Savanna ant species richness is maintained along a bioclimatic gradient of increasing latitude and decreasing rainfall in northern Australia

Abstract

Aim Using a standardized sampling protocol along a 600-km transect in northern Australia, we tested whether ant diversity within a single biome, tropical savanna, decreases with increasing latitude (as a surrogate of temperature) and decreasing rainfall, as is expected for biodiversity in general. Location Northern Australia.Methods Ants were sampled using pitfall traps on three occasions at 1-ha sand, loam and clay sites at each of five locations along the Northern Australian Tropical Transect (NATT), from 12°50 0 S (1400 mm mean annual rainfall) to 17°21 0 S (650 mm). ResultsWe recorded a total of 246 species from 37 genera. Mean observed species richness pooled across sampling periods was similar at sand (85.4) and loam (82.2) sites, but was less than half this at clay sites (40.0). Ant communities were also compositionally distinct on clay soils compared with sands and loams. Individual genera showed variable diversity patterns, ranging from a linear increase to a linear decrease in species richness along the NATT. However, total species richness was relatively uniform along the gradient. Patterns of ant species turnover were consistent with previously recognized biogeographical boundaries, with a primary disjunction between the arid and monsoonal zones in the south, and a secondary disjunction between the semi-arid and mesic zones in the north.Main conclusions Patterns of ant diversity in Australian savannas do not conform to global patterns of biodiversity declines with increasing latitude and decreasing rainfall. We believe this is due to a lack of significant temperature change across the latitudinal gradient, and, in particular, to the fauna's evolutionary history in association with aridification, which makes it unusually resilient to increasing aridity. The diversity of other important faunal groups such as termites and lizards is also exceptionally high in arid Australia and is likewise not closely linked to rainfall in Australian savannas. We predict that these taxa are far more sensitive to increasing aridity in savannas elsewhere in the world, and especially in the Neotropics, where savannas have an evolutionary association with humid rain forest rather than desert.

Plant and ant assemblages predicted to decouple under climate change

“…Our finding that species richness was correlated with MAP for plants but not ants indicates that there are different processes driving diversity for these groups. The lack of change in ant species richness across a wide climate gradient is unusual (Dunn et al., ), but mirrors results from gradients from steppe to desert in central Asia (Pfeiffer, Chimedregzen, & Ulykpan, ) and tropical to arid zone savannas in northern Australia (Andersen, Del Toro, & Parr, ). Consistent richness across the northern Australian gradient was attributed to a lack of temperature change, and the presence of a megadiverse ant fauna in the arid zone, which has been a source of species radiation back in to tropical savannas (Andersen, ; Andersen et al., ).…”

Ant diversity in Neotropical savannas: Hierarchical processes acting at multiple spatial scales

“…Soil properties may also condition ant abundance and diversity at the community level. For instance, lower abundance and smaller ant species number were found in soils with high clay content both in temperate grasslands (Boulton et al, ; Campbell & Crist, ) and in tropical savannas (Andersen et al, ), a phenomenon which appears to be related to limitations in nest excavation. In fact, it has been suggested that the local assembly of ant communities is conditioned by processes operating at the establishment phase, when ant propagules (founding queens) colonize new areas and compete for nesting sites (Andersen, ; Hakala, Seppä, & Helanterä, ).…”

The underestimated biodiversity of tropical grassy biomes

“…However, ant diversity can be similarly high in tropical savannahs, especially in Australia and the Neotropics [19,28]. For example, Australian savannahs pack up to 150 ant species per hectare, and such high diversity is maintained with increasing aridity down to at least 600 mm mean annual rainfall [28]. A remarkable 15 species from a single ant genus have been recorded in a single 10 Â 10 m savannah plot [15].…”