Post-fire grazing by feral ungulates significantly reduces perennial grass cover in north Australian mesic savannas

Abstract

Since the late 1800s, north Australian savannas have experienced a fundamental shift in grazing pressure, moving from an assemblage of native macropods to introduced ungulates. In contrast to macropods, ungulates have hard hooves, are greater in size and exhibit bulk feeding strategies, resulting in a greater impact on perennial grass cover. However, the response of perennial grasses to ungulate grazing may vary depending on rainfall, soil fertility and fire, as well as ungulate density and feeding behaviour. Interactions between grazing, drought and fire are of particular interest, as climate change is predicted to increase rainfall variability and fire frequency across northern Australia. In this study, we tested for interactions between grazing and drought, as well as fire and grazing, on two vegetation zones surrounding waterholes, through a 5 years fencing exclusion experiment. Despite drought conditions occurring over 2 years of the study, no interaction between grazing and drought was detected. This is most likely due to the relatively low ungulate density present during the study (~1 ungulate/km²). In contrast, after fire was introduced to the study sites, significantly more perennial grass cover was recorded at fenced sites relative to unfenced sites. Due to the study design the cause of change cannot be directly inferred. However, when the potential drivers are considered further (rainfall, soil properties, fire and changes in ungulate density), an interaction between grazing and fire is the most probable cause. Wetland vegetation showed a stronger response to fencing than the surrounding savanna, with this result most likely due to differences in ungulate feeding behaviour between the two vegetation zones. Our findings are important, as fire and ungulate grazing are both widespread across Australia's savannas and reduced perennial grass cover has been linked to increased cat predation on rapidly declining small, native mammal populations.