Long-term fertility control reduces overabundant koala populations and mitigates their impacts on food trees

Abstract

Overabundant herbivorous mammals can increase the mortality rates of preferred food species, causing long-term changes to plant communities. When lethal control of the herbivore is not possible, fertility control is often advocated as a management solution. There is, however, little empirical evidence that fertility control is a viable option for managing the impacts of open populations of polygynous mammals. We evaluated the effectiveness of two long-term fertility control programs implemented to reduce overabundant koala (Phascolarctos cinereus) populations and their impacts on preferred food trees (manna gum, Eucalyptus viminalis): one in Victoria (10 years) and one in South Australia (17 years). Fertility control reduced koala recruitment such that population densities more than halved at both sites. Mortality rates of manna gum trees were significantly reduced at one site, and trees with light or moderate defoliation recovered at both sites. Relationships between koala population density and canopy defoliation suggest that if management wishes to alleviate moderate browsing pressure then koala density should be maintained below 1.0 koala/ha. We conclude that population-level fertility control can significantly reduce overabundant koala populations and some of their impacts on food trees. Our empirical study demonstrates that long-term fertility control can reduce the undesirable impacts of open populations of polygynous mammals.