Multi-scale ant diversity in savanna woodlands: an intercontinental comparison

Ecological patterns and processes are highly scale-dependent, but few studies have used standardized methodology to examine how scale dependency varies across continents. This paper examines scale dependency in comparative ant species richness and turnover in savannas of Australia and Brazil, which are well-matched climatically but whose ant faunas have contrasting biogeographic origins. The study was conducted in savanna woodland near Darwin in northern Australia and Uberlândia in central Brazil. The sampling design consisted of eight 400-m line transects, four in each continent, with eight pitfall traps located on and around each of 20 trees evenly spaced along each transect. Ant richness and species turnover were compared at three spatial scales: pitfalls associated with a tree, trees within a transect and transects within a savanna. The composition of the Australian and Brazilian savanna ant faunas was broadly similar at the subfamily level, despite the very low proportion of shared genera and species. The ground and arboreal ant faunas were very distinct from each other in both savannas, but especially in Brazil. Overall ant abundance was almost three times higher in Australia than in Brazil, both on the ground and on vegetation, but overall species richness was higher in Brazil (150 species) than in Australia (93). There was no significant difference in the mean number of species per pitfall trap, but the mean species richness was significantly higher in Brazil than in Australia at both the tree and transect scales. We attribute these scale-dependent intercontinental differences to biogeographical and historical factors in Brazil that have led to a large regional pool of arboreal species of rainforest origin. Our study underlines the importance of biogeographical context when conducting comparative analyses of community structure across biogeographical scales, and highlights the importance of process acting at regional scales in determining species richness in ant communities.