Aim The roles of dispersal limitation and environmental heterogeneity in structuring tropical species composition can be better understood by accounting for dispersal barriers and possible niche differentiation effects. We make ecological and historical interpretations of dissimilarity in avian species composition across a riverine dispersal boundary in the light of environmental characteristics, species and subspecies range limits, and geographical distances.Location Lowland rain forest, western Amazon River Basin, Peru.Methods We surveyed all birds and one plant family, collected soil samples and measured forest structural characteristics and fragmentation in surrounding landscapes, at sites to the north and south of the Amazon River flood plain. We used Mantel tests, multiple regression on distance matrices, indicator species analysis and ordination methods to assess the relationships among dissimilarities in species composition, geographical distance, position relative to the river and environmental characteristics. We examined compositional variation for all bird species, for only species without range limits between sites, and for species with and without subspecies limits at the Amazon River.Results Dissimilarity in avian species composition across the river was large, despite a lack of environmental differences. Most of this dissimilarity was accounted for by species and subspecies range limits at the river. Plant species composition did not show any dissimilarity across the river. Plant species composition and forest fragmentation explained additional components of avian compositional dissimilarity not associated with the riverine boundary and involving different bird species.Main conclusions The riverine dispersal boundary, floristic heterogeneity and forest fragmentation were associated with distinctive components of avian species compositional dissimilarity, collectively explaining three-quarters of the total dissimilarity among sites. Compositional dissimilarity was consistent with historical and continuing isolation of avian populations on opposite sides of the river, and may be partly driven by niche differentiation between subspecies. Geographical distance as a measure of dispersal limitation would not have accounted for these relationships. The use of rivers in biogeographical region delineation should address their variable importance for different taxa.
Spatial heterogeneity in the plant species composition of tropical forests is expected to influence animal species abundance and composition because vegetation constitutes the primary habitat feature for forest animals. Floristic variation is tied to variation in soils, so edaphic properties should ultimately influence animal species composition as well. The study of covariation in floristic and faunistic turnover has been hindered by the difficulty of completing coordinated surveys in hyperdiverse tropical communities, but this can be overcome with the use of a few plant taxa that function as surrogates for general floristic turnover. We used avian and plant transect surveys and soil sampling in a western Amazonian upland (terra firme) forest landscape to test whether spatial variation in bird community composition is associated with floristic turnover and corresponding edaphic gradients. Partial Mantel tests and Non-metric Multidimensional Scaling showed floristic distinctiveness between two forest types closely associated with differences in soil cation concentrations, and differences in both floristic composition and cation concentrations were further linked to compositional differences in avian species, independent of geographic distances among sites. Ten percent of bird species included in Indicator Species Analyses showed significant associations with one of the two forest types. The upland forest types that we sampled, each corresponding to a different geological formation, are intermediate relative to edaphically extreme environments in the region. Models of avian diversification should take into account this environmental heterogeneity, as should conservation planning approaches that aim to represent faunal diversity.Abstract in Spanish is available in the online version of this article.
Developing conservation strategies for threatened species increasingly requires understanding vulnerabilities to climate change, in terms of both demographic sensitivities to climatic and other environmental factors, and exposure to variability in those factors over time and space. We conducted a range-wide, spatially explicit climate change vulnerability assessment for Eastern Massasauga (Sistrurus catenatus), a declining endemic species in a region showing strong environmental change. Using active season and winter adult survival estimates derived from 17 data sets throughout the species' range, we identified demographic sensitivities to winter drought, maximum precipitation during the summer, and the proportion of the surrounding landscape dominated by agricultural and urban land cover. Each of these factors was negatively associated with active season adult survival rates in binomial generalized linear models. We then used these relationships to back-cast adult survival with dynamic climate variables from 1950 to 2008 using spatially explicit demographic models. Demographic models for 189 population locations predicted known extant and extirpated populations well (AUC = 0.75), and models based on climate and land cover variables were superior to models incorporating either of those effects independently. These results suggest that increasing frequencies and severities of extreme events, including drought and flooding, have been important drivers of the long-term spatiotemporal variation in a demographic rate. We provide evidence that this variation reflects nonadaptive sensitivity to climatic stressors, which are contributing to long-term demographic decline and range contraction for a species of high-conservation concern. Range-wide demographic modeling facilitated an understanding of spatial shifts in climatic suitability and exposure, allowing the identification of important climate refugia for a dispersal-limited species. Climate change vulnerability assessment provides a framework for linking demographic and distributional dynamics to environmental change, and can thereby provide unique information for conservation planning and management.
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