Determining factors of arboreal ant mosaics in Cerrado vegetation (Hymenoptera : Formicidae)

摘要

in this paper we tested the existence of arboreal ant mosaics in Cerrado (Brazilian savanna) vegetation and their causal processes. We tested the hypotheses that mosaics could be caused by distribution of resources and habitat conditions, against the null hypothesis that the spatial distribution of ants is caused by stochastic processes. The ants were sampled in seven 20x50m plots, using baited pitfalls and hand collecting. The existence of ant mosaics was assessed using association tests, and the presence of negative associations among dominant species confirmed the mosaic. Species co-occurrence was tested comparing randomized matrices with the observed co-occurrence matrix. The association of dominant species with resources (tree species) and habitat conditions (tree cover) was also tested. The results varied in each plot, showing that mosaic distribution may be caused by: (1) dominant species preference by specific habitat conditions, (2) competitive interactions and (3) stochastic processes. The occurrence of three different explanations to ant spatial distribution leads us to suggest that using only one hypothesis to explain ant mosaics is an oversimplification of the factors involved in this pattern. Probably ant mosaics appear as the result of a complex interaction of several biological and stochastic factors, which have to be formally tested, and not simply assumed.