Breeding dynamics of overabundant koala (Phascolarctos cinereus) populations subject to fertility-control management
Freyja Watters
A D , David Ramsey B , Robyn Molsher C and Phillip Cassey A
+ Author Affiliations
- Author Affiliations
A Invasion Science and Wildlife Ecology Lab, Benham Building, Room 106, University of Adelaide, North Terrace, Adelaide, SA 5005, Australia.
B Arthur Rylah Institute for Environmental Research, Department of Environment, Land, Water and Planning, Heidelberg, Vic. 3084, Australia.
C Department for Environment and Water, PO Box 39, Kingscote, SA 5223, Australia.
D Corresponding author. Email: freyja.watters@adelaide.edu.au
Wildlife Research 48(7) 663-672 https://doi.org/10.1071/WR20162
Submitted: 11 September 2020 Accepted: 25 May 2021 Published: 10 September 2021
Abstract
Context: The koala (Phascolarctos cinereus) sometimes occurs as an overabundant folivore in south-eastern Australia, where high-density populations have caused defoliation of preferred food trees, threatening habitat. Kangaroo Island, South Australia and Budj Bim National Park, Victoria, are two regions where such eruptive population dynamics have arisen. One way of mitigating their damage is through management via fertility control.
Aims: This paper examines the outcomes of fertility control (surgical sterilisation and treatment with levonorgestrel implants) on breeding dynamics at the population level, using data obtained from two separate koala management programs conducted by State Government wildlife agencies, South Australia Department for Environment and Water and Parks Victoria.
Methods: The relationships between female body condition, reproductive status, fertility control, age and population density were examined using linear mixed effects models. Population density (koalas/ha) was estimated using annual census data and modelled in a Bayesian framework.
Key results: Body condition was a key influence on breeding success in female koalas, with sexual maturity being defined by both age and body condition, whereas the effect of reproductive experience was minimal. It is likely that reduced densities have led to decreased intraspecific competition for food resources and territory, leading to increased breeding success at one management site (BBNP) and by allowing females to begin breeding at a lower overall body condition. The reduction in densities to sustainable levels at both management sites, despite differing fertility-control methods, supports the use of the less invasive and more cost-effective levonorgestrel implants as the preferred fertility-control method.
Conclusions: In addition to contraceptive effects, broad-scale fertility control may have resulted in a compensatory higher breeding success in the untreated population, possibly in response to decreasing densities, increased resource availability or behavioural responses to population management.
Implications: Although compensatory breeding mechanisms can reduce the effectiveness of fertility control at the population level, ongoing fertility-control management can still be successful at reducing overabundant koala populations to sustainable levels with significant long-term commitment, provided that annual control targets are continuously met.
Keywords: contraception, infertility, marsupial, population control, reproduction, wildlife management.
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