Ecosystems can undergo abrupt transitions between alternative stable states when the driver crosses a critical threshold. Dynamical systems theory shows that when ecosystems approach the point of loss of stability associated with these transitions, they take a long time to recover from perturbations, a phenomenon known as critical slowing down. This generic feature of dynamical systems can offer early warning signals of abrupt transitions. However, these signals are qualitative and cannot quantify the thresholds of drivers at which transition may occur. Here, we propose a method to estimate critical thresholds from spatial data. We show that two spatial metrics, spatial variance and autocorrelation of ecosystem state variable, computed along driver gradients can be used to estimate critical thresholds. First, we investigate cellular‐automaton models of ecosystem dynamics that show a transition from a high‐density state to a bare state. Our models show that critical thresholds can be estimated as the ecosystem state and the driver values at which spatial variance and spatial autocorrelation of the ecosystem state are maximum. Next, to demonstrate the application of the method, we choose remotely sensed vegetation data (Enhanced Vegetation Index, EVI) from regions in central Africa and northeast Australia that exhibit alternative states in woody cover. We draw transects (8 × 90 km) that span alternative stable states along rainfall gradients. Our analyses of spatial variance and autocorrelation of EVI along transects yield estimates of critical thresholds. These estimates match reasonably well with those obtained by an independent method that uses large‐scale (250 × 200 km) spatial data sets. Given the generality of the principles that underlie our method, our method can be applied to a variety of ecosystems that exhibit alternative stable states.

BackgroundComparative phylogeography links historical population processes to current/ecological processes through congruent/incongruent patterns of genetic variation among species/lineages. Despite high biodiversity, India lacks a phylogeographic paradigm due to limited comparative studies. We compared the phylogenetic patterns of Indian populations of jungle cat (Felis chaus) and leopard cat (Prionailurus bengalensis). Given similarities in their distribution within India, evolutionary histories, body size and habits, congruent patterns of genetic variation were expected.Methodology/Principal FindingsWe collected scats from various biogeographic zones in India and analyzed mtDNA from 55 jungle cats (460 bp NADH5, 141 bp cytochrome b) and 40 leopard cats (362 bp NADH5, 202 bp cytochrome b). Jungle cats revealed high genetic variation, relatively low population structure and demographic expansion around the mid-Pleistocene. In contrast, leopard cats revealed lower genetic variation and high population structure with a F ST of 0.86 between North and South Indian populations. Niche-model analyses using two approaches (BIOCLIM and MaxEnt) support absence of leopard cats from Central India, indicating a climate associated barrier. We hypothesize that high summer temperatures limit leopard cat distribution and that a rise in temperature in the peninsular region of India during the LGM caused the split in leopard cat population in India.Conclusions/SignificanceOur results indicate that ecological variables describing a species range can predict genetic patterns. Our study has also resolved the confusion over the distribution of the leopard cat in India. The reciprocally monophyletic island population in the South mandates conservation attention.

The opposing effects of environmental filtering and competitive interactions may influence community assembly and coexistence of related species. Competition, both in the domain of ecological resources, and in the sensory domain (for example, acoustic interference) may also result in sympatric species evolving divergent traits and niches. Delineating these scenarios within communities requires understanding trait distributions and phylogenetic structure within the community, as well as patterns of trait evolution. We report that sympatric assemblages of Asian barbets (frugivorous canopy birds) consist of a random phylogenetic sample of species, but are divergent in both morphological and acoustic traits. Additionally, we find that morphology is more divergent than expected under Brownian evolution, whereas vocal frequency evolution is close to the pattern expected under Brownian motion (i.e. a random walk). Together, these patterns are consistent with a role for competition or competitive exclusion in driving community assembly. Phylogenetic patterns of morphological divergence between related species suggest that these traits are key in species coexistence. Because vocal frequency and size are correlated in barbets, we therefore hypothesize that frequency differences between sympatric barbets are a by-product of their divergent morphologies.

Aim Although global species richness patterns appear consistent across taxa and continents, patterns are elusive at smaller spatial scales. At regional/subcontinental scales, climatic, environmental and taxon‐specific contingencies are likely to interact to modify general richness patterns. We develop a biogeographical paradigm for the Himalayan range as representative of regions at similar spatial scales, and where historical climate fluctuations might interact with species ecology to drive species richness patterns. Location Himalayan range, Asia Methods We obtained a cell × species presence–absence matrix for babblers and murid rodents in 1° latitude × 1° longitude cells in the Himalayan range. We investigated nestedness in species richness patterns in these taxa along a distance gradient from the species‐rich eastern towards the relatively depauperate west. We also investigated the relationship between species autecology and westward extent along the Himalaya. Climate data were obtained from published sources. Results Himalayan babbler and murid assemblages are nested along an east–west axis, with assemblages in westward cells tending to be subsets of assemblages immediately to the east. Distance westward from the eastern Himalaya was related positively to altitudinal mobility of babbler assemblages, while body size increased with distance westward for murid assemblages. Main conclusions The eastern Himalaya, which was not glaciated over during glacial maxima, was a potential refugium for babbler and murid species. Following glacial retreat, species could have recolonized the Himalaya westwards to different extents based on ecological traits (size, altitudinal migration) determining ability to deal with the more seasonal west. This produces both (1) a nested species richness pattern, and (2) correlations between ‘filtering’ autecological traits and distance. Such patterns should be replicated in other regions with historical climatic refugia; investigating nestedness along distance gradients from refugia would be a powerful tool in mapping biogeographical history, especially in separating historical effects from currently proposed energy‐productivity relationships.