The importance of long-term studies in understanding effects of climate change on small mammal assemblages in arid Australia

Background/Question/Methods

Arid environments are often characterized by long periods of low rainfall that are punctuated irregularly by heavy, drought-breaking rains. Biotic resources may be depleted during the long dry spells but show spectacular albeit ephemeral increases in productivity after rain. In the central deserts of Australia, broadscale wildfires frequently occur the year following pulses of productivity when vegetation has died back, and the newly open environment then provides an easy hunting ground for invasive predators. In 1990, we set up series of experimental and monitoring plots to track fluctuations in the abundance and richness of small mammals and test hypotheses about the factors that influence them. Our study region (7000 km²), the Simpson Desert, is one of the most extreme arid regions in Australia, with rainfall averaging 100 – 150 mm a year. Heavy summer rainfalls (> 400 mm) triggered pulses of increased primary productivity in 1990-91, 2000-01, 2007-08 and 2011, and a wildfire burnt > 2500 km² of the region in 2001-02. We monitor vegetation cover and diversity, invertebrates, reptiles and mammals 4 – 6 times a year, but here present the effects of weather, fire and invasive predators (red foxes and feral cats) on small native mammals only. 

Results/Conclusions

Within six months of heavy summer rainfall 2 – 3 species of rodents showed 10 to 100-fold increases in numbers, and a fourth species invaded the desert region from peripheral refuges. One species of carnivorous marsupial and several species of raptor showed numerical increases, preying largely on the abundant rodents. By contrast, all small insectivorous marsupials declined. Numerical responses were detected in the red fox and feral cat, with these predators contributing to rapid crashes in the numbers of rodents and to prolonged low periods in their populations. All small mammals declined post-wildfire in burnt but less so in unburnt habitat. Mapping of predator activity suggests that foxes and cats focus on burnt-unburnt edges to hunt small mammals, but both rodents and small marsupials show positive behavioral responses when invasive predators are excluded. Central Australian habitats are predicted to experience longer periods of drought, increased temperatures and heavier but more episodic rainfalls as the climate changes in future, threatening to exaggerate and disrupt the boom and bust dynamics of native animals. As with the Fray Jorge program, we expect our long-term data to provide an ever-more valuable baseline to test and refine our ideas about climate change and its impacts.