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.

背景：考拉（Phascolarctos cinereus）有时在澳大利亚东南部作为一种过多的食叶动物出现，那里的高密度种群导致首选食用树落叶，威胁栖息地。南澳大利亚州的袋鼠岛和维多利亚州的布吉比姆国家公园是出现这种爆发性人口动态的两个地区。减轻其损害的一种方法是通过生育控制进行管理。

目的：本文使用从州政府野生动物机构​​、南澳大利亚州环境和水务部开展的两个独立考拉管理计划中获得的数据，研究生育控制（手术绝育和左炔诺孕酮植入物治疗）对种群水平繁殖动态的影响和维多利亚公园。

方法：使用线性混合效应模型检查女性身体状况、生殖状态、生育控制、年龄和人口密度之间的关系。人口密度（考拉/公顷）是使用年度人口普查数据估算的，并在贝叶斯框架中建模。

主要结果：身体状况是影响雌性考拉繁殖成功的关键，性成熟由年龄和身体状况共同决定，而繁殖经验的影响很小。密度的降低很可能导致种内对食物资源和领土的竞争减少，从而提高了在一个管理地点 (BBNP) 的繁殖成功率，并允许雌性在较低的整体身体条件下开始繁殖。尽管生育控制方法不同，但两个管理地点的密度降低到可持续水平，支持使用侵入性较小且更具成本效益的左炔诺孕酮植入物作为首选的生育控制方法。

结论：除了避孕效果之外，大规模的生育控制可能会导致未接受治疗的群体的补偿性更高的繁殖成功，这可能是由于密度降低、资源可用性增加或对群体管理的行为反应。

影响：虽然补偿繁殖机制会降低种群水平上生育控制的有效性，但只要持续满足年度控制目标，持续的生育控制管理仍然可以成功地将过多的考拉种群减少到可持续的水平，并具有重大的长期承诺.