Spatial scale, abundance and the species-energy relationship in British birds

1. The spatial scale of analysis may influence the nature, strength and underlying drivers of macroecological patterns, one of the most frequently discussed of which is the relationship between species richness and environmental energy availability. 2. It has been suggested that species-energy relationships are hump-shaped at fine spatial grains and consistently positive at larger regional grains. The exact nature of this scale dependency is, however, the subject of much debate as relatively few studies have investigated species-energy relationships for the same assemblage across a range of spatial grains. Here, we contrast species-energy relationships for the British breeding avifauna at spatial grains of 1 km x 1 km, 2 km x 2 km and 10 km x 10 km plots, while maintaining a constant spatial extent. 3. Analyses were principally conducted using data on observed species richness. While survey work may fail to detect some species, observed species richness and that estimated using nonparametric techniques were strongly positively correlated with each other, and thus exhibit very similar spatial patterns. Moreover, the forms of species-energy relationships using observed and estimated species richness were statistically indistinguishable from each other. 4. Positive decelerating species-energy relationships arise at all three spatial grains. There is little evidence that the explanatory power of these relationships varies with spatial scale. However, ratios of regional (large-scale) to local (small-scale) species richness decrease with increasing energy availability, indicating that local richness responds to energy with a steeper gradient than does regional richness. Local assemblages thus sample a greater proportion of regional richness at higher energy levels, suggesting that spatial turnover of species richness is lower in high-energy regions. Similarly, a crude measure of temporal turnover, the ratio of cumulative species richness over a 4-year period to species richness in a single year, is lower in high-energy regions. These negative relationships between turnover and energy appear to be causal as both total and mean occupancy per species increases with energy. 5. While total density in 1 km x 1 km plots correlates positively with energy availability, such relationships are very weak for mean density per species. This suggests that the observed association between total abundance and species richness may not be mediated by population extinction rates, as predicted by the more individuals hypothesis. 6. The sampling mechanism suggests that species-energy relationships arise as high-energy areas support a greater number of individuals, and that random allocation of these individuals to local areas from a regional assemblage will generate species-energy relationships. While randomized local species-energy relationships are linear and positive, predicted richness is consistently greater than that observed. The mismatch between the observed and randomized species-energy relationships probably arises as a consequence of the aggregated nature of species distributions. The sampling mechanism, together with species spatial aggregation driven by limited habitat availability, may thus explain the species-energy relationship observed at this spatial scale.     

Diversity patterns from sequentially restored grasslands support the ‘environmental heterogeneity hypothesis’

The ‘environmental heterogeneity hypothesis’ (EHH) has been proposed as a mechanism that enables species coexistence through resource partitioning. In accordance with this hypothesis, plant diversity is predicted to increase with variability in resources, but there has been weak support for this hypothesis from experimental studies. The objectives of this research were to 1) characterize how resource availability and heterogeneity (coefficient of variation) change as plant communities develop using sequentially restored grasslands, 2) determine if resource heterogeneity relates to plant diversity (effective number of species, richness and evenness) and 3) reveal if the strength of resource heterogeneity–diversity relationships is different among levels of resource availability. We quantified means and coefficients of variation in soil nitrate and light availability in grasslands established on former agricultural lands for different times and their relationship to plant diversity using a geostatistically‐informed design. Nitrate availability decreased exponentially with restoration age, but no directional change in nitrate heterogeneity across the chronosequence occurred due to high resource variability in some restorations. Light availability also decreased exponentially across the chronosequence, but there was no directional change in light heterogeneity. Nitrate heterogeneity was positively correlated with both plant richness and plant effective number of species at high levels of nitrate availability. However, no nitrate heterogeneity correlation was detected at low levels of nitrate availability. Light heterogeneity was positively correlated with plant effective number of species at low levels of light availability. However, no light heterogeneity correlation was detected at high levels of light availability. Plant evenness was not correlated with resource heterogeneity at any resource availability level. These results support the positive heterogeneity–diversity relationship predicted by EHH, and uniquely that this relationship develops within a decade of plant community development, but can be obscured by resource availability.     

Predicting patterns of plant species richness in megadiverse South Africa

Using new tools (boosted regression trees) in predictive biogeography, with extensive spatial 23 distribution data for >19 000 species, we developed predictive models for South African plant species richness patterns. Further, biome level analysis explored possible functional determinants of country‐wide regional species richness. Finally, to test model reliability independently, we predicted potential alien invasive plant species richness with an independent dataset. Amongst the different hypotheses generally invoked to explain species 30 diversity (energy, favorableness, topographic heterogeneity, irregularity and seasonality), results revealed topographic heterogeneity as the most powerful single explanatory variable for indigenous South African plant species richness. Some biome‐specific responses were observed, i.e. two of the five analyzed biomes (Fynbos and Grassland) had richness best explained by the “species‐favorableness” hypothesis, but even in this case, topographic heterogeneity was also a primary predictor. This analysis, the largest conducted on an almost exhaustive species sample in a species‐rich region, demonstrates the preeminence of topographic heterogeneity in shaping the spatial pattern of regional plant species richness. Model reliability was confirmed by the considerable predictive power for alien invasive species richness. It thus appears that topographic heterogeneity controls species richness in two main ways: firstly, by providing an abundance of ecological niches in contemporary space (revealed by alien invasive species richness relationships) and secondly, by facilitating the persistence of ecological niches through time. The extraordinary richness of the South African Fynbos biome, a world‐renowned hotspot of biodiversity with the steepest environmental gradients in South Africa, may thus have arisen through both mechanisms. Comparisons with similar regions of the world outside South Africa are needed to confirm the generality of topographic heterogeneity and favorableness as predictors of plant richness.     

Dissecting the species-energy relationship

Environmental energy availability can explain much of the spatial variation in species richness. Such species-energy relationships encompass a diverse range of forms, and there is intense debate concerning which of these predominate, and the factors promoting this diversity. Despite this there has been relatively little investigation of whether the form, and relative strength, of species-energy relationships varies with (i) the currency of energy availability that is used, and (ii) the ecological characteristics of the constituent species. Such investigations can, however, shed light on the causal mechanisms underlying species-energy relationships. We illustrate this using the British breeding avifauna. The strength of the species-energy relationship is dependent on the energy metric used, with species richness being more closely correlated with temperature than the Normalized Difference Vegetation Index, which is a strong correlate of net primary productivity. We find little evidence, however, for the thermoregulatory load hypothesis that high temperatures enable individuals to invest in growth and reproduction, rather than thermoregulation, increasing population sizes that buffer species from extinction. High levels of productive energy may also elevate population size, which is related to extinction risk by a negative decelerating function. Therefore, the rarest species should exhibit the strongest species-energy relationship. We find evidence to the contrary, together with little support for suggestions that high-energy availability elevates species richness by increasing the numbers of specialists or predators.     

Climate as a determinant of dung beetle response to native forest replacement by cattle pastures in South America

Anthropogenic disturbance in natural ecosystems reduces the number of species in biological communities and homogenizes their composition across different regions. Climate is one of the main abiotic determinants of species distributions and different factors were proposed as the main climatic drivers. Here we explored the role of regional climate on the local response of dung beetle assemblages to the replacement of native forest by cattle pastures in South America by simultaneously contrasting three climatic hypotheses: energy, seasonality and heterogeneity. We compiled a database by searching published studies comparing dung beetle richness and composition between both native forests and cattle pastures. We calculated the proportional difference in species richness and composition between habitat types. As explanatory variables, we used seven abiotic variables grouped into the three climatic hypotheses. Energy/Productivity: mean annual temperature (°C/year) and total annual precipitation (mm/year). Seasonality: annual thermal amplitude (°C/year), the average coefficient of variation of monthly precipitation and the coefficient of average monthly variation in temperature. Heterogeneity: coefficient of variation of mean annual temperature, coefficient of variation of mean annual precipitation. Using regression analyses and a model selection procedure, we found differences in species richness between native forests and cattle pastures were explained by the coefficient of variation of mean annual precipitation, whereas changes in species composition were explained by total annual precipitation and the coefficient of variation of mean annual precipitation. The response of dung beetle assemblages to livestock grazing in South American forests was associated with precipitation variation. The heterogeneity hypothesis better explained changes in species richness following forest replacement by cattle pastures, while both energy/productivity and heterogeneity hypotheses explained the changes in species composition.     

Plant species richness–environment relationships across the Subantarctic–Patagonian transition zone

Aim To evaluate the relative importance of climate, productivity, environmental heterogeneity, biotic associations and habitat use by cattle to account for the species richness of trees, shrubs and herbs across the Subantarctic–Patagonian transition. Location An area of c. 150 × 150 km, within the transition zone between the Subantarctic and Patagonian subregions on the eastern slope of the Andes (c. 39–42° S, 70–72° W). Methods All vascular plants found at each one of 50 (10 × 10 m) sampling plots were counted to estimate the local tree, shrub and herb species richness. Path analysis was used to evaluate the relationship between the richness of the three life‐forms and plant cover, dried litter biomass, mean annual temperature, annual precipitation, daily temperature range, substrate heterogeneity and number of faecal pats. Principal coordinates of neighbour matrices was used to model the spatial autocorrelation of the data. Results Total plant species richness showed a unimodal pattern of spatial variation across the transition. Richness responded positively to indirect effects of precipitation mediated through plant cover, but there was a negative overall effect of precipitation on richness towards the west of the transition, most strongly for trees. An increase in substrate heterogeneity promoted a local increase in herb and shrub richness; the richness of trees increased in sites with steeper slopes. Canopy closure had a direct negative impact on herb richness; it also increased the local accumulation of litter, which negatively affected shrub and herb richness. The impact of habitat use by cattle negatively affected herb richness in areas to the east of the biogeographical transition. Main conclusions We suggest that the importance of indirect climatic effects mediated by vegetation cover can account for species richness patterns across this transition, most strongly for woody species, which supports the productivity hypothesis. The southern temperate forests towards the west may represent a deviation from the predictions of the water–energy dynamics hypothesis. Dissimilar spatial patterns of variation in the richness of woody and herbaceous species, and their different responses to climatic and heterogeneity variables across the transition, suggest that plant life‐form influences the plant species richness–environment relationships.     

Environmental correlates of mammal species richness in South America: effects of spatial structure, taxonomy and geographic range

Although some consensus exists regarding the positive synergism between energy and heterogeneity in increasing species diversity, the role of environmental variability remains controversial. We examine how these factors interact to explain spatial variation in mammal species richness in South America. After taking into account the effects of spatial autocorrelation and area, elevation variability and energy mainly drive spatial variation in mammal species richness. The effect of environmental variability is less important. When different taxonomic groups of mammals are analyzed separately, three ways emerge whereby energy and heterogeneity interact to promote species richness. Heterogeneity may have no effect on species richness, habitat heterogeneity and energy availability contribute independently to species richness, or heterogeneity increases in importance with an increase in energy availability. The partition of species into range size quartiles shows that habitat heterogeneity and temporal instability in the resource supply account for the species richness pattern in the narrowest- ranging species. Habitat heterogeneity is significant also for intermediate ranging species but not for the widest-ranging species. Energy alone drives the species richness pattern in the latter species. The interplay between ecology and biogeographic history may ultimately explain these differences given that narrow- and wide-ranging species show distinct biogeographic patterns, and different taxonomic groups also unequally represent them.     

What drives the positive correlation between human population density and bird species richness in Australia?

Aim To test six hypotheses that could explain or mediate the positive correlation between human population density (HPD) and bird species richness while controlling for biased sampling effort. These hypotheses were labelled as follows: productivity (net primary productivity, NPP); inherent heterogeneity (diversity of vegetation types); anthropogenic heterogeneity (diversity of land uses); conservation policy (proportion of conservation land); increased productivity (human‐induced productivity increases); and the reduced‐slope hypothesis (which predicts that humans have a negative impact on species numbers across the full range of variation in HPD). Location Australia. Methods All data were collected at a spatial resolution of 1° across mainland Australia. Bird species richness was from 2007 atlas data and random subsampling was used to account for biased sampling effort. HPD was from the 2006 census. All other data were from government produced geographic information system layers. The most important biotic or abiotic factors influencing patterns in both species richness and HPD were assessed using simultaneous autoregressive models and an information theoretic approach. Results NPP appeared to be one of the main factors driving spatial congruence between bird species richness and HPD. Inherent habitat heterogeneity was weakly related to richness and HPD, although an interaction between heterogeneity and NPP indicated that the former may be an important determinant of species richness in low‐productivity regions. There was little evidence that anthropogenic landscape heterogeneity or human‐induced changes in productivity influenced the relationship between species richness and HPD, but conservation policy appeared to act as an important mediating factor and species richness was positively related to the proportion of conservation land only in regions of high HPD. Main conclusions The spatial congruence between bird species richness and HPD occurs because both respond positively to productivity and, in certain circumstances, habitat heterogeneity. Our results suggest that conservation policy could mediate this relationship, but further research is required to determine the importance of conservation reserves in supporting species in regions densely populated by humans.     

Relative contribution of abundant and rare species to species-energy relationships

A major goal of ecology is to understand spatial variation in species richness. The latter is markedly influenced by energy availability and appears to be influenced more by common species than rare ones; species-energy relationships should thus be stronger for common species. Species-energy relationships may arise because high-energy areas support more individuals, and these larger populations may buffer species from extinction. As extinction risk is a negative decelerating function of population size, this more-individuals hypothesis (MIH) predicts that rare species should respond more strongly to energy. We investigate these opposing predictions using British breeding bird data and find that, contrary to the MIH, common species contribute more to species-energy relationships than rare ones.     

Local-scale species-energy relationships in fish assemblages of some forested streams of the Bolivian Amazon

Productivity (trophic energy) is one of the most important factors promoting variation in species richness. A variety of species-energy relationships have been reported, including monotonically positive, monotonically negative, or unimodal (i.e. hump-shaped). The exact form of the relationship seems to depend, among other things, on the spatial scale involved. However, the mechanisms behind these patterns are still largely unresolved, although many hypotheses have been suggested. Here we report a case of local-scale positive species-energy relationship. Using 14 local fish assemblages in tropical forested headwater streams (Bolivia), and after controlling for major local abiotic factors usually acting on assemblage richness and structure, we show that rising energy availability through leaf litter decomposition rates allows trophically specialized species to maintain viable populations and thereby to increase assemblage species richness. By deriving predictions from three popular mechanistic explanations, i.e. the 'increased population size', the 'consumer pressure', and the 'specialization' hypotheses, our data provide only equivocal support for the latter.     

Species traits and the form of individual species-energy relationships

Environmental energy availability explains much of the spatial variation in species richness at regional scales. While numerous mechanisms that may drive such total species-energy relationships have been identified, knowledge of their relative contributions is scant. Here, we adopt a novel approach to identify these drivers that exploits the composite nature of species richness, i.e. its summation from individual species distributions. We construct individual species-energy relationships (ISERs) for each species in the British breeding avifauna using both solar (temperature) and productive energy metrics (normalized difference vegetation index) as measures of environmental energy availability. We use the slopes of these relationships and the resultant change in deviance, relative to a null model, as measures of their strength and use them as response variables in multiple regressions that use ecological traits as predictors. The commonest species exhibit the strongest ISERs, which is counter to the prediction derived from the more individuals hypothesis. There is no evidence that predatory species have stronger ISERs, which is incompatible with the suggestion that high levels of energy availability increase the length of the food chain allowing larger numbers of predators to exist. We find some evidence that species with narrow niche breadths have stronger ISERs, thus providing one of the few pieces of supportive evidence that high-energy availability promotes species richness by increasing the occurrence of specialist species that use a narrow range of resources.     

Environmental determinants of geographic butterfly richness pattern in eastern China

A long-standing task for ecologists and biogeographers is to reveal the underlying mechanisms accounting for the geographic pattern of species diversity. The number of hypotheses to explain geographic variation in species diversity has increased dramatically during the past half century. The oldest and the most popular one is environmental determination. However, seasonality, the intra-annual variability in climate variables has been rarely related to species richness. In this study, we assessed the relative importance of three environmental hypotheses: energy, seasonality and heterogeneity in explaining species richness pattern of butterflies in Eastern China. In addition, we also examined how environmental variables affect the relationship between species richness of butterflies and seed plants at geographic scale. All the environmental factors significantly affected butterfly richness, except sampling area and coefficient of variation of mean monthly precipitation. Energy and seasonality hypotheses explained comparable variation in butterfly richness (42.3 vs. 39.3 %), higher than that of heterogeneity hypothesis (25.9 %). Variation partitioning indicated that the independent effect of seasonality was much lower (0.0 %) than that of energy (5.5 %) and heterogeneity (6.3 %). However, seasonality performed better in explaining butterfly richness in topographically complex areas, reducing spatial autocorrelation in butterfly richness, and more strongly affect the association between butterflies and seed plants. The positive relationship between seed plant richness and butterfly richness was most likely the result of environmental variables (especially seasonality) influencing them in parallel. Insufficient sampling may partly explain the low explanatory power of environmental model (52.1 %) for geographic butterfly richness pattern. Our results have important implications for predicting the response of butterfly diversity to climate change.     

The mid‐domain effect and habitat complexity applied to elevational gradients: Moss species richness in a temperate semihumid monsoon climate mountain of China

The utility of elevational gradients as tools to test either ecological hypotheses and delineate elevation‐associated environmental factors that explain the species diversity patterns is critical for moss species conservation. We examined the elevational patterns of species richness and evaluated the effects of spatial and environmental factors on moss species predicted a priori by alternative hypotheses, including mid‐domain effect (MDE), habitat complexity, energy, and environment proposed to explain the variation of diversity. Last, we assessed the contribution of elevation toward explaining the heterogeneity among sampling sites. We observed the hump‐shaped distribution pattern of species richness along elevational gradient. The MDE and the habitat complexity hypothesis were supported with MDE being the primary driver for richness patterns, whereas little support was found for the energy and the environmental factors.     

Does resource availability,resource heterogeneity or species turnover mediate changes in plant species richness in grazed grasslands?

Grazing by large ungulates often increases plant species richness in grasslands of moderate to high productivity. In a mesic North American grassland with and without the presence of bison (Bos bison), a native ungulate grazer, three non-exclusive hypotheses for increased plant species richness in grazed grasslands were evaluated: (1) bison grazing enhances levels of resource (light and N) availability, enabling species that depend on higher resource availability to co-occur; (2) spatial heterogeneity in resource availability is enhanced by bison, enabling coexistence of a greater number of plant species; (3) increased species turnover (i.e. increased species colonization and establishment) in grazed grassland is associated with enhanced plant species richness. We measured availability and spatial heterogeneity in light, water and N, and calculated species turnover from long-term data in grazed and ungrazed sites in a North American tallgrass prairie. Both regression and path analyses were performed to evaluate the potential of the three hypothesized mechanisms to explain observed patterns of plant species richness under field conditions. Experimental grazing by bison increased plant species richness by 25% over an 8-year period. Neither heterogeneity nor absolute levels of soil water or available N were related to patterns of species richness in grazed and ungrazed sites. However, high spatial heterogeneity in light and higher rates of species turnover were both strongly related to increases in plant species richness in grazed areas. This suggests that creation of a mosaic of patches with high and low biomass (the primary determinant of light availability in mesic grasslands) and promotion of a dynamic species pool are the most important mechanisms by which grazers affect species richness in high productivity grasslands.     

Null models and spatial patterns of species richness in South American birds of prey

In this paper, we used geostatistical approaches to describe bi-dimensional spatial patterns in species richness of South American birds of prey (Falconiformes and Strigiformes). They indicated strong spatial patterns both across latitude and longitude, for the two groups. These patterns were then correlated with those expected by a bi-dimensional null model constructed to take into account South America continental edges. As considerable departures from the null model were observed, there may be other ecological or evolutionary explanations for spatial patterns in species richness. Variation seems to be related to habitat heterogeneity across the continent, especially when considering differences between habitats in the central and south-eastern portion of the continent and in the Andean region. This supports previous conclusions that habitat type and heterogeneity affect species richness and abundance at different spatial scales.     

Plant species coexistence at local scale in temperate swamp forest: test of habitat heterogeneity hypothesis

It has been suggested that a heterogeneous environment enhances species richness and allows for the coexistence of species. However, there is increasing evidence that environmental heterogeneity can have no effect or even a negative effect on plant species richness and plant coexistence at a local scale. We examined whether plant species richness increases with local heterogeneity in the water table depth, microtopography, pH and light availability in a swamp forest community at three local spatial scales (grain: 0.6, 1.2 and 11.4 m). We also used the variance partitioning approach to assess the relative contributions of niche-based and other spatial processes to species occurrence. We found that heterogeneity in microtopography and light availability positively correlated with species richness, in accordance with the habitat heterogeneity hypothesis. However, we recorded different heterogeneity-diversity relationships for particular functional species groups. An increase in the richness of bryophytes and woody plant species was generally related to habitat heterogeneity at all measured spatial scales, whereas a low impact on herbaceous species richness was recorded only at the 11.4 m scale. The distribution of herbaceous plants was primarily explained by other spatial processes, such as dispersal, in contrast to the occurrence of bryophytes, which was better explained by environmental factors. Our results suggest that both niche-based and other spatial processes are important determinants of the plant composition and species turnover at local spatial scales in swamp forests.     

Modelling geographical patterns in species richness using eigenvector-based spatial filters

Aim To test the mechanisms driving bird species richness at broad spatial scales using eigenvector‐based spatial filtering. Location South America. Methods An eigenvector‐based spatial filtering was applied to evaluate spatial patterns in South American bird species richness, taking into account spatial autocorrelation in the data. The method consists of using the geographical coordinates of a region, based on eigenanalyses of geographical distances, to establish a set of spatial filters (eigenvectors) expressing the spatial structure of the region at different spatial scales. These filters can then be used as predictors in multiple and partial regression analyses, taking into account spatial autocorrelation. Autocorrelation in filters and in the regression residuals can be used as stopping rules to define which filters will be used in the analyses. Results Environmental component alone explained 8% of variation in richness, whereas 77% of the variation could be attributed to an interaction between environment and geography expressed by the filters (which include mainly broad‐scale climatic factors). Regression coefficients of environmental component were highest for AET. These results were unbiased by short‐scale spatial autocorrelation. Also, there was a significant interaction between topographic heterogeneity and minimum temperature. Conclusion Eigenvector‐based spatial filtering is a simple and suitable statistical protocol that can be used to analyse patterns in species richness taking into account spatial autocorrelation at different spatial scales. The results for South American birds are consistent with the climatic hypothesis, in general, and energy hypothesis, in particular. Habitat heterogeneity also has a significant effect on variation in species richness in warm tropical regions.     

Latitudinal diversity gradients in bryophytes and woody plants: Roles of temperature and water availability

It remains unclear whether the latitudinal diversity gradients of micro- and macro-organisms are driven by the same macro-environmental variables. We used the newly completed species catalog and distribution information of bryophytes in China to explore their spatial species richness patterns, and to investigate the underlying roles of energy availability, climatic seasonality, and environmental heterogeneity in shaping these patterns. We then compared these patterns to those found for woody plants. We found that, unlike woody plants, mosses and liverworts showed only weakly negative latitudinal trends in species richness. The spatial patterns of liverwort richness and moss richness were overwhelmingly explained by contemporary environmental variables, although explained variation was lower than that for woody plants. Similar to woody plants, energy and climatic seasonality hypotheses dominate as explanatory variables but show high redundancy in shaping the distribution of bryophytes. Water variables, that is, the annual availability, intra-annual variability and spatial heterogeneity in precipitation, played a predominant role in explaining spatial variation of species richness of bryophytes, especially for liverworts, whereas woody plant richness was affected most by temperature variables. We suggest that further research on spatial patterns of bryophytes should incorporate the knowledge on their ecophysiology and evolution.     

Water–energy,land‐cover and heterogeneity drivers of the distribution of plant species richness in a mountain region of the European Alps

Aim To evaluate the relative importance of water–energy, land‐cover, environmental heterogeneity and spatial variables on the regional distribution of Red‐Listed and common vascular plant species richness. Location Trento Province (c. 6200 km²) on the southern border of the European Alps (Italy), subdivided regularly into 228 3′ × 5′ quadrants. Methods Data from a floristic inventory were separated into two subsets, representing Red‐Listed and common (i.e. all except Red‐Listed) plant species richness. Both subsets were separately related to water–energy, land‐cover and environmental heterogeneity variables. We simultaneously applied ordinary least squares regression with variation partitioning and hierarchical partitioning, attempting to identify the most important factors controlling species richness. We combined the analysis of environmental variables with a trend surface analysis and a spatial autocorrelation analysis. Results At the regional scale, plant species richness of both Red‐Listed and common species was primarily related to energy availability and land cover, whereas environmental heterogeneity had a lesser effect. The greatest number of species of both subsets was found in quadrants with the largest energy availability and the greatest degree of urbanization. These findings suggest that the elevation range within our study region imposes an energy‐driven control on the distribution of species richness, which resembles that of the broader latitude gradient. Overall, the two species subsets had similar trends concerning the relative importance of water–energy, land cover and environmental heterogeneity, showing a few differences regarding the selection of some predictors of secondary importance. The incorporation of spatial variables did not improve the explanatory power of the environmental models and the high original spatial autocorrelation in the response variables was reduced drastically by including the selected environmental variables. Main conclusions Water–energy and land cover showed significant pure effects in explaining plant species richness, indicating that climate and land cover should both be included as explanatory variables in modelling species richness in human‐affected landscapes. However, the high degree of shared variation between the two groups made the relative effects difficult to separate. The relatively low range of variation in the environmental heterogeneity variables within our sampling domain might have caused the low importance of this complex factor.     

Environmental correlates of leguminosae species richness in Mexico: Quantifying the contributions of energy and environmental seasonality

Explaining species richness patterns is a central issue in ecology, but a general explanation remains elusive. Environmental conditions have been proposed to be important drivers of these patterns, but we still need to better understand the relative contribution of environmental factors. Here, we aim at testing two environmental hypotheses for richness gradients: energy availability and environmental seasonality using diversity patterns of the family Leguminosae across Mexico. We compiled a data base of 502 species and 32,962 records. After dividing Mexico into 100 × 100 km grid cells, we constructed a map of variation in species richness that accounts for heterogeneity in sampling effort. We found the cells with the highest species richness of legumes are in the Neotropical region of Pacific coastal and southern Mexico, where the legume family dominates the tropical rain forests and seasonally dry tropical forests. Regression models show that energy and seasonality predictors can explain 25% and 49% of the variation in richness, respectively. Spatial autocorrelation analysis showed that richness has a strong spatial structure, but that most of this structure disappears when both energy and seasonality are used to account for richness gradient. Our study demonstrates multiple environmental conditions contribute complementarily to explain diversity gradients. Moreover, it shows that in some regions, environmental seasonality can be more important than energy availability, contradicting studies at coarser spatial scales. More basic taxonomic and floristic work is needed to help describe patterns of diversity for many groups to allow for testing the underlying mechanisms responsible for diversity gradients. Abstract in Spanish is available with online material.