The functional composition of Australia’s savanna ant communities is essentially that of its deserts. These communities are exceptionally rich, are dominated numerically and functionally by species of Iridomyrmex, and support an extremely high diversity of other arid-adapted taxa within Monomorium, Melophorus, Meranoplus and Tetramorium. One important consequence of its evolutionary origins is that savanna ant diversity is maintained along the extreme rainfall gradient from 2,000 mm on the northern coast of the Northern Territory, to 500 mm at the fringe of the northern arid zone, 800 km inland. This diversity pattern contrasts strongly with that of plants and other key faunal groups such as birds, which all decline markedly in species richness with decreasing rainfall.

 

The evolutionary history of the Australian savanna ant fauna also confers remarkable resilience to fire. Australian savannas are among the world’s most fire-prone landscapes, with more than 40 million hectares burnt each year, and in many regions most sites are burnt at least every two or three years. These fires cause inconsequential mortality for most ant species because of their soil-nesting habits. Rather, frequent fire maintains the open habitats that make the savanna ant fauna feel right at their evolutionary home.  Long-term fire exclusion sees a progressive decline in abundance and diversity of arid-adapted taxa, an increase in abundance of highly generalised, more shade-tolerant taxa (such as species of Nylanderia, Tetramorium, Pheidole and Odontomachus), and an overall reduction in diversity.

 

The savanna ant fauna of the neotropics has a totally contrasting evolutionary origin, with its biogeographical history immersed in tropical forest. Many of the key taxa have been derived from forest, as illustrated by the remarkable diversity of specialist arboreal species (especially from Cephalotes, Pseudomyrmex and Camponotus).  The dominant terrestrial species often belong to taxa that historically prefer shady rather than open habitats, and there are few specialist thermophiles. It is therefore likely that ant diversity in neotropical savaanas is far more sensitive to declining rainfall and to fire than in Australia.

 

Alan is the Chief Research Scientist at CSIRO Ecosystem Sciences Tropical Savannas in Darwin. He is a world-leading researcher in ant biodiversity and community ecology, functional group analysis and ants as bio-indicators, and fire ecology.