Pre-eradication assessment of feral cat density and population size across Kangaroo Island, South Australia

Abstract

Context Feral cats (Felis catus) are a significant threat to wildlife in Australia and globally. In Australia, densities of feral cats vary across the continent and also between the mainland and offshore islands. Densities on small islands may be at least an order of magnitude higher than those in adjacent mainland areas. To provide cat-free havens for biodiversity, cat-control and eradication programs are increasingly occurring on Australian offshore islands. However, planning such eradications is difficult, particularly on large islands where cat densities could vary considerably. Aims In the present study, we examined how feral cat densities vary among three habitats on Kangaroo Island, a large Australian offshore island for which feral cat eradication is planned. Methods Densities were compared among the following three broad habitat types: forest, forest–farmland boundaries and farmland. To detect cats, three remote-camera arrays were deployed in each habitat type, and density around each array was calculated using a spatially explicit capture–recapture framework. Key results The average feral cat density on Kangaroo Island (0.37 cats km−2) was slightly higher than that on the Australian mainland. Densities varied from 0.06 to 3.27 cats km−2 and were inconsistent within broad habitat types. Densities were highest on farms that had a high availability of macropod and sheep carcasses. The relationship between cat density and the proportion of cleared land in the surrounding area was weak. The total feral cat population of Kangaroo Island was estimated at 1629±661 (mean±s.e.) individuals. Conclusions Cat densities on Kangaroo Island are highly variable and may be locally affected by factors such as prey and carrion availability. Implications For cat eradication to be successful, resources must be sufficient to control at least the average cat density (0.37 cats km−2), with additional effort around areas of high carcass availability (where cats are likely to be at a higher density) potentially also being required.

Rapid recolonisation of feral cats following intensive culling in a semi-isolated context

“…We provide here the first feral cat density estimates from New Caledonia. At our study site, feral cat density was estimated to be relatively high compared to many places in Australia (Bengsen et al 2012;McGregor et al 2015;Hohnen et al 2020) and on two Salomon islands (Lavery et al 2020). However, it is lower than at other locations: one Salomon island (Lavery et al 2020), Great Britain (Langham and Porter 1991), Europe (Liberg 1980), New Zealand (Macdonald et al 1987), United States (Warner 1985), and highly modified landscapes in Australia (Legge et al 2017).…”

Predicting targets and costs for feral‐cat reduction on large islands using stochastic population models

“…Feral cat densities on Kangaroo Island are thought to range from 0.06 to 3.27 cat km −2 , with an average density of 0.37 cat km −2 , giving an estimated population size of 1,629 (s.e. ± 661) individuals (Hohnen et al, 2020). Taggart et al (2019) estimated that relative feral cat densities in eastern Kangaroo Island were ~10 times higher than on the adjacent mainland (Kangaroo Island relative abundance = 14.6 cats camera‐trap‐site −1 ; mainland = 1.39 cats site −1 ; 11 sites on both the Island and on the mainland).…”

Predicting targets and costs for feral-cat reduction on large islands using stochastic population models

“…Feral cat densities on Kangaroo Island are thought to range from 0.06 to 3.27 cat km −2 , with an average density of 0.37 cat km −2 , giving an estimated population size of 1629 (s.e. ± 661) individuals (Hohnen et al 2020a; Hohnen et al 2020b). Taggart et al (2019) estimated that relative feral cat densities in eastern Kangaroo Island were ~ 10 times higher than on the adjacent mainland (Kangaroo Island relative abundance = 14.6 cats camera-trap-site −1 ; mainland = 1.39 cats site −1 ; 11 sites on both the Island and mainland).…”

“…To simulate how incrementing intensities of reduction alter the projected population size, we first simulated a population not exposed to any culling to represent a ‘control’ population. We calculated the population’s stable age distribution from the base matrix A (Caswell 2001), and then multiplied this stable age structure by a starting population size of 1629 (Hohnen et al 2020b). We then expressed all subsequent projections as a proportion of this founding population size to avoid the uncertainty in initial population size estimates.…”