Interactions, Environmental Sorting and Chance: Phylostructure of a Tropical Forest Assembly
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- Gastauer, M. & Meira-Neto, J.A.A. Folia Geobot (2014) 49: 443. doi:10.1007/s12224-013-9181-1
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Density dependence, environmental sorting and chance have been discussed for the purpose of understanding, predicting and explaining the species richness, composition and structural parameters of living communities. Different ecological mechanisms occur individually in an overlapping manner, so the structure of each local community is influenced by an independent mixture of these factors. To identify which of these factors prevails in organizing the species-rich tree community from 100 plots of 10 × 10 m in a primary forest patch (the Forest of Seu Nico – FSN, from the Atlantic Forest domain), we analyzed species-environment correlations via canonical correspondence analysis and identified two different pedo-environments. We analyzed the community’s phylogenetic structure using Phylocom 4.2 software to calculate the net relatedness index (NRI) and the nearest taxon index (NTI). Furthermore, we partitioned the total phylogenetic diversity into independent α and β components (ΠST). To reveal the overlap of ecological mechanisms such as neutrality, environmental filtering and density-dependent factors, we analyzed the phylogenetic structure in both pedo-environments. The species-environment correlations observed in the FSN are weak in comparison with those found in other studies, although the permanent plot presents a short environmental gradient, dividing the plot into an upper, more acidic hillside and a lower, more fertile bottom. The overall phylogenetic structure of the FSN community shows strong and significant phylogenetic overdispersion. This overdispersion indicates that density-dependent factors, such as interspecific competition, play an important role in maintaining the species richness and community structure in megadiverse ecosystems such as the FSN when we assume traits to be conserved within evolutionary lineages. The NRI and NTI are correlated positively with the soil pH and negatively with the soil’s aluminum concentration, so the bottom plots show higher phylogenetic overdispersion and lower ΠST values than the hillside plots. This pattern can be explained by the greater importance of environmental filters in more acidic soils that form less favorable habitats, while the influence of competition and therefore also the rate of competitive exclusion are higher in the more favorable, less acidic plots.