Tropical niche conservatism and the species richness gradient of North American butterflies

Aim We explore the potential role of the ‘tropical conservatism hypothesis’ in explaining the butterfly species richness gradient in North America. Its applicability can be derived from the tropical origin of butterflies and the presumed difficulties in evolving the cold tolerance required to permit the colonization and permanent occupation of the temperate zone. Location North America. Methods Digitized range maps for butterfly species north of Mexico were used to map richness for all species, species with distributions north of the Tropic of Capricorn (Extratropicals), and species that also occupy the tropics (Tropicals). A phylogeny resolved to subfamily was used to map the geographical pattern of mean root distance, a metric of the evolutionary development of assemblages. Regression models and general linear models examined environmental correlates of overall richness and for Extratropicals vs. Tropicals, patterns in summer vs. winter, and patterns in northern vs. southern North America. Results Species in more basal subfamilies dominate the south, whereas more derived clades occupy the north. There is also a ‘latitudinal’ richness gradient in Canada/Alaska, whereas in the conterminous USA richness primarily varies longitudinally. Overall richness is associated with broad‐ and mesoscale temperature gradients. The richness of Tropicals is strongly associated with temperature and distance from winter population sources. The richness of Extratropicals in the north is most strongly correlated with the pattern of glacial retreat since the more recent Ice Age, whereas in the south, richness is positively associated with the range of temperatures in mountains and the presence of forests but is negatively correlated with the broad‐scale temperature gradient. Main conclusions The tropical conservatism hypothesis provides a possible explanation for the complex structure of the species richness gradient. The Canada/Alaska fauna comprises temperate, boreal and tundra species that are nevertheless constrained by cold climates and limited vegetation, coupled with possible post‐Pleistocene recolonization lags. In the USA tropical species are constrained by temperature in winter as well as recolonization distances in summer, whereas temperate‐zone groups are richer in cooler climates in mountains and forests, where winter conditions are more suitable for diapause. The evolution of cold tolerance is key to both the evolutionary and ecological patterns.     

Local‐ to continental‐scale variation in the richness and composition of an aquatic food web

Aim We investigated patterns of species richness and composition of the aquatic food web found in the liquid‐filled leaves of the North American purple pitcher plant, Sarracenia purpurea (Sarraceniaceae), from local to continental scales. Location We sampled 20 pitcher‐plant communities at each of 39 sites spanning the geographic range of S. purpurea– from northern Florida to Newfoundland and westward to eastern British Columbia. Methods Environmental predictors of variation in species composition and species richness were measured at two different spatial scales: among pitchers within sites and among sites. Hierarchical Bayesian models were used to examine correlates and similarities of species richness and abundance within and among sites. Results Ninety‐two taxa of arthropods, protozoa and bacteria were identified in the 780 pitcher samples. The variation in the species composition of this multi‐trophic level community across the broad geographic range of the host plant was lower than the variation among pitchers within host‐plant populations. Variation among food webs in richness and composition was related to climate, pore‐water chemistry, pitcher‐plant morphology and leaf age. Variation in the abundance of the five most common invertebrates was also strongly related to pitcher morphology and site‐specific climatic and other environmental variables. Main conclusions The surprising result that these communities are more variable within their host‐plant populations than across North America suggests that the food web in S. purpurea leaves consists of two groups of species: (1) a core group of mostly obligate pitcher‐plant residents that have evolved strong requirements for the host plant and that co‐occur consistently across North America, and (2) a larger set of relatively uncommon, generalist taxa that co‐occur patchily.     

Large‐scale patterns of tree species richness and the metabolic theory of ecology

The metabolic theory of ecology (MTE) endeavours to explain ecosystem structure and function in terms of the effects of temperature and body size on metabolic rate. In a recent paper (Wang et al., 2009, Proceedings of the National Academy of Sciences USA, 106 , 13388), we tested the MTE predictions of species richness using tree distributions in eastern Asia and North America. Our results supported the linear relationship between log‐transformed species richness and the inverse of absolute temperature predicted by the MTE, but the slope strongly depends on spatial scale. The results also indicate that there are more tree species in cold climate at high latitudes in North America than in eastern Asia, but the reverse is true in warm climate at low latitudes. Qian & Ricklefs (2011, Global Ecology and Biogeography, 20 , 362–365) recently questioned our data and some of the analyses. Here we reply to them, and provide further analyses to show that their critiques are primarily based on unsuitable data and subjective conjecture.     

Carboniferous biogeography of the bryozoan Archimedes in North America

The fenestrate bryozoan genus Archimedes apparently originated and then diversified in Early Carboniferous seas of eastern North America. Its highest species richness and greatest abundance are reached in uppermost Lower to lowest Upper Carboniferous (Chesterian) rocks of eastern North America. Following the unconformity that marks the mid‐Carboniferous eustatic event, however, it is known in North America only in Upper Carboniferous basinal deposits of the Oquirrh Mountains and in nearby regions of Utah and Nevada. It is hypothesized that Archimedes’ life‐history patterns caused the biogeographical change: dependence on fragmentation as a means of recruitment did not allow Archimedes to keep up with its rapidly migrating preferred back‐barrier environment during the numerous eustatic transgressive and regressive cycles that caused the shoreline to sweep across North America during the Late Carboniferous. The persistence of Archimedes in the Oquirrh Basin and vicinity is inferred to have resulted from its preferred back‐barrier environment migrating only locally along the relatively steep basinal slope present there during the Late Carboniferous eustatic cycles.     

Energy gradients and the geographic distribution of local ant diversity

Geographical diversity gradients, even among local communities, can ultimately arise from geographical differences in speciation and extinction rates. We evaluated three models—energy-speciation, energy-abundance, and area—that predict how geographic trends in net diversification rates generate trends in diversity. We sampled 96 litter ant communities from four provinces: Australia, Madagascar, North America, and South America. The energy-speciation hypothesis best predicted ant species richness by accurately predicting the slope of the temperature diversity curve, and accounting for most of the variation in diversity. The communities showed a strong latitudinal gradient in species richness as well as inter-province differences in diversity. The former vanished in the temperature-diversity residuals, suggesting that the latitudinal gradient arises primarily from higher diversification rates in the tropics. However, inter-province differences in diversity persisted in those residuals—South American communities remained more diverse than those in North America and Australia even after the effects of temperature were removed.     

Latitude,tree species diversity and the metabolic theory of ecology

The latitudinal diversity gradient (LDG) has been known for over a century, but its origin remains poorly understood. Because both latitude and species richness are broadly related to temperature, environmental temperature has been proposed as a driver of the LDG. Recently, Wang et al. (2009, Proceedings of the National Academy of Sciences USA, 106 ,13388–13392) used datasets compiled from tree distributions in eastern Asia and North America to compare the species richness?temperature relationship between the two regions at several spatial scales and framed their analyses in the context of the metabolic theory of ecology. Here, we show that their datasets lack comparability between eastern Asia and North America and that some aspects of their analyses probably biased their results, casting doubt on some of their conclusions.     

The biogeography of aquatic macrophytes in North America since the Last Glacial Maximum

Aim To document the post‐glacial migration of the major aquatic macrophytes of North America. Location North America north of Mexico. Methods Aquatic macrophyte pollen were extracted from the North American Pollen Database. The modern pollen distribution was mapped and related to the climate to document the geographical and climatic constraints on these taxa. The fossil pollen were mapped at 2‐ka intervals for the past 21 ka. Results Numerous genera were present in ice‐free Alaska during the Last Glacial Maximum, and south of the Laurentide Ice Sheet in the southeast. Those taxa with the widest modern climatic ranges migrated rapidly into ice‐marginal areas, first in the west and then in the east of North America. Subsequent changes in the range and abundance were smaller. Main conclusions There were four migration routes of aquatic macrophytes during the late‐glacial and post‐glacial periods: a southward migration from Alaska between 14–13 and ka, a northern migration in the west at the same time into the ice‐free Cordilleran region, and movements east and west of Appalachia as early as 19 ka for some taxa into the lower Mississippi and into the upper Mississippi and Great Lakes by 11 ka. As the Laurentide ice sheet wasted, aquatic taxa with the broadest contemporary temperature tolerances rapidly occupied ice‐marginal environments.     

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.     

Reconciling topographic and climatic effects on widespread and range-restricted species richness

Aim To identify the reasons behind differing geographical species richness patterns of range‐restricted and widespread species. Location The Western Hemisphere. Methods We used regression to determine the strongest environmental predictors of richness for widespread and range‐restricted mammal species in 10,000 km² quadrats in the continental Americas. We then used range‐placement models to predict the expected correlation between range‐restricted and widespread species richness were they to be determined by identical, random, or contrasting environmental factors. Finally, to determine the reasons underlying deviations from these predictions, we divided the Americas into 5% quantiles based on temperature and topographic heterogeneity and correlated richness of these two assemblages across quantiles – an approach that avoids constraints on statistical testing imposed by low potential for range overlap among range‐restricted species. Results Minimum annual temperature was the strongest predictor of widespread species richness while topographic heterogeneity was the best, although weak, predictor of range‐restricted species richness in conventional regression analysis. Our models revealed that the observed correlation between range‐restricted and widespread species richness was similar to what would be observed if both range‐restricted and widespread species richness were determined by temperature. Patterns of range‐restricted and widespread species richness were highly correlated across temperature quantiles, but range‐restricted species uniquely showed an increasing pattern across heterogeneity quantiles. Main conclusions Species richness gradients among range‐restricted species differ from those of widespread species, but not as extensively or for the reasons reported previously. Instead, these assemblages appear to share some but not all underlying environmental determinants of species richness. Our new approach to examining species richness patterns reveals that range‐restricted and widespread species richnesses share a common response to temperature that conventional analyses have not previously revealed. However, topographic heterogeneity has assemblage‐specific effects on range‐restricted species.     

Northernmost North American Pinus contorta var. latifolia (lodgepole pine) sociations and vegetation diversity relative to its central range east of the Rocky Mountains

Lodgepole pine (Pinus contorta var. latifolia) stands were sampled in central Yukon, Canada (61.5–64°N latitude), which represented the northernmost 9% of the tree's North American range. Within this area, lodgepole pine occupied only ? 2% of the landscape. This study determined: 1) what forest sociations occurred (i.e. structural dominance‐types); 2) how plant growth form composition and richness differed from the central portion of the species’ geographical range; and 3) if stands were biased towards occurring on more thermally favorable south‐facing slopes. Five lodgepole pine sociations were recognized among 100 relevés: Rhododendron groenlandicum (Labrador tea); Cladonia arbuscula (green reindeer lichen); Calamagrostis purpurascens (purple reedgrass); Hylocomium splendens (stairstep moss) and Alnus viridis (green alder, n = 4 relevés). Rhododendron stands were proportionally more common on low gradient sites and had more total plant cover than the other sociations. Cladonia and Calamagrostis stands were typically associated with dry coarse‐textured soils and warm dry sites, respectively; whereas the composition of the Hylocomium sociation reflected the detrimental influences of atypically dense forest canopies on understory vascular plants. Only the Calamagrostis sociation was unique to the study region. Species richness among common northern lodgepole pine sociations averaged 16–19 taxa per relevé (p > 0.05). Northern compared to central range (n = 1394) relevés were compositionally different based on little overlap of their datasets in the ordination space. Northern vegetation had less (p < 0.001) total plant (129% vs 184%), deciduous shrub (9% vs 26%), broad‐leaved herb (5% vs 25%), and bryophyte (27% vs 54%) cover; had greater macro‐lichen cover (13% vs 5%) and lower floristic richness (11 vs 24 taxa) and was less than half as phytosociological diverse. Lodgepole pine stands in the northernmost portion of their range were not biased towards occurring on south‐facing slopes, which suggested an ecological potential for range expansion.     

Summer vegetation, deglaciation and the anomalous bird diversity gradient in eastern North America

Aim Geographic variation in the species richness of birds has been shown to be strongly associated with annual water and energy levels (actual evapotranspiration, AET) at the global scale. However, the gradient in eastern North America appears to be anomalous, because richness is greatest around the Great Lakes, whereas AET is highest in the south‐eastern US. Here I examine if birds may be responding to vegetation produced during the breeding season rather than to annual production. Location North America east of longitude 98° W. Methods The bird richness pattern was examined using climatic variables, remotely sensed estimates of annual and seasonal plant biomass, and time since areas were exposed by the retreating Laurentide ice sheet from 20,000 to 6000 yr bp . Results Average summer GVI (Global Vegetation Index, derived from NDVI) was found to be positively linearly associated with richness, explaining 82% of the variance, whereas the relationships between richness and annual measures of both AET and GVI were curvilinear. The pattern of retreat of the Laurentide ice sheet explained an additional 6% of the variance in richness, consistent with a previous analysis of Canadian birds. Main conclusions In eastern North America, a seasonal variable associated with plant production explains the diversity gradient rather than the annual measures, but it does not undermine a general conclusion that bird diversity is closely linked with plant biomass. Further, both contemporary and historical factors appear to influence the gradient, and an association between bird richness and the geographic pattern of glacial retreat is detectable in both climatic and plant‐biomass models of bird diversity.     

Elucidating the global elapid (Squamata) richness pattern under metabolic theory of ecology

Environmental determinants of global patterns in species richness are still uncertain. The Metabolic Theory of Ecology (MTE) proposes that species richness patterns can be explained by environmental temperature acting on the metabolism of ectothermic organisms. However, the generality of this theory has been questioned due to its low fit to the geographic variation in species richness of different taxonomic groups. Here, we investigated whether the MTE drives elapid richness, testing the non-stationarity of the relationship between the natural logarithm of species richness (ln S) and the inverse function of temperature (1/kT) using a geographically weighted regression (GWR). The relationship between ln S and 1/kT varied systematically over space and showed non-stationarity. Few tropical locations were consistent with MTE predictions, whereas other regions fitted differently. Although the slope of the GWR model ranged from low to high, the temperature did not predict species richness strongly on average and did not limit the upper values of richness. The response of richness to temperature in some areas might reflect a recent history of colonization and diversification of species across tropical and subtropical regions. In regions not affected by temperature, species richness should be structured by other biotic and abiotic interactions. This scenario reveals that the non-stationarity of the relationship would be linked to idiosyncrasies in the sample sites, which can drift the magnitude or change the relationship between species richness and temperature throughout space.     

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

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

Specialists in ancient trees are more affected by climate than generalists

Ancient trees are considered one of the most important habitats for biodiversity in Europe and North America. They support exceptional numbers of specialized species, including a range of rare and endangered wood‐living insects. In this study, we use a dataset of 105 sites spanning a climatic gradient along the oak range of Norway and Sweden to investigate the importance of temperature and precipitation on beetle species richness in ancient, hollow oak trees. We expected that increased summer temperature would positively influence all wood‐living beetle species whereas precipitation would be less important with a negligible or negative impact. Surprisingly, only oak‐specialist beetles with a northern distribution increased in species richness with temperature. Few specialist beetles and no generalist beetles responded to the rise of 4°C in summer as covered by our climatic gradient. The negative effect of precipitation affected more specialist species than did temperature, whereas the generalists remained unaffected. In summary, we suggest that increased summer temperature is likely to benefit a few specialist beetles within this dead wood community, but a larger number of specialists are likely to decline due to increased precipitation. In addition, generalist species will remain unaffected. To minimize adverse impacts of climate change on this important community, long‐term management plans for ancient trees are important.     

Divergent biogeography of native and introduced soil macroinvertebrates in North America north of Mexico

To improve understanding of the biogeographical consequences of species introduction, we examined whether introduced soil macroinvertebrates differ from natives in the relationship between species richness and key environmental predictors, and whether such differences affect the relationship between native and introduced species richness. For North America north of Mexico, we summarized jurisdiction occurrence data for seven macroinvertebrate taxa with strong influences on soil biodiversity or processes. We analysed the relationships of native and introduced species richness to each other using linear regression; to latitude using Gaussian regressions; and, using the residuals of the richness–latitude regressions, to distance from coasts, human population density, and human population size using regression and correlation. We found weak to strong positive relationships between native and introduced species richness. This variation was related to divergent relationships of native and introduced species with latitude, human population density, and distance from coasts. Native species richness declined with increasing latitude for all taxa, as did introduced species richness for taxa with predominantly lower‐latitude origins (ants, termites, non‐lumbricid earthworms). In contrast, introduced species richness peaked at higher latitudes for four taxa of predominantly Palearctic origins (weevils, ground beetles, lumbricid earthworms, isopods). Partitioning introduced taxa within these groups based on region of origin, we found that Palearctic taxa were distributed at higher latitudes than non‐Palearctic taxa. Thus source region appears to strongly influence introduced species richness–latitude relationships. Compared to natives, introduced species exhibited more positive relationships with human population density and negative relationships with distance from coasts, but did not differ in relationships with human population size. Thus coastal, densely populated regions are likely to have a higher proportion of introduced soil macroinvertebrate species. These differences between distribution of native and introduced species tend to weaken positive correlations between native and introduced species richness, especially for taxa dominated by Palearctic introductions.     

The role of disjunction and postglacial population expansion on phylogeographical history and genetic diversity of the circumboreal plant Chamaedaphne calyculata

In the last decade a number of studies has illustrated quite different phylogeographical patterns amongst plants with a northern present‐day geographical distribution, spanning the entire circumboreal region and/or circumarctic region and southern mountains. These works, employing several marker systems, have brought to light the complex evolutionary histories of this group. Here I focus on one circumboreal plant species, Chamaedaphne calyculata (leatherleaf), to unravel its phylogeographical history and patterns of genetic diversity across its geographical range. A survey of 29 populations with combined analyses of chloroplast DNA (cpDNA), internal transcribed spacer (ITS) and AFLP markers revealed structuring into two groups: Eurasian/north‐western North American, and north‐eastern North American. The present geographical distribution of C. calyculata has resulted from colonization from two putative refugial areas: east Beringia and south‐eastern North America. The variation of chloroplast DNA (cpDNA) and ITS sequences strongly indicated that the evolutionary histories of the Eurasian/north‐western North American and the north‐eastern North American populations were independent of each other because of a geographical disjunction in the distribution area and ice‐sheet history between north‐eastern and north‐western North America. Mismatch analysis using ITS confirmed that the present‐day population structure is the result of rapid expansion, probably since the last glacial maximum. The AFLP data revealed low genetic diversity of C. calyculata (P = 19.5%, H = 0.085) over the whole geographical range, and there was no evidence of loss of genetic diversity within populations in the continuous range, either at the margins or in formerly glaciated and nonglaciated regions. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 761–775.     

Weak links: 'Rapoport's rule' and large-scale species richness patterns

Many hypotheses have been proposed to explain regional species richness patterns. Among these, ‘Rapoport's rule’ has sparked considerable controversy by stating that the latitudinal gradient in species richness can be explained indirectly as a function of narrower geographic ranges for species at low latitudes. Annual climatic variability, or deviation from mean climatic conditions, has been hypothesized to moderate this phenomenon. Furthermore, taxa that avoid much of this seasonality, such as temperate zone insects that enter diapause or species that migrate, were predicted to show reduced latitudinal gradients in richness. I test the suggested link between ‘Rapoport's rule’ and species richness for two higher level insect taxa as well as for the class Mammalia. Although these taxa exhibit the well-known latitudinal gradient in species richness, simple annual climatic variability and deviation from mean annual climatic conditions provide very poor predictions of species richness in each of them. Potential evapotranspiration, a measurement of ambient climatic energy, explains most of the observed variance in regional species richness patterns for all three taxa, consistent with the species richness-energy hypothesis. I find no support for an indirect link between ‘Rapoport's rule’ and terrestrial species richness patterns in North America.     

Dispersal ability rather than ecological tolerance drives differences in range size between lentic and lotic water beetles (Coleoptera: Hydrophilidae)

Aim In aquatic ecosystems, standing (lentic) and running (lotic) waters differ fundamentally in their stability and persistence, shaping the comparative population genetic structure, geographical range size and speciation rates of lentic versus lotic lineages. While the drivers of this pattern remain incompletely understood, the suite of traits making up the ability of a species to establish new populations is instrumental in determining such differences. Here we explore the degree to which the association between habitat type and geographical range size results from differences in dispersal ability or fundamental niche breadth in the members of the Enochrus bicolor complex, an aquatic beetle clade with species across the lentic–lotic divide. Location Western Mediterranean, with a special focus on North Africa, the Iberian Peninsula and Sicily. Methods DNA sequences for four loci were obtained from species of the E. bicolor complex and analysed using phylogenetic inference. Dispersal and establishment abilities were assessed in lentic–lotic species pairs of the complex, using flight wing morphometrics and thermal tolerance ranges as surrogates, respectively. Results There were clear differences in range size between the lotic and lentic taxa of the complex, which appears to have had a lotic origin with two transitions to standing waters. Only small differences were observed in temperature tolerance and acclimation ability between the two lotic–lentic sister species studied. By contrast, wing morphometrics revealed clear, consistent differences between lotic and lentic Enochrus species pairs, the latter having a higher dispersal capacity. Main conclusions We hypothesize that there have been two habitat shifts from lotic to lentic waters, which have allowed marked expansions in geographical range size in western Mediterranean species of the E. bicolor complex. Differences in dispersal rather than in establishment ability appear to underlie differences in geographical range extent, as transitions to lentic waters were associated with changes in wing morphology, but not in thermal tolerance range. In this lineage of water beetles, selection for dispersal in geologically short‐lived lentic systems has driven the evolution of larger range sizes in lentic taxa compared with those of their lotic relatives.     

The influence of habitat heterogeneity and latitude on gamma diversity of the Nearctic Simuliidae,a ubiquitous group of stream‐dwelling insects

Among the most prominent, large‐scale patterns of species richness are the increases in richness with decreasing latitude and with increasing habitat heterogeneity. Using the stream‐dwelling larval and pupal stages of North American black flies (Diptera: Simuliidae), we address 3 broad questions about species richness: (i) Does a significant latitude–richness relationship exist? (ii) How does habitat heterogeneity influence gamma diversity? (iii) What is the sign (positive or negative) of the latitude–richness and the heterogeneity–richness relationships? We found no evidence that habitat heterogeneity influences gamma diversity. The estimated peak species richness for black flies in North America was at 50–53°N, which also corresponds with peak generic richness. All plesiomorphic, extant lineages of the Simuliidae in the Western Hemisphere are found in cool mountainous environments of North America, suggesting that peak richness at 50–53°N might be a signature of this phylogenetic pattern and a reflection of underlying historical processes.     

Type and spatial structure of distribution data and the perceived determinants of geographical gradients in ecology: the species richness of African birds

Aim Studies exploring the determinants of geographical gradients in the occurrence of species or their traits obtain data by: (1) overlaying species range maps; (2) mapping survey‐based species counts; or (3) superimposing models of individual species’ distributions. These data types have different spatial characteristics. We investigated whether these differences influence conclusions regarding postulated determinants of species richness patterns. Location Our study examined terrestrial bird diversity patterns in 13 nations of southern and eastern Africa, spanning temperate to tropical climates. Methods Four species richness maps were compiled based on range maps, field‐derived bird atlas data, logistic and autologistic distribution models. Ordinary and spatial regression models served to examine how well each of five hypotheses predicted patterns in each map. These hypotheses propose productivity, temperature, the heat–water balance, habitat heterogeneity and climatic stability as the predominant determinants of species richness. Results The four richness maps portrayed broadly similar geographical patterns but, due to the nature of underlying data types, exhibited marked differences in spatial autocorrelation structure. These differences in spatial structure emerged as important in determining which hypothesis appeared most capable of explaining each map's patterns. This was true even when regressions accounted for spurious effects of spatial autocorrelation. Each richness map, therefore, identified a different hypothesis as the most likely cause of broad‐scale gradients in species diversity. Main conclusions Because the ‘true’ spatial structure of species richness patterns remains elusive, firm conclusions regarding their underlying environmental drivers remain difficult. More broadly, our findings suggest that care should be taken to interpret putative determinants of large‐scale ecological gradients in light of the type and spatial characteristics of the underlying data. Indeed, closer scrutiny of these underlying data — here the distributions of individual species — and their environmental associations may offer important insights into the ultimate causes of observed broad‐scale patterns.