Importance of getting the numbers right: quantifying the rapid and substantial decline of an abundant marsupial, Bettongia penicillata

Abstract

Context A reliable measure of population size is fundamental to ecology and conservation but is often difficult to obtain. The woylie, Bettongia penicillata, provides an example where ‘getting the numbers right’ has important implications in verifying and quantifying the recent unexpected, rapid and substantial declines across much of its range. Initial estimates prompted a conservation-status upgrade for the species to Endangered by the Australian Government. The present paper constitutes the foundational paper addressing the first steps of a decline diagnosis framework intended to identify the causes of the recent declines. Aims To verify whether the declines in woylie trap-capture rates are representative of population change; better quantify the size of the largest woylie populations; and review what is understood about the ecology of the woylie and identify key knowledge gaps that may be relevant to identifying the causes of the recent declines. Methods Monitoring data from live-cage trapping (transects and grids), sandpads, woylie diggings and nest-density surveys and spotlighting were collated. Population measures derived from trapping data included capture rates, number of individuals, abundance estimates based on capture–mark–recapture modelling and density using spatially explicit capture–recapture models (SECR). Key results The declines in woylie trap-capture rates were verified as real population declines and corresponded closely with other measures of abundance derived from the same trapping data as well as with independent measures. A 95% decline occurred in the largest extant woylie populations (in the Upper Warren region, Western Australia) between 2002 and 2008. At a species level, woylies declined ~90% (1999–2006), from a peak of ~200 000 individuals in 1999. Conclusions An accurate formal conservation status is an important factor in promoting the conservation of a species. It is recommended that the woylie be considered for Critically Endangered status under the Australian EPBC Act. Implications Adequate and effective monitoring of species is critical to detecting and quantifying population changes in a timely manner. Having an accurate measure of population size can have a significant impact on the effectiveness of conservation and management efforts.

Ongoing unraveling of a continental fauna: Decline and extinction of Australian mammals since European settlement

“…Together with a program of translocations, the fox-baiting program resulted in the first removal because of conservation action of a listed threatened species from Australian national and state lists-the woylie, Bettongia penicillata. Unfortunately, within the last decade, the population size of this species has again plummeted, even in the fox-baited area, and the woylie is again listed as threatened (110), probably because fox baiting has perversely led to increased predation pressure by feral cats (40). Notwithstanding this case of reversal of success, fox-baiting programs have been extended to some large areas in southeastern Australia and at least some threatened native mammal species are now increasing in response (111).…”

HEMATOLOGIC CHARACTERISTICS OF THE WOYLIE (BETTONGIA PENICILLATA OGILBYI)

“…We describe the hematologic characteristics of the woylie (Bettongia penicillata ogilbyi), an Australian macropod that has recently undergone a dramatic population decline (approximately 90% between 1999 and2010;Wayne et al, 2009Wayne et al, , 2013Groom, 2010). Based on the available evidence, it was hypothesized that predators or disease may be the primary causes of the decline (Wayne, 2006).…”

First in, first served: uptake of 1080 poison fox baits in south-west Western Australia

“…A baiting regime should be reassessed to improve efficacy when either control of the target is reduced or bait uptake by non-targets is suspected. In Western Australia, there has been a long history of fox control with initially promising results Burrows and Christensen 2002); however, major declines in monitored prey species within baited areas have been observed recently (Wayne et al 2011(Wayne et al , 2013. Similar observations in other long-term baited areas have raised the question of whether baiting is still effective (Walsh et al 2012).…”