Danielle A. Risbey

Bio: Danielle A. Risbey is an academic researcher. The author has contributed to research in topics: Population & Feral cat. The author has an hindex of 5, co-authored 6 publications receiving 602 citations.

The impact of cats and foxes on the small vertebrate fauna of Heirisson Prong, Western Australia. II. A field experiment.

Abstract: The hypothesis that predation by feral cats and introduced foxes reduces population sizes of small, native vertebrates was supported by results of a predator-removal experiment at Heirisson Prong, a semi-arid site in Western Australia. The methods of control used against cats and foxes to protect native mammals reintroduced to Heirisson Prong produced three broad ‘predator zones’: a low-cat and low-fox zone, where foxes were eradicated and spotlight counts of cats declined after intensive cat control; a high-cat and low-fox zone where spotlight counts of cats increased three-fold after foxes were controlled; and a zone where numbers of cats and foxes were not manipulated. Small mammals and reptiles were monitored for one year before and three years after predator control began. Captures of small mammals increased in the low-cat and low-fox zone, but where only foxes were controlled captures of small mammals declined by 80%. In the absence of cat and fox control, captures of small mammals were variable over the sampling period, lower than where both cats and foxes were controlled, yet higher than where only foxes were controlled. The capture success of reptiles did not appear to be related to changes in predator counts. This study presents the first experimental evidence from mainland Australia that feral cats can have a negative impact on populations of small mammals.

The impact of cats and foxes on the small vertebrate fauna of Heirisson Prong, Western Australia. I. Exploring potential impact using diet analysis

Abstract: The diets of cats (Felis catus) and foxes (Vulpes vulpes) killed during predator control at a semi-arid site in Western Australia were studied to see which prey species may be affected by predation from these introduced predators. The number of items, biomass and frequency of occurrence of each food type in the gut contents from 109 fetal cats, 62 semi-feral cats and 47 foxes were used to calculate an Index of Relative Importance for each food category for each predator. Mammals were the most important prey group for all three predators, with rabbit being the most highly ranked prey species. The diets of feral and semi-feral cats were similar in dietary diversity but differed in the frequency of occurrence of some food categories. Native rodents, birds and reptiles occurred more frequently and were ranked higher in the diet of feral cats, and food scraps occurred more frequently in the diet of semi-feral cats. The diet of foxes was less diverse than that of either group of cats. Invertebrates and sheep carrion were more important prey categories for foxes than for cats. In the summer-autumn period, foxes ate more sheep carrion and invertebrates than they did in winter-spring. The diet of feral cats was more diverse in summer-autumn, including a greater range of invertebrates and more rodents, birds and reptiles than in the winter-spring period. We predict that cats are more likely to have an impact on small vertebrates at this site and that the control of cats could lead to recoveries in the populations of native rodents, birds and reptiles. By contrast, the control of foxes alone may lead to a rise in cat numbers and a consequent detrimental impact on small vertebrate populations.

Control of Feral Cats for Nature Conservation. II. Population Reduction by Poisoning

Abstract: A feral cat population was substantially reduced by poisoning at a semi-arid site in Western Australia. The control programme was designed to protect two species of endangered native mammals that had recently been reintroduced to the site. Fetal cats were poisoned with carcasses of laboratory mice, each impregnated with 4.5 mg of sodium monofluoroacetate (1080). Baits were placed at 100-m intervals along the track system each night for four consecutive nights. Kill rates were assessed by monitoring survival of radio-collared cats and by spotlight counts of cats before and after baiting. All radio-collared cats were killed and there was a 74% reduction in spotlight counts of cats after baiting. Bait removal varied with the abundance of rabbits, the primary prey item for cats in this area. Effectiveness of control operations against fetal cats is maximised by baiting at times of low prey abundance. Monitoring the changing abundance of the primary prey species provides important information for timing control operations against feral cats.

Control of feral cats for nature conservation. III. Trapping

Abstract: We present comparative success of various trapping methods trialed during control of feral cats at a site for the reintroduction of threatened mammals at Shark Bay, Western Australia. Our results come from 31 703 trap-nights that caught 263 cats (an average of 0.83 per 100 trap-nights). Cats differed markedly in their vulnerability to trapping depending on whether they primarily scavenged at rubbish tips or around human settlement or whether they hunted for their food in the bush. Cage traps were an effective means of controlling the former, with 9.4 cats captured per 100 trap-nights. Scavenging cats included a higher proportion of sub-adults and kittens and lower proportion of adult males than hunting cats. Variation between years in capture success for hunting cats was largely explained by the abundance of rabbits relative to that of cats and whether the rabbit population was increasing or decreasing. These factors accounted for a nine-fold difference in trap success. The number of cats caught in any particular trapping session could be explained by location (rubbish tip or town versus bush), trapping effort (typically greater effort yielded higher captures), abundance of cats at the site (captures were highest when cats were abundant), and season (captures were highest in the first half of the year when the young of the year were becoming independent). Concealed foot-hold traps, in a range of possible sets, provided effective methods for capturing cats that hunt, except where capture of non-target species was a critical limiting factor. Cage traps caught cats at a comparable rate to foot-hold traps for standard sets, but caught a significantly different cohort. Concealed foot-hold traps caught a higher percentage of adult cats, particularly males, than did cage traps. Mouse carcases and rabbit pieces were significantly more effective as lures when rabbits (the major food of cats at the site) were at low densities, whereas the success of commercial scent lures was unrelated to food availability. Significantly more cats than expected were caught using food as an attractant at times of food shortage (late summer, autumn and early winter) for both scavenging and hunting cats. In contrast, scent lures caught significantly more cats than expected in spring and summer when cats were defending access to mates and/or territory. Hence, no single trap type, trap set, or lure provided unequivocally superior performance over others. Control is likely to be best achieved by a variety of trapping methods and lure types used in combination, supplementing well timed poisoning efforts. Trap success is likely to be maximised by trapping at times when the dominant prey of cats are scarce relative to the number of cats and are decreasing in abundance.

Control of Feral Cats for Nature Conservation. I. Field Tests of Four Baiting Methods

Abstract: Four methods of baiting were evaluated on a radio-collared population of feral cats on Heirisson Prong, Shark Bay, Western Australia. Dried-meat baits, baiting rabbits to kill cats through secondary poisoning, a fishmeal-based bait and a bait coated in the flavour enhancer Digest were tested. All proved to be ineffective for controlling feral cats. Future research should explore baits more ‘natural’ in appearance and the effect of visual lures, and possibly bait over a larger area to increase the number of cats exposed to baits.

The impact of free-ranging domestic cats on wildlife of the United States

Abstract: Anthropogenic threats, such as collisions with man-made structures, vehicles, poisoning and predation by domestic pets, combine to kill billions of wildlife annually. Free-ranging domestic cats have been introduced globally and have contributed to multiple wildlife extinctions on islands. The magnitude of mortality they cause in mainland areas remains speculative, with large-scale estimates based on non-systematic analyses and little consideration of scientific data. Here we conduct a systematic review and quantitatively estimate mortality caused by cats in the United States. We estimate that free-ranging domestic cats kill 1.4–3.7 billion birds and 6.9–20.7 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the majority of this mortality. Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact.

Top predators as biodiversity regulators: the dingo Canis lupus dingo as a case study.

Abstract: Top-order predators often have positive effects on biological diversity owing to their key functional roles in regulating trophic cascades and other ecological processes. Their loss has been identified as a major factor contributing to the decline of biodiversity in both aquatic and terrestrial systems. Consequently, restoring and maintaining the ecological function of top predators is a critical global imperative. Here we review studies of the ecological effects of the dingo Canis lupus dingo, Australia's largest land predator, using this as a case study to explore the influence of a top predator on biodiversity at a continental scale. The dingo was introduced to Australia by people at least 3500 years ago and has an ambiguous status owing to its brief history on the continent, its adverse impacts on livestock production and its role as an ecosystem architect. A large body of research now indicates that dingoes regulate ecological cascades, particularly in arid Australia, and that the removal of dingoes results in an increase in the abundances and impacts of herbivores and invasive mesopredators, most notably the red fox Vulpes vulpes. The loss of dingoes has been linked to widespread losses of small and medium-sized native mammals, the depletion of plant biomass due to the effects of irrupting herbivore populations and increased predation rates by red foxes. We outline a suite of conceptual models to describe the effects of dingoes on vertebrate populations across different Australian environments. Finally, we discuss key issues that require consideration or warrant research before the ecological effects of dingoes can be incorporated formally into biodiversity conservation programs.

The impact of cats and foxes on the small vertebrate fauna of Heirisson Prong, Western Australia. II. A field experiment.

Abstract: The hypothesis that predation by feral cats and introduced foxes reduces population sizes of small, native vertebrates was supported by results of a predator-removal experiment at Heirisson Prong, a semi-arid site in Western Australia. The methods of control used against cats and foxes to protect native mammals reintroduced to Heirisson Prong produced three broad ‘predator zones’: a low-cat and low-fox zone, where foxes were eradicated and spotlight counts of cats declined after intensive cat control; a high-cat and low-fox zone where spotlight counts of cats increased three-fold after foxes were controlled; and a zone where numbers of cats and foxes were not manipulated. Small mammals and reptiles were monitored for one year before and three years after predator control began. Captures of small mammals increased in the low-cat and low-fox zone, but where only foxes were controlled captures of small mammals declined by 80%. In the absence of cat and fox control, captures of small mammals were variable over the sampling period, lower than where both cats and foxes were controlled, yet higher than where only foxes were controlled. The capture success of reptiles did not appear to be related to changes in predator counts. This study presents the first experimental evidence from mainland Australia that feral cats can have a negative impact on populations of small mammals.