Does resilience to fire confer resilience to grazing in savanna ant communities of Northern Australia?

Abstract

Introduction: Evolutionary traits acquired in response to one type of disturbance will potentially confer resilience to other disturbances that have similar environmental impacts, even if the biota has no evolutionary history of such disturbances. In grassy ecosystems the environmental impacts of grazing have important similarities to those of fire through the removal of grass biomass, and we hypothesise that high resilience to frequent fire confers high resilience to grazing. Aims: We test this hypothesis by investigating the resilience of highly fire-resilient ant communities to grazing in a mesic Australian savanna, which has not historically experienced such high levels of mammalian grazing. Methods: We sampled ants using pitfall traps at Annaburroo Station in the Australian seasonal tropics using ten plot triplets, with each triplet representing no, low and heavy grazing. Grazing has had a major impact on the basal area of perennial grasses and the cover of bare ground. We considered large (> 4 mm) ants only, which tend to be particularly sensitive to disturbance. Results: We recorded 28 species of ‘large’ ants from 14 genera. Neither ant species richness nor overall composition varied significantly with grazing, and only one of the eight most common species responded to grazing. Discussion: Ant communities at Annaburoo Station are highly resilient to livestock grazing. The limited number of relevant studies suggest that ant communities in Australian savannas more generally have higher resilience than those in southern rangelands where fire frequency is relatively low. This supports our hypothesis that an evolutionary history of frequent fire confers resilience to grazing. Many more studies are required, but we suggest that resilience to grazing might be related more to evolutionary history in relation to fire, a more pervasive remover of plant biomass globally, than to grazing. Implications for insect conservation: Grazing by cattle is the dominant land-use in Australian savannas and the Australian savanna ant fauna is one of the richest on Earth; our findings of high resilience is therefore good news for insect conservation. Such good news is likely to have wide applicability to other fire-prone grassy ecosystems subject to livestock grazing.