Functional structure in ant communities along a latitudinal gradient in the Brazilian Atlantic Forest hotspot

Background/Question/Methods

General principles about community structure can be expressed thought functional traits and be used to create predictive models in Ecology. Increased attention has been paid to factors accounting for the functional diversity of species assemblages. We analyze the interaction between leaf-litter ant species richness, functional and morphological structures within the context of a latitudinal gradient in one of the world's most species-rich ecosystems. We surveyed 26 Atlantic rain forest localities, representing 3,000 km of tropical rainforest along twenty degrees of latitude in eastern Brazil. At each site, fifty 1-m2 leaf-litter samples were collected along a transect of 1,200 m and ants were extracted using mini-Winklers extractors for 48 hs. First, we describe the relationship between species richness, functional diversity and latitude. Second, we test for non-random distribution of morphological similarity between co-occurring species at three scales (plot, site, and regional pools). Third, we examine the effect of climatic variables (temperature, precipitation and temperature range) and site characteristics (elevation and area) on local ant species richness, functional and taxonomic beta diversities.

Results/Conclusions

Altogether we recorded 550 ant species from 1,300 1-m2 samples of leaf-litter in 26 areas. Observed species richness showed an inverse latitudinal pattern, even after correcting for species abundance. Functional diversity components (richness, divergence, evenness, specialization) showed different responses to the Atlantic Forest latitudinal gradient. Functional richness and functional evenness were not related to latitude. Functional specialization of the communities showed a classical latitudinal pattern in diversity; functional divergence showed a weak positive relationship. The taxonomic beta diversity and functional beta diversity measures showed a general decay in similarity with geographical or environmental distance along the Atlantic Forest. However, the structure of assemblages remains unchanged along the gradient, in terms of functional complementary and guild structure. Overall, wide-community analysis shows random morphological structuring along the latitudinal gradient. At sample scale, we found overdispersion at the northeastern Forest sites and random structure at the southern half of the Atlantic Forest. Within-guilds morphological structure (1-m2 scale, Atlantic Forest species pool) suggests general overdispersion along the latitudinal gradient. At the site level (1-m2 scale, community species pool) we found morphological overdispersion along the gradient; we found also significant morphological aggregation in a few guilds (fungus-grower, generalists, medium hypogaeic or epigaeic predators, small hypogaeic predators). We conclude that splitting components of diversity is appropriate to elucidate processes of community assembly and may have important consequences in understanding biotic communities along environmental gradients.