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.
Question How do soil fertility, neighbourhood host tree composition and bark characteristics influence community attributes of vascular epiphytes in a central Amazonian forest? Location Adolpho Ducke Forest Reserve, Amazonas, Brazil. Methods The abundances of all species of vascular epiphytes were recorded from 300 host trees with DBH >30 cm. Phorophytes were distributed among 30 sample plots established in lowland, slope and upland habitats, and bark characteristics were classified into five types (rough, peeling, fissured, smooth, rugose). The epiphytic species were also classified into three functional types (true epiphytes, hemi‐epiphytes and nomadic vines) for separate analyses in an effort to highlight differences in ecological constraints in relation to neighbourhood phorophyte composition and edaphic variation. Results More than 21 000 individuals representing 122 species distributed in 66 genera and 20 families of vascular epiphytes were recorded from 300 phorophytes during the course of the study. Multiple regressions demonstrated that neighbourhood phorophyte composition and edaphic variables, nitrogen (N) and phosphorus (P), together explained 65% of the variation in epiphytic species composition. However, edaphic factors were the only significant predictors of epiphytic species richness, accounting for 70% of this variation. Nearly one‐third of the true epiphytes occurred only on phorophytes in soils with high P content. Null models demonstrate that a vast majority of the epiphyte species (76% in positive and 96% in negative associations) failed to exhibit a statistically significant preference for bark texture. Conclusion Neighbourhood phorophyte composition, in concert with N and P availability, accounts for the vast majority of the variation in vascular epiphyte composition and abundance in central Amazonian forests. However, only N and P were shown to be significant predictors of epiphyte richness. Such results suggest that both biotic and abiotic variables influence epiphyte community structure (composition, abundance and richness) at differing intensities. Weak associations between bark type and epiphyte taxa suggest that epiphyte community variation may also be influenced by other unmeasured physical and environmental factors such as phorophyte architecture or microclimatic differences. Nonetheless, these results corroborate with other studies which report significant changes in epiphyte composition and richness as a result of experimentally manipulated ground soil fertility.
Variation in the spatial structure of communities in terms of species composition (beta diversity) is affected by different ecological processes, such as environmental filtering and dispersal limitation. Large rivers are known as barriers for species dispersal (riverine hypothesis) in tropical regions. However, when organisms are not dispersal limited by geographic barriers, other factors, such as climatic conditions and geographic distance per se, may affect species distribution. In order to investigate the relative contribution of major rivers, climate and geographic distance on Passeriformes beta diversity, we divided Amazonia into 549 grid cells (1° of latitude and longitude) and obtained data of species occurrence, climate and geographic position for each cell. Beta diversity was measured using taxonomic, phylogenetic and functional metrics of composition. The influence of climatic variables, geographic distance and rivers on these metrics was tested using regression analyses. Passerine beta diversity is characterized mainly by the change in species taxonomic identity and in phylogenetic lineages across climatic gradients and over geographic distance. However, species with similar traits are found throughout the entire Amazonia. The size of rivers was proportional to their effect on species composition. However, climate and geographic distance are relatively more important than rivers for Amazonian taxonomic and phylogenetic species composition.
Environmental filtering is a major mechanism structuring ecological communities. However, it is still not clear how different abiotic drivers composing the environmental filter interact with each other to determine local species assemblage and create spatial patterns in species distribution. Here, we evaluated the effects of two strong and uncorrelated environmental variables (salinity and sediment properties) on the β‐diversity of an estuarine macrobenthic community while accounting for spatial effects. Our results show that the benthic community composition has a strong spatial structure along the estuary, which can be greatly explained by salinity and sediment variation. Salinity is most associated with species replacement (turnover), whereas sediment is more important for species loss (nestedness). However, the effects of sediment variation on nestedness are mainly detected at a smaller spatial scale (estuarine sectors), whereas the effects of salinity on species turnover are stronger as spatial scale increases (entire estuary). Our findings suggest that environmental filters can drive both turnover and nestedness components of β‐diversity, but that their relative importance depends on the spatial scale of investigation. Although abiotic drivers associated with detrimental effects (sediment) usually result in nestedness, larger spatial scales encompass abiotic drivers associated with different suitable conditions (salinity), increasing the relative importance of the replacement component of species β‐diversity.
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.
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