spatial patterns in biodiversity, and particularly the latitudinal gradient, are increasingly well documented for a range of taxa, the proposed mechanisms underlying these gradients remain controversial. In essence, the multitude of mechanisms proposed to explain diversity gradients can be reduced to three categories: historical, ecological, or null. Most significant in raising the temperature of recent discussions is the question of the relative importance of each of these major categories. Historical mechanisms are those that suggest that earth history (e.g., the opening of the Drake Passage and the cooling of Antarctica) and phylogenetic history have played major roles in generating current patterns in diversity, and tend to emphasise regional (and especially longitudinal) differences therein (Qian and Ricklefs 2004; Ricklefs 2004). Explanations involving ecological mechanisms often downplay the significance of such regional differences and give most attention to covariation between current diversity and variables such as energy and water availability, and to the ultimate mechanisms underlying this covariation (Hawkins et al. 2003; Currie and Francis 2004). By contrast, null models, and specifically the geometric constraints model, argue that the expected pattern of latitudinal variation in richness is not a uniform one, but rather a mid-domain peak, which is almost inevitably the outcome of the random placement of a set of variable species ranges within a bounded domain (Colwell et al. 2004; but see also Zapata et al. 2003). It is deviation from the mid-domain expectation that is then argued to be of most interest. In many cases the historical and ecological mechanisms might be difficult to disentangle, such as the historical effects of the establishment of the Antarctic Circumpolar Current, and its consequences for energy availability in the region today (Clarke 2003). Nonetheless, juxtaposing these three major mechanisms raises several questions that could substantially inform the debate in many ways, but have enjoyed far less attention than debating the relative merits of each of them. The geometric constraints model suggests that, to the extent that there is symmetry in the continuity of land (or water) about the equator, declines in richness from the tropical peak should also be symmetrical, with any asymmetries in the latter matching those in the former. Indeed, most texts and reviews dealing with latitudinal diversity gradients only briefly mention hemisphere-related differences and focus instead on the general decline of diversity away from the tropics in both directions (e.g., Brown and Lomolino 1998; Willig et al. 2003). However, that diversity gradients in the two hemispheres might in many cases be highly asymmetric has long been appreciated (Gaston 1996). Although several historical hypotheses suggest reasons why this asymmetry should exist (reviewed in Brown and Lomolino 1998), differences in present ecological factors, such as temperature gradients and rainfall variation, might also explain such asymmetry. If ecological factors are important, then these asymmetries should show up not only in diversity patterns, but also at other levels in the ecological and genealogical hierarchies. From the perspective of ecological explanations for such spatial variation, the questions, then, are how common and strong are such asymmetries, how common are they in patterns of diversity, and what, if any, might be the ecological, rather than null or historical, mechanisms responsible for them?

North American mammals follow a well-established latitudinal diversity gradient in species richness. However, the degree to which species in different mammal clades follow the same latitudinal gradient—and to which each clade contributes to the pattern observed for all mammals remains unknown. Here, we separate the overall mammalian latitudinal diversity gradient by mammal orders and investigate the impact of climate and topography on the distribution of each major mammal clade. We joined an equal-area grid (100 × 100 km cells) of continental North America embedded with environmental variables (n = 10) with mammalian species ranges (n = 753). We used spatial regression models to quantify the relationship between species richness and latitude for all mammals, all mammals excluding select clades, and for each individual subordinate clade (n = 9). We used multiple linear regression and simultaneous autoregressive regression models to determine which environmental variables best explained patterns of species richness for each mammal order. Whereas North American mammals altogether exhibit a strong latitudinal diversity gradient in species richness, most orders deviate from the species richness pattern observed for all mammals and their gradients are weak or entirely absent. Bats (Chiroptera) exhibit the strongest latitudinal gradient—their removal from the pattern for all mammals substantially weakens the total mammalian gradient, more so than when rodents are removed. Environmental variables explain patterns of species richness well for some clades, but poorly for others. The gradient we observe for North American mammals today is likely a combined product of multiple diversification events, dispersals, and climatic and tectonic histories.

AimFor most higher‐order taxa, species diversity peaks sharply in the moist tropics and declines rapidly at higher latitudes. However, the mechanisms driving this latitudinal gradient are numerous, remain uncertain and are even undocumented in some important major clades such as the grasses. Grasses are a cosmopolitan, important plant family with more than 11,000 species world‐wide. Our aims were: to investigate the latitudinal distributions of species richness for different grass lineages, and the grass family as a whole; and to test hypotheses proposed in general for the latitudinal diversity gradient or specifically as determinants of grass species richness patterns at the global scale.LocationGlobal.MethodsWe used the most comprehensive global database of grass distributions currently available to calculate species richness for 340 political regions of the world. Using generalized additive models we used these data to model the latitudinal gradients of species richness for different grass lineages and for the grass family as a whole. We constructed multiple regression models to include climatic, productivity, topographic, habitat and geographic variables.ResultsAn unusual, shallow latitudinal diversity gradient arises because different grass lineages exhibit contrasting latitudinal patterns. This reflects differential specialization of grass lineages to arid and cool environments, the legacy of historical effects, most notably the Gondwanan origin of the grasses, and a strong association between grasses and topographically heterogeneous, mountainous regions.Main conclusionsThe grasses are one of the relatively few higher‐order lineages that exhibits an atypical latitudinal gradient; this has arisen because of climatic specialization of particular grass lineages to cold and arid environments. Key additional roles have been inferred for historical biogeography and topographical heterogeneity in determining global patterns of grass species richness. These mechanisms have generally been under‐appreciated and are probably important for many other lineages.

The most pervasive species-richness pattern, the latitudinal gradient of diversity, has been related to Rapoport's rule, i.e., decreasing latitudinal extent of species' ranges toward the equator. According to this theory, species can have narrower tolerances in more stable climates, leading to smaller ranges and allowing coexistence of more species. We show, using a simple geometric model, that the postulated decrease of species' potential range sizes toward the tropics would itself lead to a latitudinal gradient opposite to that observed. In contrast, an increase in extent of potential ranges toward the tropics would lead to the observed diversity gradient. Moreover, in the presence of geographic barriers constraining actual species' ranges, Rapoport's rule emerges if the latitudinal trend in extents of potential ranges (as defined by climatic tolerance) is opposite to that postulated or if variability in potential range extents decreases toward the poles. A strong implicit latitudinal diversity gradient (i.e., higher concentration of midpoints of species' potential ranges in the tropics), however, produces both observed macroecological patterns without the contribution of any latitudinal trends in species climatic tolerances or in potential range sizes. Our model underscores the necessity of discriminating theoretical processes and principles from the patterns we observe, and it is well supported by data on global distribution of species' range sizes.

Opinion about latitudinal diversity gradients in freshwater fauna has varied over past decades. Global data have been compiled for diversity of Ephemeroptera, Plecoptera, and Trichoptera at the site scale, but no assessment of global variability in regional diversity has been done for any taxon except fish. Global variation cannot be inferred from site-scale data because the relationship between site and regional diversity is not necessarily linear. We have assembled global data on regional diversity for 7 freshwater taxa. Here, we plot diversity against regional area, develop diversity–area regressions, and then plot the residuals against latitude to investigate latitudinal diversity gradients. The existence and directions of gradients vary substantially among taxa. For Ephemeroptera and Plecoptera, significant latitudinal gradients exist, and diversity is greater at higher latitudes. For Trichoptera and Caudata, latitudinal gradients are not apparent, although taxa within the Caudata show distinct patterns. For Odonata, Osteichthyes, and Anura, latitudinal gradients are highly significant, and diversity is greatest at low latitudes. A clear distinction between the life cycles of tropically diverse taxa and of other taxa (excluding fish) is a long terrestrial phase in the tropically diverse taxa (e.g., Odonata). We discuss reasons for the gradients and this contrast in ecological and evolutionary contexts (e.g., habitat complexity and cross-habitat adaptation).

A latitudinal gradient in biodiversity has existed since before the time of the dinosaurs, yet how and why this gradient arose remains unresolved. Here we review two major hypotheses for the origin of the latitudinal diversity gradient. The time and area hypothesis holds that tropical climates are older and historically larger, allowing more opportunity for diversification. This hypothesis is supported by observations that temperate taxa are often younger than, and nested within, tropical taxa, and that diversity is positively correlated with the age and area of geographical regions. The diversification rate hypothesis holds that tropical regions diversify faster due to higher rates of speciation (caused by increased opportunities for the evolution of reproductive isolation, or faster molecular evolution, or the increased importance of biotic interactions), or due to lower extinction rates. There is phylogenetic evidence for higher rates of diversification in tropical clades, and palaeontological data demonstrate higher rates of origination for tropical taxa, but mixed evidence for latitudinal differences in extinction rates. Studies of latitudinal variation in incipient speciation also suggest faster speciation in the tropics. Distinguishing the roles of history, speciation and extinction in the origin of the latitudinal gradient represents a major challenge to future research.

ABSTRACTAim The latitudinal diversity gradient, in which taxonomic richness is greatest at low latitudes and declines towards the poles, is a pervasive feature of the biota through geological time. This study utilizes fossil data to examine how the latitudinal diversity gradient and associated spatial patterns covaried through the major climate shifts at the onset and end of the late Palaeozoic ice age.Location Data were acquired from fossil localities from around the world.Methods Latitudinal patterns of diversity, mean geographical range size and macroevolutionary rates were constructed from a literature‐derived data base of occurrences of fossil brachiopod genera in space and time. The literature search resulted in a total of 18,596 occurrences for 991 genera from 2320 localities.Results Climate changes associated with the onset of the late Palaeozoic ice age (c. 327 Ma) altered the biogeographical structure of the brachiopod fauna by the preferential elimination of narrowly distributed, largely tropical genera when glaciation began. Because the oceans were left populated primarily with widespread genera, the slope of the diversity gradient became gentle at this time, and the gradient of average latitudinal range size weakened. In addition, because narrowly distributed genera had intrinsically high rates of origination and extinction, the gradients of both of these macroevolutionary rates were also reduced. These patterns were reversed when the ice age climate abated in early Permian time (c. 290 Ma): narrowly distributed genera rediversified at low latitudes, restoring steep gradients of diversity, average latitudinal range size and macroevolutionary rates.Main conclusions During late Palaeozoic time, these latitudinal gradients for brachiopods may have been linked by the increased magnitude of seasonality during the late Palaeozoic ice age. Pronounced seasonality would have prevented the existence of genera with narrow latitudinal ranges. These results for the late Palaeozoic ice age suggest a climatic basis for the present‐day latitudinal diversity gradient.