Ant assemblages of Brazil nut trees Bertholletia excelsa in forest and pasture habitats in the Southwestern Brazilian Amazon

In the Southwestern Brazilian Amazon, the Brazil nut tree Bertholletia excelsa is the most conspicuous remnant element of original forest ecosystems in human-modified landscapes. They often exist in pastures that have very different environmental characteristics than forests. In our study, we surveyed the ant fauna of Brazil nut trees in forest and pasture habitats in the Southwestern Brazilian Amazon. Specifically, we addressed the following questions: (i) How do arboreal ant assemblages of Brazil nut tree crowns change based on habitat type? (ii) What is the contribution of forest ant species from Brazil nut tree crowns to ant assemblages in pasture Brazil nut tree crowns? (iii) What features of pasture Brazil nut trees lead to a greater number of arboreal ant species? We selected 20 trees in each habitat type (forest and pasture) and collected ant samples both at the soil surface and at tree crowns, sampling a total of 184 ant species. Species composition was primarily determined by sampling layer. We found that the number of ant species was 63% lower in pasture than in forest tree crowns. The dissimilarity between the ant assemblages of tree crown and soil surface remained almost the same in both habitats. There was relatively little overlap in species composition in the forest tree crowns versus the pasture tree crowns, with species replacement component highly contributing to the differences. We conclude that scattered Brazil nut trees plays limited role on the conservation of its associated biodiversity in human-modified landscapes such as pastures.