Irruptive dynamics of the brush-tailed bettong (

Abstract

Context The fluctuating fortunes of the brush-tailed bettong have seen this species classified as ‘Rare or Likely to Become Extinct’ in the 1970s, delisted and hailed as a conservation success in the 1990s, and re-listed as ‘Critically Endangered’ in 2008. Key actions to recover this species, broad-scale fox control and reintroduction to fox-free habitat, have had variable success. Aims To monitor the reintroduction of bettong to fox-free habitat of Wadderin Sanctuary in the eastern wheatbelt of Western Australia. Methods Growth of the population was monitored over a 12-year period by regular trapping throughout the sanctuary. Key results The population increased monotonically to peak at 305 individuals (0.71 ha−1) at 7.5 years following release, before subsequent decline. Population growth was accompanied by a significant decline in mean condition and a reduction in the proportion of females with pouch young or lactating, presumably owing to resources per head gradually declining. The proportion of large pouch young relative to total young carried by females declined as woylie numbers increased, suggesting that, increasingly, pouch young did not go to full term. Declines in population number, condition and reproduction were exacerbated by dry seasons. Bettongs established successfully, despite a succession of single feral cats within the 427-ha sanctuary. The removal of the last cat, 8 years following the establishment of bettongs, had no obvious impact on bettong numbers, as, at this point, their population was stabilising because of declining body condition and reduced reproductive output. Conclusions The observed pattern of population growth of bettongs was consistent with an herbivore irruption consequent of a release of a species to new habitat. Decline following peak numbers appeared as a result of density-dependent resource limitation (declining resources per head with increased abundance) interacting with years of low rainfall. Implications The dynamics of irruption and decline of herbivores are relevant to management of reintroduced populations to fenced predator-free sites and, potentially, to unfenced populations following release from predation.