Tropical terrestrial habitats are renowned for their exceptional biodiversity, and one contributing factor is vertical stratification. Differences in abiotic and biotic conditions from the ground to the top of trees favour the occurrence of distinct species assemblages on the ground and on trees. Our study shows that the degree of this distinction increases with increasing latitude in Brazilian savannas.
Above: General of a Brazilian savanna (cerrado) area. (Novo São Joaquim, MT, Brazil).
Photo: Heraldo Vasconcelos.
The study was motivated by our interest in understanding how ant diversity and community structure varies along climatic gradients within Brazilian Cerrado, the world’s richest and most threatened tropical savanna. The cerrado ant fauna includes an unusually high diversity of specialist arboreal species, reflecting its relatively recent origin from tropical forest.
We sampled ants from 32 savanna sites along a latitudinal gradient of decreasing temperatures and increasing rainfall during the warmest quarter. We used a standardized sampling protocol on both the ground or in trees, allowing us to evaluate how ant species richness and composition varies both vertically (ground vs arboreal) and horizontally (across a climatic gradient). The fieldwork took several years to complete.
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Vasconcelos, H. L., Neves, K. C., & Andersen, A. N. (2023). Vertical stratification of ant assemblages varies along a latitudinal gradient in Brazilian savanna. Journal of Biogeography, 50, 1331– 1340. https://doi.org/10.1111/jbi.14628
Ground-foraging ants have lower thermal tolerances than those in trees and so we expected them to be more sensitive to climatic variation. This proved to be the case, such that species richness differences between vertical strata were much greater at the wetter and colder (higher latitude) than at the drier and hotter (lower latitude) sites. Furthermore, the turnover of species between vertical strata also decreased with latitude, due to a sharp increase in the proportion of ground-dwelling species that foraged in trees at lower latitudes. We also expected that the differences in species composition would be greater horizontally than vertically. To our surprise we found the opposite! The mean composition dissimilarity between ant assemblages from different strata was greater than the dissimilarity between assemblages from the same stratum for horizontal distances up to 1,500 km.
Ectatomma brunneum, a ground-dwelling ant species foraging in the arboreal stratum of a savanna area (Brasilia, DF, Brazil). Photo: Alexandre Ariel.
Our study is important for showing that local (vertical) variation in microclimate is an important driver of geographical variation in community structure. It is also important for understanding stratum-specific responses to climate change. In particular, a predicted hotter and drier climate in our study region can be expected to have a greater impact on ground than on arboreal ants, and ground and arboreal assemblages are likely to become less distinct from each other.
As part of our continued interest in the biogeography of ant diversity, we are pursuing the stratum-specific macroecology of Neotropical ant communities by investigating how the relationships between functional, phylogenetic, and taxonomic diversity vary between habitat strata along geographical climate gradients.
Arboreal pitfall traps installed onto the branches of a savanna tree.
Photo: Heraldo Vasconcelos.
Heraldo L. Vasconcelos and Karen C. Neves, Institute of Biology, Federal University of Uberlândia (UFU), Brazil
Alan N. Andersen, Darwin University, Darwin, Northern Territory, Australia