Andrew J. Bengsen

Using a general index approach to analyze camera-trap abundance indices†

ABSTRACT The lack of variance estimates constrain the utility of abundance indices calculated from camera-trap data. We adapted a General Index model, which allows variance estimation, to analyze camera-trap observations of feral pigs (Sus serofa) for population monitoring in a tropical rainforest. We tested whether the index would respond to population manipulation, and found that it decreased by 57% following removal of 24 pigs and remained low in the following period. Our method is useful for monitoring other large animals in difficult landscapes, and the model can be used to enhance the value of existing data sets.

Estimating and indexing feral cat population abundances using camera traps

Context The ability to monitor changes in population abundance is critical to the success of pest animal management and research programs. Feral cats (Felis catus) are an important pest animal, but current monitoring techniques have limited sensitivity or are limited in use to particular circumstances or habitats. Recent advances in camera-trapping methods provide the potential to identify individual feral cats, and to use this information to estimate population abundances using capture–mark–recapture (CMR) methods. Aims Here, we use a manipulative study to test whether camera-trapping and CMR methods can be used to estimate feral cat abundances. Methods We established a grid of infrared cameras and lure stations over three pastoral properties on Kangaroo Island, Australia, for 15 days. We then reduced the population abundance with an intensive trapping program and repeated the camera survey. We estimated population abundances using robust design CMR models, and converted abundance estimates to densities using home-range data from GPS tracking. We also calculated relative abundance indices from the same data. Key results The CMR methods produced credible estimates of the change in population abundance, with useful confidence intervals, showing a statistically identifiable population decline from at least 0.7 cats km–2 before trapping down to 0.4 cats km–2 after trapping. The indexing method also showed a statistically identifiable decrease in abundance. Conclusions Camera-trapping and CMR methods can provide a useful method for monitoring changes in the absolute abundance of feral cat populations. Camera-trap data may also be used to produce indices of relative abundance when the assumptions of CMR models cannot be met. Implications These methods are widely applicable. The ability to reliably estimate feral cat abundances allows for more effective management than is generally available.

Applying home-range and landscape-use data to design effective feral-cat control programs

Abstract Context. Effective feral-cat (Felis silvestris catus) management requires a sound understanding of the ways cats use their environment. Key characteristics of landscape use by cats vary widely among different regions and different conditions. Aims. The present study aimed to describe the most important characteristics of landscape use by feral cats on a large, human-populated island, and to use this information to guide the development of feral-cat management programs. Methods. We used GPS tracking collars to record the movements of 13 feral cats at two sites on Kangaroo Island, South Australia, for between 20 and 106 days. We described home-range extents by using local convex hulls, and derived management suggestions from examination of home-range and movement data. Key results. Median feral-cat home range was 5.11 km2, and this did not differ between sexes or sites. Cats at a fragmented pastoral site tended to favour woody vegetation over open paddocks, but habitat preferences were less clear at a bushland site. Cats that preferentially used treelines at the pastoral site were almost twice as likely to be recorded close to a tree-line junction as expected. Conclusions. Control programs for feral cats on Kangaroo Island should deploy control devices at a density no less than 1.7 devices km–2. Spatial coverage should be as large as practicable or repeated frequently. Infrequent programs covering small areas can be expected only to provide short-term reductions in cat abundance. Implications. The information gained from the present study will contribute to the development of strategic sustained management plans for feral cats on Kangaroo Island. The principles from which we inferred management guidelines are applicable to other regions and species.

Target-specificity of feral pig baits under different conditions in a tropical rainforest

Context The mitigation of feral pig (Sus scrofa) impacts in north Queensland’s World Heritage tropical rainforests is constrained by the lack of an effective and target-specific poison baiting method. Aims This study aimed to determine whether easily implemented bait presentation methods or seasonal variation in bait acceptability could be used to selectively reduce the consumption of feral pig baits by non-target species. Methods We exposed manufactured feral pig baits to pigs and non-target species in the field, and compared bait encounter, sampling and consumption rates for different functional groups of species among three different types of bait presentation and composition. We then exposed baits under different seasonal conditions and related bait encounter and consumption by different functional groups to seasonally variable phenomena. Key results Shallow burial greatly reduced bait consumption by most non-target species, but not dingoes (Canis lupus dingo). Nocturnal bait distribution and seasonal baiting were less useful. Pigs showed substantial seasonal variation in physiological condition, suggesting that pigs should be more susceptible to consuming novel foods, such as baits, after periods of low rainfall. However, few pigs consumed the manufactured baits used in this study. Conclusions Manufactured baits are not currently suitable for widespread use in the region. However, shallow burial should provide an effective method of reducing non-target bait-take if baits can be made more attractive and acceptable to pigs and less acceptable to dingoes. Implications Future efforts to enable effective feral pig control in the region should focus on developing baiting materials that are more attractive to pigs and unappealing to dingoes.

Effects of coordinated poison-baiting programs on survival and abundance in two red fox populations

Abstract Context. Poison-baiting programs coordinated among neighbouring landholders should provide the most effective and efficient tool for controlling fox (Vulpes vulpes) populations and impacts in mixed agricultural landscapes, but the effects of such programs on fox mortality and abundance have not been well described. Aims. This study aimed to describe the effects of coordinated fox-control programs conducted by landholders on fox mortality and abundance, and to evaluate the likely impacts of reduced landholder participation rates on the proportion of the fox population exposed to baits. Methods. The effects of two baiting programs on fox abundance were evaluated using camera-trap surveys and abundance-induced heterogeneity models. The proportion of foxes surviving baiting was estimated by tracking the fate of 19 GPS-collared individuals. The benefits of coordinated baiting were examined using simulated scenarios based on local fox movements and bait distribution patterns. Key results. Examination of actual and simulated fox home ranges demonstrated that coordinated baiting increases exposure of the target population to baits. However, 69% of foxes captured on properties that later baited were estimated to have survived the baiting period. Camera-trap surveys across baited and unbaited properties showed no detectable decline in average fox abundance after baiting. Conclusions. Coordinated baiting increases the proportion of the fox population encountering baited properties. However, high fox survival and the absence of detectable declines in abundance after baiting showed that even well coordinated baiting programs can produce suboptimal results if many of the foxes using baited properties fail to locate and ingest lethal baits. Implications. Baiting programs aiming to reduce the density and impacts of foxes in agricultural landscapes should strive to maximise participation among neighbours. Programs may also benefit from taking steps to improve bait-encounter and consumption rates within properties, for example, by deploying baits at sufficient intensity to provide all foxes with access to at least one bait within their short-term home range. Future research should aim to identify optimal baiting intensities within properties and conditions to maximise bait uptake.