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Reducing wildlife predation by domestic cats: An approach based on the precautionary principle

Grayson, J. (2016) Reducing wildlife predation by domestic cats: An approach based on the precautionary principle. PhD thesis, Murdoch University.

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Pet cats kill a range of suburban wildlife, including some native mammals, birds and lizards. The dense cat populations sustained in suburbs by people exacerbate the problem. However, there is sparse evidence of suppression of populations of any native species in suburbia as a result of cat predation and accurate estimates of predation rates are difficult. Such uncertainty as to whether or not cat predation poses a serious risk to remnant wildlife populations in suburbia is no reason for inaction until the question is resolved, because serious environmental impacts including species decline or local extinction could occur before definitive evidence is available. Therefore, it is appropriate to invoke the precautionary principle, which requires (i) detailed consultation to choose and implement precautionary measures to anticipate possible environmental damage, and (ii) concurrent research to reduce uncertainty as to the exact impact and whether precautionary measures should be continued or reduced. In this study I apply a precautionary approach to the question of whether or not predation by pet cats influences passerine species richness or community composition in suburban Perth, Western Australia. In keeping with the twin tenets of the recautionary principle the study involved an assessment of community attitudes and practices regarding the husbandry of pet cats and their impact on wildlife in general (consultation), and a detailed study of factors (including the density of pet cats) influencing passerine species richness or community composition across metropolitan Perth (reducing uncertainty).

To assess the attitudes and practices of the general public towards cat legislation and other issues relating to pet cats, I designed and issued a survey to 2,000 residents within the City of Melville, a local government municipality in Perth. The response rate was 63%. Respondents were questioned upon their knowledge of cat issues and their attitudes and practices toward sterilisation of pet cats; legislation regulating cat
ownership and the putative impact cats have upon wildlife. Age, gender and cat ownership status of respondents were investigated to determine if such factors influenced responses. Cat-owners, particularly women, knew more about cat issues. Non-owners were more supportive than cat-owners of the introduction of cat control measures and were more concerned about the possible impacts cats exert upon suburban and remnant wildlife. Women, regardless of cat-ownership status, were more supportive of sterilisation, whereas men were more supportive of the introduction of cat control measures. Age was positively related to the implementation of control measures, with older respondents showing most support. Over 70% of respondents, both cat-owners and non-owners, supported the introduction of cat legislation that promoted sterilisation,
restricted the number of cats per household and their roaming behaviour, and mandated licensing of pet cats. However, only a minority of cat-owners or non-owners supported the concept that local governments should enforce cat-free zones where ownership of pet cats was prohibited.

To find definitive evidence of the impact of cats upon suburban fauna, I utilised data collected by members of Birds Australia for the ‘Suburban Bird Survey’ that covered 57 sites throughout suburban Perth, extending onto the Darling Scarp. Using these data, I tested the influence of eight variables including cat density, dog density, housing density, age of suburb, distance to, and size of, nearest bushland less than or greater than 5 ha on passerine species richness, passerine species composition and the presence/absence of 15 selected passerines that were recorded in 20 to 80% of sites. Garden vegetation factors including structure and floristics were also tested in 18 of these sites.

Cat density was not a significant predictor of any of the dependent variables tested. Rather, community composition of passerines declined with increasing housing density and distance to nearest bushland, and increased with size of nearest bushland > 5 ha. These independent variables, particularly housing density, significantly affected small to medium size insectivores. There were no clear results that predicted the presence/absence of the 15 selected passerines, although housing density appeared to be the most likely predictor. Garden vegetation was not a significant predictor for the presence or absence of any of the 15 selected species, although gardens with low bird pollinated plants were more likely to contain Yellow Rumped Thornbill, whereas gardens dominated by fruiting vegetation, tall, bird pollinated and deciduous vegetation
were less likely to contain any of the 15 selected species.

Overall, the possible cat control measures supported by 70% or more of owners and non-owners would protect wildlife by reducing dumping of unwanted cats, limiting cat densities in suburbia and enabling identification of nuisance animals. Given this high level of community support, these measures should be implemented. However, they are not a panacea for wildlife conservation in the suburbs. While cat predation might be significant adjacent to remnant bushland or other areas of conservation significance, blaming cats for bird conservation issues in long-established suburbs may be a scapegoat for high residential densities, inappropriate landscaping at a range of scales or poor conservation of remnant bushland.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Calver, Michael
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