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Assessing Lamb Predation by Feral Pigs in Australia's Semi-Arid Rangelands

David Choquenot, Brian Lukins and Greg Curran
Journal of Applied Ecology
Vol. 34, No. 6 (Dec., 1997), pp. 1445-1454
DOI: 10.2307/2405260
Stable URL: http://www.jstor.org/stable/2405260
Page Count: 10
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Assessing Lamb Predation by Feral Pigs in Australia's Semi-Arid Rangelands
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Abstract

1. Feral pigs prey upon newborn lambs in the semi-arid rangelands of eastern Australia. In this study, two experiments were used to investigate the relationship between feral pig density and the rate of lamb predation. The differential susceptibility of single and twin-born lambs to pig predation was also examined. 2. In Experiment 1, the density of pig populations on four sites subjected to different levels of pig control was monitored quarterly (range: 0.7-6.4 pigs km-2) between 1989 and 1991. The rate of lamb loss was indexed by udder-scoring a sample of lambing ewes to estimate the proportion whose lambs had died. The index of lamb loss varied significantly with pig density, the relationship taking the form of an asymptotic increase in loss at progressively higher pig densities. Inclusion of a variable indicating whether pig density increased or decreased over the lambing season (summarizing the cumulative effects of recent seasonal conditions) explained no additional variation in lamb loss. This suggests that the availability of alternative foods did not influence the propensity of pigs to prey upon lambs. 3. Experiment 2 was conducted on three sites with respective pig densities of 0.4, 2.4 and 5.8 pigs km-2. On each site, two paddocks were established, one of which was electrically fenced to preclude pigs. Approximately 300 pregnant ewes at each site were ultrasonically scanned to determine whether they carried single or twin lambs, and then placed randomly in either the paddock to which pigs had access or the paddock from which they were excluded. The proportion of lambs born that survived was contrasted between the two paddocks at each site to provide a direct estimate of predation rate. The rate of lamb predation increased with feral pig density, reaching a maximum proportional rate of 0.29 on the site where pig density was highest. Comparison of predation rates indicated that twin lambs were on average 5-6 times more likely to be preyed upon by pigs than were single lambs. 4. The relationship between pig density and the index of predation rate estimated in Experiment 1 was modified to predict actual predation rates by substituting the maximum rate measured in Experiment 2. The resulting relationship, in combination with a measure of stochastic year-to-year variation in predation rates, was used to construct distributions describing the probability of sustaining different rates of lamb predation at given feral pig densities. 5. This information enables sheep farmers to contrast the risk of lamb predation with their lamb production objectives and the costs of holding feral pig density below levels corresponding to acceptable rates of lamb predation. By valuing the reduced risk of sustaining high levels of lamb predation and the costs associated with achieving it, a sheep farmer can incorporate reduction in costs and benefits of pig control directly into economic analyses used to manage their enterprise.

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