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AIM: Ecological patterns and process change across spatial, temporal and taxonomic scales. This confounds comparisons between modern and fossil communities, which are sampled across very different scales, especially temporal ones. We use a recent bone dataset (i.e., “death assemblages”) from a modern ecosystem to explore spatial, temporal and taxonomic scaling in biodiversity assessments. Our ultimate goal is to create a model based on these scaling relationships to facilitate meaningful comparisons between modern and fossil communities. LOCATION: Amboseli National Park, southern Kenya. TIME PERIOD: Mid‐1960 s to present day. MAJOR TAXA STUDIED: Large mammals (>1 kg). METHODS: We implemented a random placement null model and used model selection methods to investigate how species richness at Amboseli scales as a function of time and area [i.e., the species–time–area relationship (STAR) model]. We then analysed how the model coefficients change at different taxonomic scales (i.e., genus, family, order). RESULTS: In agreement with previous studies, we find species richness scales positively with time and area but with a negative interaction between the two. Rates of richness turnover decrease as taxonomic scale increases. MAIN CONCLUSIONS: We hypothesize that decreasing rates of turnover with increasing spatial and/or temporal scale are caused by taking progressively larger samples from a species pool that is changing at a slower rate relative to turnover at the scale of sampling. Because increasing area and time are simply alternative ways of uncovering the species pool, increased time‐averaging of communities results in a more spatially averaged ecological signal. Increasing taxonomic scale causes turnover rates to decrease because of how lower‐level taxa are aggregated into coarser, higher‐level ones. The STAR model presents a framework for extrapolating and comparing richness between small‐scale modern and large‐scale fossil communities, as well as a means to understand the general processes involved with changing scale.

Aim Accelerating rates of anthropogenic introductions are leading to a dramatic restructuring of species distributions globally. However, the extent to which invasions alter the imprint of evolutionary history in species geographical ranges remains unclear. Here, we provide a global assessment of how the introduction, establishment and spread of alien species alters the phylogenetic signal in geographical range size using birds as a model system. Location Global. Time period Contemporaneous. Taxa Birds. Methods We compare the phylogenetic signal in alien range size with that of native distributions of species globally (n = 9,993) and across different stages in the invasion pathway, from introduced (n = 965) to established species (n = 359). Using stochastic simulations, we test whether differences in phylogenetic signal arise from nonrandom patterns of species introduction, establishment or spread. Results Geographical range size in birds exhibits an intermediate phylogenetic signal, driven by the spatial clustering of closely related species. Nonrandom introductions, biased towards wide‐ranging species from particular clades and regions, produce an anomalously strong phylogenetic signal in the native range size of introduced species. In contrast, the phylogenetic signal in alien range size is substantially weaker than for native distributions. This weak phylogenetic signal cannot be explained by a lack of time for dispersal but is instead regulated by phylogenetic correlations across species in the location and number of introduction events. Main conclusions We demonstrate that the effects of anthropogenic introductions on the phylogenetic signal in range size vary across different stages in the invasion pathway. The process of transport and introduction amplifies the phylogenetic signal in the pool of potential invaders, whereas the subsequent pattern of spread decouples variation in alien range size from phylogenetic ancestry. Together, our findings suggest that evolutionary relatedness is likely to be a relatively weak predictor of the spread of invasive species.

Aim Protected areas have become pivotal to the modern conservation planning toolbox, but a limited understanding of marine macroecology is hampering their efficient design and implementation in pelagic environments. We explored the respective contributions of environmental factors and human impacts in capturing the distribution of an assemblage of commercially valuable, large-bodied, open-water predators (tunas, marlins and mackerels). Location Western Australia. Time period 1997–2006. Major taxa studied Pelagic fishes. Methods We compiled 10 years of commercial fishing records from the Sea Around Us Project and derived relative abundance indices from standardized catch rates while accounting for confounding effects of effort, year and gear type. We used these indices to map pelagic hotspots over a 0.5°-resolution grid and built random forests to estimate the importance of 33 geophysical, oceanographic and anthropogenic predictors in explaining their locations. We additionally examined the spatial congruence between these hotspots and an extensive network of marine reserves and determined whether patterns of co-occurrence deviated from random expectations using null model simulations. Results First, we identified several pelagic hotspots off the coast of Western Australia. Second, geomorphometrics explained up to 50% of the variance in relative abundance of pelagic fishes, and submarine canyon presence ranked as the most influential variable in the North bioregion. Seafloor complexity, geodiversity, salinity, temperature variability, primary production, ocean energy, current regimes and human impacts were also identified as important predictors. Third, spatial overlap between hotspots and marine reserves was limited, with most high-abundance areas primarily found in zones where anthropogenic activities are subject to few regulations. Main conclusions This study reveals geomorphometrics as valuable indicators of the distribution of mobile fish species and highlights the relevance of harnessing static topography as a key element in any blueprint for ocean zoning and spatial management.

Aim Understanding the relative contribution of different species interactions in shaping community assembly has been a pivotal aim in community ecology. Biotic interactions are acknowledged to be important at local scales, although their signal is assumed to weaken over longer distances. We examine the relationship between positive, neutral and negative pairwise bird abundance distributions and the phylogenetic and functional distance between these pairs after first controlling for habitat associations. Location France and Finland. Time period 1984 to 2011 (Finland), 2001 to 2012 (France). Major Taxa studied Birds. Methods We used results from French and Finnish land bird monitoring programmes, from which we created three independent datasets (French forests, French farmlands and Finnish forests). Separately for the three datasets, we fitted linear mixed-effects models for pairwise abundance values across years per point count station to infer the association between all common species pairs, while controlling for geographical distribution and habitat associations, and saved pairwise regression coefficients for further analyses. We used a null model approach to infer whether the observed associations (effect sizes) differ from random. Finally, using quantile regression, we analysed the relationships between functional dissimilarity/phylogenetic distance and effect sizes. Results Our results show both negative and positive species interactions, although negative interactions were twice as common as positive interactions. Closely related species were more likely to show strong associations, both negative and positive, than more distant species across broad spatial scales. For functional dissimilarity, the results varied across datasets. Main conclusions Our results emphasize the potential of functional and phylogenetic proximity in generating both negative and positive species associations, which can produce pervasive patterns from local to geographical scales. Future assembly studies should refrain from strict dichotomies, such as compensatory dynamics versus environmental forcing, and instead consider the possibility of positive interactions.

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

Aim Species distribution modelling typically relies completely or partially on climatic variables as predictors, overlooking the fact that these are themselves predictions with associated uncertainties. This is particularly critical when such predictors are interpolated between sparse station data, such as in the tropics. The goal of this study is to provide a new set of satellite-based climatic predictor data and to evaluate its potential to improve modelled species–climate associations and transferability to novel geographical regions. Location Rain forests areas of Central Africa, the Western Ghats of India and South America. Methods We compared models calibrated on the widely used WorldClim station-interpolated climatic data with models where either temperature or precipitation data from WorldClim were replaced by data from CRU, MODIS, TRMM and CHIRPS. Each predictor set was used to model 451 plant species distributions. To test for chance associations, we devised a null model with which to compare the accuracy metric obtained for every species. Results Fewer than half of the studied rain forest species distributions matched the climatic pattern better than did random distributions. The inclusion of MODIS temperature and CHIRPS precipitation estimates derived from remote sensing each allowed for a better than random fit for respectively 40% and 22% more species than models calibrated on WorldClim. Furthermore, their inclusion was positively related to a better transferability of models to novel regions. Main conclusions We provide a newly assembled dataset of ecologically meaningful variables derived from MODIS and CHIRPS for download, and provide a basis for choosing among the plethora of available climate datasets. We emphasize the need to consider the method used in the production of climate data when working on a region with sparse meteorological station data. In this context, remote sensing data should be the preferred choice, particularly when model transferability to novel climates or inferences on causality are invoked.

Veech (2013, Global Ecology and Biogeography, 22, 252–260) introduced a formula to calculate the probability of two species co-occurring in various sites under the assumption of statistical independence between the two distributional patterns. He presented his model as a new procedure, a ‘pairwise approach’, different from analyses of whole presence–absence matrices to examine patterns of co-occurrence. Here I show that: (1) Veech's method is identical to Fisher's exact test, a standard procedure for measuring the statistical association between two discrete variables; (2) in a broad sense, the pairwise approach is very similar to early analyses of spatial association, such as the one advanced by Forbes in 1907; (3) implicit in Veech's formula is a sampling scheme that is indistinguishable from well-known matrix-level null models that randomize the distribution of species among equiprobable sites; (4) pairwise co-occurrence patterns can be analysed using any matrix-level null model, so pairwise comparisons are not limited to using Veech's formula. The methodological distinction that Veech proposed between pairwise and matrix-level approaches does not in fact exist, although the conceptual distinction between the two approaches is still a debated topic.

Aim Studies of species turnover commonly assume that turnover is a critical determinant of species richness patterns. But the concordance in patterns of turnover and species richness along gradients is poorly known. Here we characterize elevational patterns of species turnover and test whether turnover and species richness are strongly related. Location Sixty-two elevation gradients world-wide, from 17° S to 43° N. Methods We used elevational range data for six terrestrial vertebrate groups to characterize species turnover between neighbouring elevational bands. We measured turnover as Simpson's dissimilarity, a metric that is unaffected by measured differences in species richness among recorded samples. To assess differences from random patterns, elevational turnover was compared with three null models (hard, soft and no boundaries). Lastly, elevational turnover was compared with the combined species richness of neighbouring elevational bands. Analyses were conducted at three grain sizes (200, 400 and 800 m elevation). Results We found no consistent, repeated patterns in elevational turnover. Variability among gradients was very high, with most datasets displaying multiple but inconsistently located peaks. Concordance between null predictions and empirical turnover was poor (average r2 for 200, 400 and 800m grains were: hard boundaries 0.06, 0.12 and 0.15; soft boundaries 0.06, 0.11 and 0.14; unbounded 0.03, 0.07 and 0.10; respectively), although many empirical values fell within the confidence intervals of the null model. Correlations of turnover and species richness were generally poor, but increased with analysis grain (average r2 = 0.19, 0.33 and 0.54, respectively). Main conclusions Turnover cannot serve as a general explanation for richness patterns within elevational gradients. Elevational turnover patterns are highly idiosyncratic, change with scale, and are often indistinguishable from random patterns. Despite the common assertion that the highest species richness occurs where distinct, dominant communities turn over on mountains (e.g. low- and high-elevation communities at a middle ecotone), we found no strong support for such Clementsian-structured patterns.

Aim: Approaches to calculating beta diversity (β) include classical measures based on alpha (α) and gamma (γ) diversity, and multivariate distance-based measures. Species-area relationships cause measurements of γ to vary, making comparisons of classical β among regions contingent on sampling effort. A recent null-modelling approach has attempted to account for variation in γ by calculating the degree to which β deviates from a random expectation. Here, we clarify the mathematical links between classical and multivariate approaches to measuring β, to derive predictions regarding the reliability of classical, null-model and multivariate approaches. Next, we use four ecological datasets and simulated data to test the consistency of these approaches across sampling effort and γ. We focus on an issue that arises when making comparisons among regions, namely that even small changes to the area sampled can differentially increase measured γ in each region, potentially causing artefacts in β that are driven by methodology rather than biology. Innovation: Comparisons among regions using classical and null-model measures change dramatically as sampling effort and γ increase. This change is understood for classical β because of species-area relationships, but not for null-model measures, making comparisons among regions impossible using the null-model approach. Multiple-site dissimilarity shows a similar sensitivity to γ as classical measures. In contrast, pairwise multivariate distances show no systematic effect of sampling effort and γ: increasing the number of sample plots decreases variability but does not alter mean β. Main conclusions: Multivariate pairwise distances are independent of sample size, offering the most robust comparison among regions. The widespread influence of sampling effort and γ indicate that only scale-dependent measures of classical and multiple-site β are comparable, whereas null-model β may not be comparable among regions. However, in cases where γ is well known, multiple-site dissimilarity metrics offer several advantages, and should be strongly considered.

Aim Variation in species composition among sites (β-diversity) generally decreases with increasing latitude, but the underlying mechanisms are ambiguous. Although both local and large-scale processes may drive this pattern, they act all through influencing species abundance distribution (SAD) and spatial pattern of species. A null model incorporating SAD is often used to calculate expected β-diversity, which accounts for most variation in β-diversity. However, a recent study has shown that the deviation of observed β-diversity from expected values (β-deviation) increases with latitude. The latitudinal gradients in β-deviation may be related to both latitudinal differences in SADs and the degrees of spatial aggregation. Our study aims to (1) investigate how β-deviation varies with SAD and spatial aggregation, and (2) separate the contributions of SAD and aggregation in explaining latitudinal gradients in β-deviation. Location Global. Methods 197 forest plots (each containing 10 subplots) distributed along latitudinal gradients were used. Two β-diversity models were derived for communities with randomly and nonrandomly distributed species. The two models were used to simulate relationships of β-deviation with SAD and aggregation, and to separate the contributions of these two factors in explaining latitudinal gradients in β-deviation. Results β-deviation increased with the degree of aggregation and peaked at intermediate species abundance. The fraction of β-deviation linked to SAD increased with latitude in global and regional analyses, whereas the fraction of β-deviation linked to aggregation was only significantly correlated with latitude in New World south. The degree of aggregation increased with latitude in New World south, but not in global extent and New World north. Main conclusions The latitudinal gradients in β-deviation are primarily explained by latitudinal differences in SADs. Additionally, the expected β-diversity is determined solely by SAD. Therefore, we conclude that latitude-β-diversity gradients at local spatial scales appear to be explained by latitudinal differences in SADs.

Aim Because of the negative impact that ongoing biodiversity loss may have on ecosystem properties that are critical for humans, understanding the relationship between extinction and functional diversity over time is of critical importance for conservation. However, empirical evidence concerning the sensitivity of vertebrate community function to species loss is very limited. Here we assess documented prehistoric and historic extinctions of birds on Pacific islands in an effort to quantify the consequences of extinctions for functional structure and diversity in natural communities over broad spatial scales. Location Forty-four islands from across the Pacific. Methods We estimated functional aspects of island bird communities before and after Holocene extinctions based on body size, foraging niche, diet and activity period. We used four separate metrics to measure ecological function: functional diversity (FD), functional richness (FRic), functional evenness and functional divergence. We employed null models to separate the effects of observed extinctions from changes expected due to declining species richness. Results We find that Holocene bird extinctions led to substantial changes in community-level functional diversity. Observed declines in FD and FRic were predictable from the pre-extinction composition of communities, and did not differ from null model expectations. Across all islands, we observed non-random changes in functional trait composition, with shifts away from ground-level foraging, granivory and herbivory after extinctions. Extinctions have resulted in the loss of up to 80% of original functional diversity on some islands and caused a sharp decrease in the variety of ecological functions provided by birds. Main conclusions Our findings illustrate the significant losses of functional diversity that are already taking place on many islands and demonstrate its close connection with the loss of species. Accounting for the functional roles of species allows a more integrative understanding of ecological function and helps to bridge species and ecosystem perspectives in conservation science.

Aim To explore the effects of the introduction of exotic and translocated species and possible future extirpation of native species on the functional diversity (FD) of freshwater fish assemblages. Location Japanese archipelago. Methods We examined spatio-temporal changes in species richness, FD, functional richness (the number of trait-based functional groups), and the functional group composition between historical and current fish assemblages for 27 eco-regions, and compared the relative effects of the introduction of exotic and translocated species on FD. We also used a null model approach to determine the assembly patterns and the extent of functional redundancy. Finally, we determined the effect of the loss of endangered species on FD by comparing the observed losses with simulated random loss. Results Through the introductions of non-native species, the species richness, FD and functional richness of the fish assemblages increased 2.4-, 1.6- and 2.1-fold, respectively. The functional group composition also changed largely through the additions of new functional groups. Exotic species had a significantly greater effect size than translocated species, but there were no differences in the overall net effects of exotic and translocated species. Null modelling approaches showed that the observed FD was higher than expected by chance (i.e. trait divergent) in both historical and current assemblages. There was also low functional redundancy. In our simulation, FD decreased in proportion to the loss of species, independent of whether the species were endangered. Main conclusions We demonstrated that both exotic and translocated species may change FD and functional group composition, which might have dramatic consequences for ecosystem processes. We suggest that the future extirpation of even a few native species can cause a substantial loss of FD. Our findings emphasize the need to improve conservation strategies based on species richness and conservation status, and to incorporate translocated species into targets of the management of non-native species.

Aim Determining the relative influence of niche-based and neutral processes in driving the spatial turnover of community composition is a central challenge in community ecology. Spatial patterns of functional turnover, or functional beta diversity, may capture important signals of niche-based assembly processes, but these patterns have not been quantified for communities across broad geographic and environmental gradients. Here, we analyse continental-scale patterns of species and functional beta diversity in relation to space and the environment to assess the relative importance of niche-based and neutral community assembly mechanisms. Location Eastern North America. Methods We use a continental-scale forest plot dataset and functional trait data to quantify spatial patterns of species and functional beta diversity. We use redundancy analysis-based variance partitioning to evaluate the influence of space, soil and climate on beta-diversity metrics. We use a null model approach to test for non-random functional beta diversity given the observed patterns of species turnover across spatial scales. Results Species and functional beta diversity increased with increasing geographic distance (i.e. distance decay of community similarity). Results of variance partitioning analysis show that species and functional beta diversity were spatially structured and significantly related to environmental, particularly climatic, variation. Results of null model analysis show that functional beta diversity was lower than expected based on species turnover at fine scales ( 1800 km). Main conclusions The observed patterns of functional beta diversity support a niche-based model of community assembly, driven by the deterministic filtering of species across environmental gradients based on their functional traits.

Aim Ecologists have generally agreed that -diversity is driven at least in part by ecological processes and mechanisms of community assembly and is a key determinant of global patterns of species richness. This idea has been challenged by a recent study based on an individual-based null model approach, which aims to account for the species pool. The goal of the present study is twofold: (1) to analyse data sets from different parts of the world to determine whether there are significant latitude-diversity gradients after accounting for the species pool, and (2) to evaluate the validity of the null model. Location Global. Methods A total of 257 forest plots, each being 0.1 ha in size and having 10 0.01-ha subplots, were used. We conducted four sets of analyses. A modified version of Whittaker's -diversity index was used to quantify -diversity for each forest plot. A randomization procedure was used to determine expected -diversity. Results The number of individuals per species, which characterizes species abundance distribution, alone explains 56.884.2% of the variation in observed -diversity. Species pool (-diversity) explained only an additional 2.615.2% of the variation in observed -diversity. Latitude explains 18.6% of the variation in raw deviation in Gentry's global data set, and explains 11.011.6% of the variation in standardized deviation in the global and three regional analyses. Latitude explains 33.246.2% of the variation in the number of individuals per species. Main conclusions Species abundance distribution, rather than species pool size, plays a key role in driving latitude-diversity gradients for -diversity in local forest communities. The individual-based null model is not a valid null model for investigating -diversity gradients driven by mechanisms of local community assembly because the null model incorporates species abundance distributions, which are driven by mechanisms of local community assembly and in turn generate -diversity gradients.

Aim We conduct a nested analysis of bathymetric distributions in neogastropods to test the hypothesis that bathyal and abyssal populations represent a source–sink system. Abyssal assemblages are predicted to be significantly nested subsets of bathyal assemblages, and to be characterized by low density, which drives extinction, and a high incidence of species with larval dispersal for continued immigration to maintain diversity. Location Bathyal and abyssal regions of the eastern North Atlantic Ocean. Methods We used published indices of beta diversity to distinguish the components of species dissimilarity among sites that are caused by turnover and nestedness. We used BINMATNEST and its most conservative null model to test specifically for nestedness along a depth gradient. Results Both turnover and nestedness affect beta diversity, but dissimilarity due to turnover predominates at depths of less than 3000 m, and dissimilarity due to nestedness at depths greater than 3000 m. Nestedness increases significantly down-slope at depths greater than 1000 m. The rank order of nestedness is significantly predicted by macrofaunal density and by the proportion of neogastropod species with dispersing larvae. Main conclusions Bathymetric patterns of beta diversity in the deep-sea benthos have been interpreted largely as a consequence of species turnover. Our results indicate that beta diversity in neogastropods is composed of two separate processes, turnover and nestedness, and that their relative importance changes with depth. Dissimilarity among sites due to nestedness, coupled with information on standing stock and life history, suggests that at least part of the abyssal neogastropod assemblage is maintained by source–sink dynamics.

Aim To describe and analyse the body size frequency distributions (BSFDs) of avian assemblages at several spatial scales in the Afrotropics. We also tested if the variation in median body size across assemblages at different spatial scales was related to environmental variables and whether purely stochastic processes could explain BSFDs. Location The Afrotropical biogeographic realm. Methods Avian body masses for 1960 species where analysed at continental, biome, ecoregion and local spatial scales with standard metrics. Variation in median assemblage body size was modelled as a function of environmental and spatial explanatory variables to assess non-random assemblage structure. We tested if BSFDs of smaller spatial scale distributions are random subsets of the larger spatial scale assemblages in which they are embedded, and used three different null model randomizations to investigate the influence of stochastic processes on BSFDs. Results The African avifauna's continental BSFD is unimodal and right-skewed. BSFDs generally become less skewed and less modal with decreasing spatial scale. The best-fit model explained 71% of median body size values at the ecoregion scale as a function of latitude, latitude2, longitude, species richness and species range size. BSFDs at smaller scales show non-random assembly from larger scale BSFDs distributions. Main conclusion African avifaunal BSFDs are quantitatively dissimilar to African mammal BSFDs, which are bimodal at all spatial scales. Much of the change in median body size with spatial scale can be captured by a range-weighted null model, suggesting that differential turnover between smaller- and larger-bodied species might explain the shift in the central tendency of the BSFD. At the local scale, energy may well contribute to structuring BSFDs, but this pattern is less pronounced at larger spatial scales.

Aim To develop a new probabilistic model that can be used to test for statistically significant pair-wise patterns of species co-occurrence. The model gives the probability that two species would co-occur at a frequency less than (or greater than) the observed frequency if the two species were distributed independently of one another among a set of sites. The model can be used to classify species associations as negative, positive or random. Innovation Historically, the analysis of species co-occurrence has involved the use of data randomization. An observed species presence–absence matrix is compared with randomized matrices to determine if the observed matrix has structure, either an excess or deficit of species positively or negatively associated with each other. The computer algorithms used to randomize matrices can sometimes produce Type I and Type II errors (when the randomization algorithm produces a biased set of all possible matrices) due to the randomization process itself. The probabilistic model does not rely on any data randomization, hence it has a very low Type I error rate and is powerful having a low Type II error rate. Main conclusions When applied to 10 different data sets the probabilistic model revealed significant positive and negative species associations in most of the data sets. Compared with previous analyses the model tended to find fewer significant associations; this may indicate a generally low rate of Type I error in the model. The model is easy to implement and requires no special software. The model could potentially transform the way that ecologists test for species co-occurrence in a wide range of ecological studies.

Aim A long-standing challenge in ecology is to identify the suite of factors that lead to turnover in species composition in both space and time.These factors might be stochastic (e.g. sampling and priority effects) or deterministic (e.g. competition and environmental filtering).While numerous studies have examined the relationship between turnover and individual drivers of interest (e.g.primary productivity, habitat heterogeneity,or regional ‐‘gamma’‐ diversity),few studies have disentangled the simultaneous influences of multiple stochastic and deterministic processes on both temporal and spatial turnover. If turnover is governed primarily by stochastic sampling processes, removing the sampling effects of gamma diversity should result in non-significant relationships between turnover and environmental variables.Conversely,if deterministicprocessesgovernturnoverpatterns,removing sampling effects will have little influence on turnover gradients. Here, we test these predictions. Location The United States. Methods Continental-scale,multidecadal data were used to quantify spatial and temporal turnover in avian community composition within 295 survey routes. A series of regression and structural equation models were coupled with a null model to construct statistical models describing turnover patterns. Results Examining explanatory variables alone or in combination showed that spatial and temporal turnover increased together, decreased with primary productivity and increased with habitat heterogeneity.The relationships between turnover and all variables became weaker when sampling effects were removed,but relationships with primary productivity and habitat heterogeneity remained relatively strong. In addition, spatial turnover increased strongly with spatial gamma diversity after sampling effects were removed.

Energy equivalence, the notion that population energy flux is independent of body mass, has become a key concept in ecology. We argue that energy equivalence is not an ecological ‘rule’, as claimed, but a flawed concept beset by circular reasoning. In fact, the independence of mass and energy flux is a null hypothesis. We show that our mechanistic understanding of size–density relationships (SDRs) follows directly from this null model and the assumption that energy limits abundance. Paradoxically, without this assumption energy equivalence has no meaning and we lack a mechanistic understanding for SDRs. We derive an expression for the strength (r 2) of SDRs under the null model, which provides a framework within which to compare published SDRs. This confirms that tight correlations between mass and abundance are a trivial consequence of the span of body masses considered. Our model implies that energy flux varies by five to six orders of magnitude among similarly sized mammals and to a far greater extent in birds. We conclude that the energetic paradigm can be strengthened by considering alternative, non-energetic, hypotheses.

Aims With the aim of understanding why some of the world's forests exhibit higher tree beta diversity values than others, we asked: (1) what is the contribution of environmentally related variation versus pure spatial and local stochastic variation to tree beta diversity assessed at the forest plot scale; (2) at what resolution are these beta-diversity components more apparent; and (3) what determines the variation in tree beta diversity observed across regions/continents? Location World-wide. Methods We compiled an unprecedented data set of 10 large-scale stem-mapping forest plots differing in latitude, tree species richness and topographic variability. We assessed the tree beta diversity found within each forest plot separately. The non-directional variation in tree species composition among cells of the plot was our measure of beta diversity. We compared the beta diversity of each plot with the value expected under a null model. We also apportioned the beta diversity into four components: pure topographic, spatially structured topographic, pure spatial and unexplained. We used linear mixed models to interpret the variation of beta diversity values across the plots. Results Total tree beta diversity within a forest plot decreased with increasing cell size, and increased with tree species richness and the amount of topographic variability of the plot. The topography-related component of beta diversity was correlated with the amount of topographic variability but was unrelated to its species richness. The unexplained variation was correlated with the beta diversity expected under the null model and with species richness. Main conclusions Because different components of beta diversity have different determinants, comparisons of tree beta diversity across regions should quantify not only overall variation in species composition but also its components. Global-scale patterns in tree beta diversity are largely coupled with changes in gamma richness due to the relationship between the latter and the variation generated by local stochastic assembly processes.

Aim To quantify the vulnerability of habitats to invasion by alien plants having accounted for the effects of propagule pressure, time and sampling effort. Location New Zealand. Methods We used spatial, temporal and habitat information taken from 9297 herbarium records of 301 alien plant species to examine the vulnerability of 11 terrestrial habitats to plant invasions. A null model that randomized species records across habitats was used to account for variation in sampling effort and to derive a relative measure of invasion based either on all records for a species or only its first record. The relative level of invasion was related to the average distance of each habitat from the nearest conurbation, which was used as a proxy for propagule pressure. The habitat in which a species was first recorded was compared to the habitats encountered for all records of that species to determine whether the initial habitat could predict subsequent habitat occupancy. Results Variation in sampling effort in space and time significantly masked the underlying vulnerability of habitats to plant invasions. Distance from the nearest conurbation had little effect on the relative level of invasion in each habitat, but the number of first records of each species significantly declined with increasing distance. While Urban, Streamside and Coastal habitats were over-represented as sites of initial invasion, there was no evidence of major invasion hotspots from which alien plants might subsequently spread. Rather, the data suggest that certain habitats (especially Roadsides) readily accumulate alien plants from other habitats. Main conclusions Herbarium records combined with a suitable null model provide a powerful tool for assessing the relative vulnerability of habitats to plant invasion. The first records of alien plants tend to be found near conurbations, but this pattern disappears with subsequent spread. Regardless of the habitat where a species was first recorded, ultimately most alien plants spread to Roadside and Sparse habitats. This information suggests that such habitats may be useful targets for weed surveillance and monitoring.

Aim Richness gradients are frequently correlated with environmental characteristics at broad geographic scales. In particular, richness is often associated with energy and climate, while environmental heterogeneity is rarely its best correlate. These correlations have been interpreted as evidence in favour of environmental determinants of diversity gradients, particularly energy and climate. This interpretation assumes that the expected-by-random correlation between richness and environment is zero, and that this is equally true for all environmental characteristics. However, these expectations might be unrealistic. We investigated to what degree basic evolutionary/biogeographical processes occurring independently of environment could lead to richness gradients that correlate with environmental characteristics by chance alone. Location Africa, Australia, Eurasia and the New World. Methods We produced artificial richness gradients based on a stochastic simulation model of geographic diversification of clades. In these simulations, species speciate, go extinct and expand or shift their distributions independently of any environmental characteristic. One thousand two hundred repetitions of this model were run, and the resulting stochastic richness gradients were regressed against real-world environmental variables. Stochastic species–environment relationships were then compared among continents and among three environmental characteristics: energy, environmental heterogeneity and climate seasonality. Results Simulations suggested that a significant degree of correlation between richness gradients and environment is expected even when clades diversify and species distribute stochastically. These correlations vary considerably in strength; but in the best cases, environment can spuriously account for almost 80% of variation in stochastic richness. Additionally, expected-by-chance relationships were different among continents and environmental characteristics, producing stronger spurious relationships with energy and climate than with heterogeneity. Main conclusions We conclude that some features of empirical species– environment relationships can be reproduced just by chance when taking into account evolutionary/biogeographical processes underlying the construction of species richness gradients. Future tests of environmental effects on richness should consider structure in richness–environment correlations that can be produced by simple evolutionary null models. Research should move away from the naive nonbiological null hypotheses that are implicit in traditional statistical tests.

Aim Geographic distributions of species are constrained by several factors acting at different scales, with climate assumed to be a major determinant at broad extents. Recent studies, however, have challenged this statement and indicated that climate may not dominate among the factors governing geographic distributions of species. Here, we argue that these results are misleading due to the lack of consideration of the geographic area that has been accessible to the species. Location North America. Methods We generated null distributions for 75 North American endemic and 19 non-endemic bird species. For each species, climatic envelopes of observed and null distributions were modelled using neural networks and generalized linear models, and seven climatic predictors. Values of the area under the receiver–operating characteristic curve (AUC) based on models of observed distributions were compared with corresponding AUC values for the null distributions. Results More than 82% of the endemic species showed AUC higher for the observed than for the null distributions, while 63% of the non-endemic species showed such a pattern. Main conclusions We demonstrate a dominant climatic signal in shaping North American bird distributions. Our results attest to the importance of climate in determining species distributions and support the use of climate-envelope models for estimating potential distributional areas at the appropriate spatial scales.

Aim The study aims to decipher the co-occurrence of understorey plant assemblages and, accordingly, to identify a set of species groups (diversity deconstruction) to better understand the multiple causal processes underlying post-fire succession and diversity patterns in boreal forest. Location North-eastern Canadian boreal forest (49°07′–51°44′ N; 70°13′– 65°15′ W).

Aim Species distribution models (SDMs) are used to infer niche responses and predict climate change-induced range shifts. However, their power to distinguish real from chance associations between spatially autocorrelated distribution and environmental data at continental scales has been questioned. Here, this is investigated at a regional (10 km) scale by modelling the distributions of 100 plant species native to the United Kingdom. Location UK Methods SDMs fitted using real climate data were compared to those utilising simulated climate gradients. The simulated gradients preserve the exact values and spatial structure of the real ones, but have no causal relationships with any species and so represent an appropriate null model. SDMs were fitted as generalised linear models or by the Random Forest machine learning algorithm and were either non-spatial or included spatially-explicit trend surfaces or autocovariates as predictors. Results GLMs erroneously detected significant effects (P

Aim To test two hypotheses: (i) the Rapoport effect is valid for a river continuum, particularly regarding the altitudinal distribution of mayflies over the large-scale continuum from alpine stream to medium-sized lowland river, and (ii) the observed Rapoport effect is based on high nestedness of the meta-assemblage, implying the presence of a common species pool and species-thinned nested subsets. Location Geometric centre of Eurasia, south-western Siberia, Russia. Methods Regression analysis of breadth and midpoints of species altitudinal ranges, species richness and altitude. Comparison of the observed patterns with the null model predicted by the mid-domain effect and conventional null hypothesis. Nestedness analysis. Results The Rapoport effect was explicitly demonstrated: species altitudinal ranges expanded with increase in the ranges’ midpoints and average per-altitude ranges increased with increase in altitude and concurrent decrease in species richness. The significance of each trend was confirmed with respect to both the null models applied. Nestedness analysis has revealed that the observed Rapoport effect is based on nested organization of the meta-assemblage over the river altitudinal/longitudinal gradient: a common species pool at low (but not the lowest) altitudes and nested subsets of this pool at higher altitudes. Main conclusions This study for the first time credibly shows the altitudinal Rapoport effect in freshwaters. This finding demonstrates cross-habitat-type consistency of the Rapoport effect, but does not suggest that the Rapoport effect is an obligatory attribute of river systems or lotic organisms. High nestedness underlying the observed Rapoport effect together with some previous studies indicate that the Rapoport effect in altitudinal and bathymetric gradients in general may be based on nested organization. The study highlights a deficiency of cross-habitat-type analyses of macroecological patterns (i.e. over terrestrial, freshwater and marine habitats).

Aim: A recent upsurge of interest in the island biogeography of exotic species has followed from the argument that they may provide valuable information on the natural processes structuring island biotas. Here, we use data on the occurrence of exotic bird species across oceanic islands worldwide to demonstrate an alternative and previously untested hypothesis that these distributional patterns are a simple consequence of where humans have released such species, and hence of the number of species released. Location: Islands around the world. Methods: Statistical analysis of published information on the numbers of exotic bird species introduced to, and established on, islands around the world. Results: Established exotic birds showed very similar species-area relationships to native species, but different species-isolation relationships. However, in both cases the relationship for established exotics simply mimicked that for the number of exotic bird species introduced. Exotic bird introductions scaled positively with human population size and island isolation, and islands that had seen more native species extinctions had had more exotic species released. Main conclusion: The island biogeography of exotic birds is primarily a consequence of human, rather than natural, processes. © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd.

Aim This research aims to understand the factors that shape elevational diversity gradients and how those factors vary with spatial grain. Specifically, we test the predictions of the species‐productivity hypothesis, species‐temperature hypothesis, the metabolic theory of ecology and the mid-domain effects null model. We also examine how the effects of productivity and temperature on richness depend on spatial grain. Location Deciduous forests along an elevational gradient in Great Smoky Mountains National Park, USA. Methods We sampled 22 leaf litter ant assemblages at three spatial grains, from 1-m 2 quadrats to 50 × 50 m plots using Winkler samplers. Results Across spatial grains, warmer sites had more species than did cooler sites, and primary productivity did not predict ant species richness. We found some support for the predictions of the metabolic theory of ecology, but no support for the mid-domain effects null model. Thus, our data are best explained by some version of a species‐temperature hypothesis. Main conclusions Our results suggest that temperature indirectly affects ant species diversity across spatial grains, perhaps by limiting access to resources. Warmer sites support more species because they support more individuals, thereby reducing the probability of local extinction. Many of our results from this elevational gradient agree with studies at more global scales, suggesting that some mechanisms shaping ant diversity gradients are common across scales.

Discerning the processes influencing geographical patterns of species richness remains one of the central goals of modern ecology. Traditional approaches to exploring these patterns have focused on environmental and ecological correlates of observed species richness. Recently, some have suggested these approaches suffer from the lack of an appropriate null model that accounts for species ranges being constrained to occur within a bounded domain. Proponents of these null geometric constraint models (GCMs), and the mid-domain effect these models produce, argue their utility in identifying meaningful gradients in species richness. This idea has generated substantial debate. Here we discuss what we believe are the three major challenges in the application of GCMs. First, we argue that there are actually two equally valid null models for the random placement of species ranges within a domain, one of which actually predicts a uniform distribution of species richness. Second, we highlight the numerous decisions that must be made to implement a GCM that lead to marked differences in the predictions of the null model. Finally, we discuss challenges in evaluating the importance of GCMs once they have been implemented.

Aim Recently, a flurry of studies have focused on the extent to which geographical patterns of diversity fit mid-domain effect (MDE) null models. While some studies find strong support for MDE null models, others find little. We test two hypotheses that might explain this variation among studies: small-ranged groups of species are less likely than large-ranged species to show mid-domain peaks in species richness, and mid-domain null model predictions are less robust for smaller spatial extents than for larger spatial extents. Location We analyse data sets from elevational, riverine, continental and other domains from around the world. Methods We use a combination of Spearman rank correlations and binomial tests to examine whether differences within and among studies and domains in the predictive power of MDE null models vary with spatial scale and range size. Results Small-ranged groups of species are less likely to fit mid-domain predictions than large-ranged groups of species. At large spatial extents, diversity patterns of taxonomic groups with large mean range sizes fit MDE null model predictions better than did diversity patterns of groups with small mean range sizes. MDE predictions were more explanatory at larger spatial extents than at smaller extents. Diversity patterns at smaller spatial extents fit MDE predictions poorly across all range sizes. Thus, MDE predictions should be expected to explain patterns of species richness when ranges and the scale of analysis are both large.

Aim Phylogenetic conservatism or heritability of the geographical range sizes of species (i.e. the tendency for closely related species to share similar range sizes) has been predicted to occur because of the strong phylogenetic conservatism of niche traits. However, the extent of such heritability in range size is disputed and the role of biology in shaping this attribute remains unclear. Here, we investigate the level of heritability of geographical range sizes that is generated from neutral models assuming no biological differences between species. Methods We used three different neutral models, which differ in their speciation mode, to simulate the life-history of 250,000 individuals in a square lattice of 50 × 50 cells. These individuals can speciate, reproduce, migrate and die in the metacommunity according to stochastic events. We ran each model for 3000 steps and recorded the range size of each species at each step. The heritability of geographical range size was assessed using an asymmetry coefficient between range sizes of sister species and using the coefficient of correlation between the range sizes of ancestors and their descendants. Results Our results demonstrated the ability of neutral models to mimic some important observed patterns in the heritability of geographical range size. Consistently, sister species exhibited higher asymmetry in range sizes than expected by chance, and correlations between the range sizes of ancestor–descendant species pairs, although often weak, were almost invariably positive. Main conclusions Our findings suggest that, even without any biological trait differences, statistically significant heritability in the geographical range sizes of species can be found. This heritability is weaker than that observed in some empirical studies, but suggests that even here a substantial component of heritability may not necessarily be associated with niche conservatism. We also conclude that both present-day and fossil data sets may provide similar information on the heritability of the geographical range sizes of species, while the omission of rare species will tend to overestimate this heritability.

Aim To t est the Rapoport effect using null models and data sets taken from the literature. We propose an improvement on an existing method, testing the Rapoport effect in elevational and latitudinal distributions when distributions are restricted by sampling. Location Global. Methods First, we hypothesized that real range size distributions are similar to those expected by null assumptions (expected by only imposing boundaries to species distributions). When these distributions were different from those expected under the null assumptions, we tested the hypothesis that these distributions correspond to those expected when a Rapoport effect occurs. We used two simulation methods, random and pseudo-random, which differed only in that the latter one assumes fixed species mid-points, coinciding with real mid-points. Observed correlations between range size and mid-point were compared with the frequency distribution of 1000 simulations, using both simulation methods. We compared the correlation curves generated by 1000 simulations with those of the observed distributions, testing whether correlations indicated a Rapoport effect.

Aim We studied pteridophyte species richness between 100 m and 3400 m along a Neotropical elevational gradient and tested competing hypotheses for patterns of species richness. Location Elevational transects were situated at Volcan Barva in the Braulio Carrillo National Park and La Selva Biological Station (100–2800 m) and Cerro de la Muerte (2700–3400 m), both on the Atlantic slope of Costa Rica, Central America. Method We analysed species richness on 156 plots of 20 × 20 m and measured temperature and humidity at four elevations (40, 650, 1800 and 2800 m). Species richness patterns were regressed against climatic variables (temperature, humidity, precipitation and actual evapotranspiration), regional species pool, area and predicted species number of a geometric null model (the mid-domain effect, MDE). Results The species richness of the 484 recorded species showed a hump-shaped pattern with elevation with a richness peak at mid-elevations (c. 1700 m). The MDE was the single most powerful explanatory variable in linear regression models, but species richness was also associated strongly with climatic variables, especially humidity and temperature. Area and species pool were associated less strongly with observed richness patterns. Main conclusions Geometric models and climatic models exclusive of geometric constraints explained comparable amounts of the elevational variation in species richness. Discrimination between these two factor complexes is not possible based on model fits. While overall fits of geometric models were high, large- and small-ranged species were explained by geometric models to different extents. Species with narrow elevational ranges clustered at both ends of the gradient to a greater extent than predicted by the MDE null models used here. While geometric models explained much of the pattern in species richness, we cannot rule out the role of climatic factors (or vice versa) because the predicted peak in richness from geometric models, the empirical peak in richness and the overlap in favourable environmental conditions all coincide at middle elevations. Mid-elevations offer highest humidity and moderate temperatures, whereas at high elevations richness is reduced due to low temperatures, and at low elevations by reduced water availability due to high temperatures.

Aim: To investigate the relationship between geographical range size and abundance (population density) in Australian passerines. Location: Australia (including Tasmania). Methods: We analysed the relationship between range size and local abundance for 272 species of Australian passerines, across the whole order and within families. We measured abundance as mean and maximum abundance, and used a phylogenetic generalized least-squares regression method within a maximum-likelihood framework to control for effects of phylogeny. We also analysed the relationship within seven different habitat types. Results: There was no correlation between range size and abundance for the whole set of species across all habitats. Analyses within families revealed some strong correlations but showed no consistent pattern. Likewise we found little evidence for any relationship or conflicting patterns in different habitats, except that woodland/forest habitat species exhibit a negative correlation between mean abundance and range size, whilst species in urban habitats exhibit a significant positive relationship between maximum abundance and range size. Despite the general lack of correlation, the raw data plots of range size and abundance in this study occupied a triangular space, with narrowly distributed species exhibiting a greater variation in abundances than widely distributed species. However, using a null model analysis, we demonstrate that this was due to a statistical artefact generated by the frequency distributions for the individual variables. Conclusions: We find no evidence for a positive range size-abundance relationship among Australian passerines. This absence of a relationship cannot be explained by any conflicting effects introduced by comparing across different habitats, nor is it explained by the fact that large proportions of Australia are arid. We speculate that the considerable isolation and evolutionary age of Australian passerines may be an explanatory factor.

Aim To test whether congeneric species are significantly associated with one another in space, either positively or negatively. Also, to provide a framework for a causal investigation of co-occurrence patterns by a parallel comparison of interactions in geographical and ecological data matrices. Location For the analysis of congeneric species’ co-occurrences we used 30 matrices covering a wide range of taxa and geographical areas, while for the causal investigation we used the distribution of 50 terrestrial isopod species on 20 islands and 264 sampling stations in the central Aegean archipelago, as well as a number of ecological variables for each sampling station. Methods We developed a software program (cooc) that incorporates the species-by-species approach to co-occurrence analysis using EcoSim's output of prior null model analysis of co-occurrence. We describe this program in detail, and use it to investigate one of the most common assembly rules, namely, the decreased levels of co-occurrence among congeneric species pairs. For the causal analysis, we proceed likewise, cross-checking the results from the geographical and the ecological matrices. There is only one possible combination of results that can support claims for direct competition among species. Results We do not get any strong evidence for widespread competition among congeneric species, while most communities investigated do not show significant patterns of species associations. The causal analysis suggests that the principal factors behind terrestrial isopod species associations are of historical nature. Some exceptional cases are also discussed. Main conclusions Presence/absence data for a variety of taxa do not support the assembly rule that congeneric species are under more intense competition compared to less related species. Also, these same data do not suggest strong interactions among species pairs, regardless of taxonomic status. When significant species associations can be seen in such matrices, they mainly reflect the effects of history or of habitat requirements.

Aim The elevational gradients of species richness have been well documented; however, studies about species turnover (i.e. beta diversity) in such gradients are limited. We aimed to evaluate the relationship between species turnover and elevation in rodents, and particularly to assess two assumptions at the local scale: (1) species turnover is higher at an intermediate elevation; and (2) turnover decreases at higher elevations as a result of species with larger elevational ranges; and a third assumption at a regional scale; (3) beta diversity changes positively with the size of the elevational range. Location We selected 13 studies from the literature that evaluated the relationship between elevation and species richness in rodents, which include diverse sites around the world. Methods We constructed presence/absence matrices of species occurring at 500-m intervals, in order to measure spatial species turnover, using Whittaker's (βw) and Lennon's (βsim) indices for pairwise comparisons between each pair of elevation intervals along the gradient (local scale), and for each elevational gradient (regional scale). The relationship between species turnover and elevation at the local scale was evaluated with a LOWESS (locally weighted sums of squares) regression analysis, to determine if the overall trend was linear or curvilinear. The expected distribution of species turnover along the elevational gradients under a mid-domain null model was evaluated for both indices, and the predicted values were statistically compared with the observed species turnover. Regression analysis was used to evaluate the relationship at the regional scale. Results Only one gradient showed spatial autocorrelation. At the local scale, species turnover showed the highest values at an intermediate elevation, which varied between 1000 and 2500 m depending on the particular mountain, for both indices. LOWESS analysis showed a curvilinear trend for most of the gradients, with nonsignificant regression results. The expected distribution of species turnover by the null mid-domain model showed a U-shaped pattern for βw, while no change (zero values) was predicted with βsim. Observed and predicted species turnover were statistically different. A positive and significant linear relationship was found at the regional scale. Conclusions Our results suggest that the presence of clearly defined species assemblages is reflected in the variation of turnover with elevation. Accordingly, (1) a common increment of species turnover around mid-elevations along the gradients was found for the rodents studied, whether measured independently (βsim) or not (βw), of species richness; (2) the trend was dependent on the size of the gradient: it was evident only for elevational gradients exceeding 1900 m; (3) some gradients did not show an increase in the elevational range of species at higher elevations, contrary to Stevens’ rule expectations; (4) the expected species turnover distribution significantly contrasted with the empirical results, confirming that the extent and location of the species’ elevational ranges that overlap along the gradient determine the trend in species turnover; and (5) larger mountains showed higher beta diversity values compared to smaller ones; the extent and location of the species’ ranges also determine the species turnover at this scale. The primarily general descriptions presented are useful information towards elucidating and understanding the factors governing patterns of diversity on mountains.

Aim Local-regional (LR) species diversity plots were conceived to assess the contribution of regional and local processes in shaping the patterns of biological diversity, but have been used also to explore the scaling of diversity in terms of its alpha, beta, and gamma components. Here we explore the idea that patterns in the geographical ranges of species over a continent can determine the shape of small region to large region (SRLR) plots, which are equivalent to LR plots when comparing the diversity of sites at two regional scales. Location To test that idea, we analysed the diversity patterns at two regional scales for the mammals of North America, defined as the mainland from Alaska and Canada to Panama. Method We developed a theoretical model relating average range size of species over a large-scale region with its average regional point species diversity (RPD). Then, we generated a null model of expected SRLR plots based on theoretical predictions. Species diversities at two scales were modelled using linear and saturation functions for Type I and Type II SRLR relationships, respectively. We applied the models to the case of North American mammals by examining the regional diversity and the RPD for 21 large-scale quadrats (with area equal to 160,000 km 2 ), arranged along a latitudinal gradient. Results Our model showed that continental and large-scale regional patterns of distribution of species can generate both types of SRLR relationship, and that these patterns can be reflected in LR plots without invoking any kind of local processes. We found that North American nonvolant mammals follow a Type I SRLR relationship, whereas bats follow a Type II pattern. This difference was linked to patterns in which species of the two mammalian groups distribute in geographical space. Conclusion Tr aditional LR plots and the new SRLR plots are useful tools in exploring the scaling of species diversity and in showing the relationship between distribution and diversity. Their usefulness in comparing the relative role of local and regional processes is, however, very limited.

A recent explanation for diversity gradients proposes a ‘null model’ based on how species ranges are constrained by the geometry of bounded domains. We conduct a test of this hypothesis by comparing patterns predicted by two two-dimensional geometric models against observed diversity patterns for terrestrially feeding Nearctic birds. Consistent with previous tests in two-dimensional space, we find empirical support for the hypothesis to be very weak. We also point out a fundamental conceptual flaw in the hypothesis with respect to the key assumption that ranges can exist independently of the environment in which they are embedded that undermines its theoretical basis as well. We conclude that the mid-domain effect has little empirical support and no theoretical support for its existence, and recommend that it be eliminated as a potential explanation for diversity gradients.

Null models are pattern-generating models that deliberately exclude a mechanism of interest, and allow for randomization tests of ecological and biogeographic data. Although they have had a controversial history, null models are widely used as statistical tools by ecologists and biogeographers. Three active research fronts in null model analysis include biodiversity measures, species co-occurrence patterns, and macroecology. In the analysis of biodiversity, ecologists have used random sampling procedures such as rarefaction to adjust for differences in abundance and sampling effort. In the analysis of species co-occurrence and assembly rules, null models have been used to detect the signature of species interactions. However, controversy persists over the details of computer algorithms used for randomizing presence-absence matrices. Finally, in the newly emerging discipline of macroecology, null models can be used to identify constraining boundaries in bivariate scatterplots of variables such as body size, range size, and population density. Null models provide specificity and flexibility in data analysis that is often not possible with conventional statistical tests.

Aim To test historical and current influences on the distributions of sympatric colour morphotypes in the coral reef fish genus Hypoplectrus. Location The Caribbean and surrounding tropical waters. These areas cover the entire distribution of the genus. Methods A large and extensive database of Hypoplectrus sightings was used to establish the distribution of colour morphotypes and test a long-standing hypothesis regarding their origin. First, we considered the evidence for the previously proposed ‘population centre’ hypothesis, which suggests that current morphotype distributions reflect past conditions where these colour forms evolved in allopatry. Using morphotype sighting data, the existence of clusters in occurrence and density was tested. Second, we examined whether the observed patterns of morphotype co-occurrence deviate from random expectations using null model simulations, within subregions of the distribution of the genus, to infer ecological influences on distribution. Results There is considerable variation in morphotype distribution, with even widespread morphotypes showing geographical clustering. There is also little evidence to suggest past or current geographical isolation, with only one of the 11 morphotypes (Hypoplectrus chlorurus) showing a density distribution that is consistent with the population centre hypothesis. Null model analyses show that variation in local morphotype co-occurrence is typically significantly lower than expected under random dispersal conditions. Main conclusions Our results strongly suggest that morphotype co-occurrence is not random, but there is no evidence to suggest a past allopatric radiation in Hypoplectrus colour. Current distributions are likely to be driven by competitive interactions and/or habitat preferences. Our study highlights the value of the Hypoplectrus species complex as a system for the study of speciation in the marine environment, and implies that these closely related morphotypes have ecological relevance rather than being simple colour variants of a single polymorphic species.