Breeding dynamics of overabundant koala (Phascolarctos cinereus) populations subject to fertility-control management

Fertility control can effectively reduce overabundant koala populations to sustainable levels, even when compensatory breeding mechanisms may partially offset its effects. A study analyzing data from two management programs—one using surgical sterilization and the other levonorgestrel implants—found that both methods successfully lowered population densities in Budj Bim National Park (Victoria) and Kangaroo Island (South Australia). The research revealed that female koalas' breeding success is strongly influenced by body condition, with sexual maturity determined by age and body weight rather than reproductive experience. Reduced population density likely decreased intraspecific competition for food and territory, leading to increased breeding success in untreated individuals at one site. Despite these compensatory responses, sustained fertility control remains a viable long-term strategy, provided annual targets are met. The study also supports levonorgestrel implants as the preferred method due to their lower invasiveness and cost-effectiveness.

Watters, F., Ramsey, D., Molsher, R., & Cassey, P. (2021). Breeding dynamics of overabundant koala (Phascolarctos cinereus) populations subject to fertility-control management. Wildlife Research, 48(7), 663–672. https://doi.org/10.1071/wr20162