Assessing lamb predation by feral pigs in Australia's semi-arid rangelands.
Two experiments were used to investigate the relationship between feral pig density and the rate of lamb predation and the differential susceptibility of single and twin-born lambs to pig predation in eastern Australia. In experiment 1, the density of pig populations on 4 sites subjected to different levels of pig control was monitored quarterly (range: 0.7-6.4 pigs/km2) 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 increased significantly with pig density. Experiment 2 was conducted on three sites with respective pig densities of 0.4, 2.4 and 5.8 pigs/km2. On each site, 2 paddocks were established, 1 with electric fencing to prevent access by 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 paddock. The rate of lamb predation increased with 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. The estimated results from experiment 1 were modified to predict actual predation rates by substituting the maximum rate measured in experiment 2. The resulting relationship was used to construct distributions describing the probability of sustaining different rates of lamb predation at given feral pig densities. This information enables sheep farmers to contrast the risk of lamb predation with their lamb production objectives and the costs of reducing feral pig density.