To determine the effect of rivers, environmental conditions, and isolation by distance on the distribution of species in Amazonia. Location: Brazilian Amazonia. Time period: Current. Major taxa studied: Birds, fishes, bats, ants, termites, butterflies, ferns + lycophytes, gingers and palms. We compiled a unique dataset of biotic and abiotic information from 822 plots spread over the Brazilian Amazon. We evaluated the effects of environment, geographic distance and dispersal barriers (rivers) on assemblage composition of animal and plant taxa using multivariate techniques and distance- and raw-data-based regression approaches. Environmental variables (soil/water), geographic distance, and rivers were associated with the distribution of most taxa. The wide and relatively old Amazon River tended to determine differences in community composition for most biological groups. Despite this association, environment and geographic distance were generally more important than rivers in explaining the changes in species composition. The results from multi-taxa comparisons suggest that variation in community composition in Amazonia reflects both dispersal limitation (isolation by distance or by large rivers) and the adaptation of species to local environmental conditions. Larger and older river barriers influenced the distribution of species. However, in general this effect is weaker than the effects of environmental gradients or geographical distance at broad scales in Amazonia, but the relative importance of each of these processes varies among biological groups.

The spatial distribution of species is affected by dispersal barriers, local environmental conditions and climate. However, the effect of species dispersal and their adaptation to the environment across geographic scales is poorly understood. To investigate the distribution of species from local to broad geographic scales, we sampled termites in 198 transects distributed in 13 sampling grids in the Brazilian Amazonian forest. The sampling grids encompassed an area of 271 500 km2 and included the five major biogeographic regions delimited by Amazonian rivers. Environmental data for each transect were obtained from local measurements and remote sensing. Similar to previous studies, termite species composition at the local scale was mostly associated with measures of soil texture and chemistry. In contrast, termite species composition at broad geographic scales was associated with soil nutrients, and the geographic position of the transects. Between 17 and 30% of the variance in termite species composition could be attributed exclusively to the geographic position of the transects, but could not be attributed to measured environmental variables or the presence of major rivers. Isolation by distance may have strong effects on termite species composition due to historic processes and the spatially structured environments along distinct geological formations of Amazonia. However, in contrast to many taxa in Amazonia, there is no evidence that major rivers are important barriers to termite dispersal.

Aim: To investigate the contributions of ecosystem productivity, temperature-dependent metabolism and climatic tolerances to the latitudinal gradient in species richness and changes in species composition in tropical and subtropical regions. Location: Global. Time period: Contemporary. Major taxa studied: Termites. Methods: We compiled termite occurrence data from papers using the standardized sampling protocol proposed 20 years ago by Jones and Eggleton. Three continents, 28 countries and 494 sampling plots were included in our database. We used structural equation models to determine the direct and indirect effects of productivity, and the average and extreme values of temperature and precipitation on termite species richness. We also compared the observed association between diversity and temperature with predictions from the metabolic theory of ecology and compared how the similarity in species composition relates to geographic distance and environmental differences in tropical versus subtropical regions. Results: Termite species richness was, directly and indirectly, related to average and extreme temperatures, but not to productivity. However, the association of species richness and temperature was much stronger than predicted by the metabolic theory of ecology. Moreover, species composition changed more rapidly with geographic distance and temperature differences between plots in the tropics than in the subtropics. Main conclusions: These results suggest that the increase of species richness toward the equator cannot be explained by productivity gradients. Instead, the increase in the speciation rates and the narrow species niches along the temperature gradient in tropical regions likely determine the distribution of termites at broad spatial scales.

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