Wildlife Research - CSIRO Publishing

C S I R O

P U B L I S H I N G

Wildlife Research Volume 26, 1999 © CSIRO Australia 1999

A journal for the publication of original scientific research in the biology and management of wild native or feral introduced vertebrates

w w w. p u b l i s h . c s i r o . a u / j o u r n a l s / w r All enquiries and manuscripts should be directed to Wildlife Research CSIRO PUBLISHING PO Box 1139 (150 Oxford St) Collingwood Telephone: 61 3 9662 7622 Vic. 3066 Facsimile: 61 3 9662 7611 Australia Email: [email protected]

Published by CSIRO PUBLISHING for CSIRO Australia and the Australian Academy of Science

© CSIRO 1999

Wildlife Research, 1999, 26, 671–673

Fox control and rock-wallaby population dynamics – assumptions and hypotheses Jim Hone Applied Ecology Research Group, University of Canberra, Canberra, ACT 2601, Australia.

Abstract Previous studies have demonstrated that the abundance of rock-wallabies can increase after fox control. The assumptions made to explain the increases are examined. Testable hypotheses are described to explain the observed increases. Testing of the hypotheses is encouraged.

It has been reported that after baiting with sodium monofluoroacetate (compound 1080) against foxes (Vulpes vulpes) the abundance of rock-wallaby (Petrogale lateralis) populations increased, compared with populations at sites with no baiting with 1080 (Kinnear et al. 1988, 1998). The original hypothesis of Kinnear et al. (1988) centred on ‘relaxation of predation pressure’ to generate a response of rock-wallaby populations. These studies have rightly been of major interest for rock-wallaby conservation and predation control generally in Australia. This note aims to examine key assumptions in the studies of fox control and rock-wallaby population dynamics and propose new hypotheses on the topic and encourage the testing of the hypotheses. The implications for predation control in general are examined briefly. Kinnear et al. (1998) also reviewed and disputed the statistical analyses by Hone (1994) of some of the data presented by Kinnear et al. (1988). The new analyses are more comprehensive than those reported by Hone (1994) and are welcomed. Romesburg (1981) argued forcefully that testing of a statistical hypothesis was fundamental to gaining reliable knowledge in wildlife science and hence achieving better wildlife management. We now have more reliable knowledge of the effect of fox baiting on rock-wallaby abundance. The focus here is on the assumed mechanism(s) causing a population response rather than whether there was a response. It was assumed (Kinnear et al. 1988, 1998) that fox baiting with 1080 poison killed individual foxes that caused a decline in fox abundance, that reduced fox predation that caused the observed increase in rock-wallaby abundance. It is known that some foxes died from 1080 baiting and from shooting (Kinnear et al. 1988), and from other studies, for example McIlroy and King (1990), that 1080 kills individual foxes. It is also known from, for example, Thompson and Fleming (1994) and Saunders et al. (1995), that fox baiting with 1080 poison can kill a substantial percentage of foxes in an area. No data were presented, however, in the rock-wallaby studies on trends in fox abundance as a result of the fox baiting, and no data have been provided to show trends in fox predation, in particular that it actually declined. An incident of predation was described, and interesting and supporting data are presented by Kinnear et al. (1998) on changes in habitat use by wallabies. Hence, the assumed mechanism generating the population response, relaxation of predation pressure, remains an assumption. The important role of monitoring fox abundance as part of rock-wallaby management, by fox control, was recognised in the case study described by Olsen (1998, p. 125). If fox abundance did not decrease, in the case study, then issues of baiting effectiveness and immigration needed to be addressed. Immigration and the consequences for baiting frequency were discussed by Kinnear et al. (1988, 1998). However, there may be no detectable change in fox abundance after 1080 baiting, if only a small number or percentage of foxes do the killing of rock-wallabies, and those particular foxes and only those, are killed by 1080 baiting. A phenomenon, which may be analogous, of a few individual foxes killing most lambs was noted briefly by Rowley (1970) and 10.1071/WR98083

1035-3712/99/050671

672

J. Hone

occurs with feral pigs (Sus scrofa) killing lambs. In the case of pigs, most of the killing is by a few old boars (Pavlov and Hone 1982). The advantages of monitoring prey and predator abundance and actual predation were demonstrated in recent studies of the recovery of kokako (Callaeas cinerea wilsoni) populations after control of introduced mammals in parts of New Zealand (Innes et al. 1999). Then causal links between predators and prey were shown clearly and not assumed. In future studies, fox predation and fox abundance should be monitored to provide the necessary data to explain the mechanisms of prey population responses and not only describe the responses. The experimental nature of the rock-wallaby study and the important conservation lessons to be learned from it, for predation control in general, suggest that four additional hypotheses could be tested. First, if fox baiting started at the current non-baiting sites then the wallaby populations therein should also increase in a manner similar to those at the current baited sites. The response may be slow or non-existant, however, if there is an effect of very small population size – the Allee effect (Begon et al. 1996). Second, the mean rate of increase of the wallaby populations at the two baited sites in Kinnear et al. (1998) was 22% per year. The observed rate of increase was not compared with a maximum rate of increase expected if all predation, or another limiting factor(s), was removed by killing foxes. An estimate of the maximum rate of increase can be obtained from the intrinsic rate of increase (rm) and bodyweight equation of Sinclair (1996). If mean adult bodyweight of the rock-wallabies is 4 kg (Strahan 1995), the predicted intrinsic rate of increase is 0.89 per year, which is an annual finite rate of increase (l) of 2.44. The mean observed rate of increase (l = 1.22) is much lower than the predicted, suggesting that some factor is still limiting population increase even when fox baiting occurs. McNab (1987) suggested that marsupials would have a lower rm than eutherians because of their lower basal metabolic rate. The Sinclair equation is based on eutherians and marsupials. However, the difference between the expected and observed rates of increase may be more than that caused by metabolic rates, so either substantial predation is still occurring, or some other factor(s) is limiting rates of increase. Sinclair et al. (1998) suggest that the effect of predation on rock-wallabies may differ as habitats and weather vary. Two other hypotheses can be advanced and need evaluating and testing. For example, in theory, the effect of 1080 baiting could be mediated by the poison having a greater effect on an alternate predator species (to be identified). The second predator species may kill more prey (wallabies) than foxes in the absence of 1080 baiting and could be sensitive to 1080 baiting. Alternatively, if the predator and prey share common microparasites and/or macroparasites (to be identified) then reducing predator abundance may, in theory, indirectly increase prey abundance by reducing the limiting effect of the shared parasites. Such changes in shared parasites may, or may not, generate changes in habitat use, so this hypothesis, while possible, may be unlikely to be supported. Kinnear et al. (1988) described the wallabies as being healthy, but that may not be incompatible with parasites having a population effect if the parasites are aggregated into a few individual hosts. These alternative hypotheses could be evaluated and tested in future research to obtain more reliable knowledge. In conclusion, the topic of predation control should not simply be about patterns and numbers. A prey species may be lost while statistical and scientific merits are debated. That would be a waste of scientific and conservation resources. The gaining of reliable knowledge of mechanisms and patterns can help achieve scientific and conservation objectives. Acknowledgments I thank G. Saunders and P. O’Brien for useful comments on a draft manuscript. References Begon, M., Harper, J. L., and Townsend, C. R. (1996). ‘Ecology. Individuals, Populations and Communities.’ 3rd Edn. (Blackwell Science: Oxford.)

Fox control and rock-wallaby population dynamics

673

Hone, J. (1994). ‘Analysis of Vertebrate Pest Control.’ (Cambridge University Press: Cambridge.) Innes, J., Hay, R., Flux, I., Bradfield, P., Speed, H., and Jansen, P. (1999). Successful recovery of North Island kokako (Callaeas cinerea wilsoni) populations, by adaptive management. Biological Conservation 87, 201–214. Kinnear, J., Onus, M. L., and Bromilow, R. N. (1988). Fox control and rock-wallaby population dynamics. Australian Wildlife Research 15, 435–450. Kinnear, J. E., Onus, M. L., and Sumner, N. R. (1998). Fox control and rock-wallaby population dynamics II. An update. Wildlife Research 25, 81–88. McIlroy, J. C., and King, D. R. (1990). Appropriate amounts of 1080 poison in baits to control foxes. Australian Wildlife Research 17, 11–13. McNab, B. K. (1987). The reproduction of marsupial and eutherian mammals in relation to energy expenditure. Symposium of the Zoological Society of London 57, 29–39. Olsen, P. (1998). ‘Australia’s Pest Animals. New Solutions to Old Problems.’ (Bureau of Resource Sciences & Kangaroo Press: Canberra.) Pavlov, P. M., and Hone, J. (1982). The behaviour of feral pigs, Sus scrofa, in flocks of lambing ewes. Australian Wildlife Research 9, 101–109. Romesburg, H. C. (1981). Wildlife science: gaining reliable knowledge. Journal of Wildlife Management 45, 293–313. Rowley, I. (1970). Lamb predation in Australia: incidence, pre-disposing conditions, and the identification of wounds. CSIRO Wildlife Research 15, 79–123. Saunders, G., Coman, B., Kinnear, J., and Braysher, M. (1995). ‘Managing Vertebrate Pests: Foxes.’ (Bureau of Resource Sciences: Canberra.) Sinclair, A. R. E. (1996). Mammal populations: fluctuation, regulation, life history theory and their implications for conservation. In ‘Frontiers of Population Ecology’. (Eds R. B. Floyd, A. W. Shepherd and P. J. de Barro.) pp. 127–154. (CSIRO: Melbourne.) Sinclair, A. R. E., Pech, R. P., Dickman, C. R., Hik, D., Mahon, P., and Newsome, A. E. (1998). Predicting effects of predation on conservation of endangered prey. Conservation Biology 12, 564–575. Strahan, R. (1995). ‘The Mammals of Australia.’ (Reed Books: Sydney.) Thompson, J. A., and Fleming, P. J. S. (1994). Evaluation of the efficacy of 1080 poisoning of red foxes using visitation to non-toxic baits as an index of fox abundance. Wildlife Research 21, 27–39.

Manuscript received 28 October 1998; accepted 23 March 1999

http://www.publish.csiro.au/journals/wr