Using aspects of predator-prey theory to evaluate helicopter shooting for feral pig control

Abstract

Because it achieves rapid reductions in pig density, helicopter shooting is perceived to be a cost-effective option for feral pig control. In order to evaluate the cost effectiveness of the technique and develop predictive models of variation in costs, functional response models derived from predator–prey theory were fitted to 3 data-sets describing variation in kill rates with feral pig density. The data-sets were collected during shooting programs conducted on the Mary River floodplain in northern Australia, and on the Macquarie Marshes and Paroo River floodplain in western New South Wales. Fitted models indicated that variation in kill rates with pig density took the form of a Type 3 functional response for all 3 data sets, kill rates approaching a constant maximum at high pig densities and declining toward 0 at pig densities greater than 0. While maximum kill rates were similar for the 3 shooting programs (average 60.49 kill h–1, range 49.64–76.28), densities below which no pigs would theoretically be killed varied significantly (average 2.79 pigs km–2, range 1.34–5.02). Similar maximum kill rates for the 3 shooting programs indicates that, once located, the time taken by shooting teams to dispatch pigs was relatively constant (0.023 h). Variation in threshold densities below which no more pigs would theoretically be shot, indicates that as the density of pigs was reduced, their vulnerability to shooting teams differed between the 3 shooting programs. This may have reflected differences between sites in either the capacity of resident pigs to learn to evade shooting teams or, more likely, the availability of refuge habitat. For 2 of the shooting programs, too few data were available to estimate the effect of declining pig density on kill rate, precluding detailed examination of differences in the efficiency with which pigs were found (search efficiency). Using estimates of pig density below which no pigs would theoretically be shot to set a limit to the effectiveness of shooting programs, models predicting variation in hours per kill with pig density were derived from each data set. These models demonstrated that hours per kill increased exponentially as shooting reduced pig populations below threshold densities of approximately 2–6 pigs km–2. Generalised models relating variation in cost per kill to pig density for the 3 shooting programs are described.

Effectiveness of commercial harvesting in controlling feral-pig populations

“…Nevertheless, the decline in harvest rate with increasing density reported here is expected from predator-prey theory. It fits a Type II functional response (Holling 1959) that is reported for several wildlife populations (Caughley and Sinclair 1994) and is observed for harvesting kangaroos in Queensland (Pople 2006) and shooting pigs by helicopter (Hone 1990;Choquenot et al 1999). Unless more harvesters enter the industry, harvest off-take will remain effectively limited (or saturated) at higher pig densities, reducing the harvest rate.…”

Feral swine damage and damage management in forested ecosystems

“…First, as illustrated by numerous examples throughout this review paper, the value of using a variety of techniques in an integrated fashion cannot be overstated. This is particularly true for feral swine because they often become ''educated'' and learn to evade capture or removal when using one technique exclusively (Choquenot et al, 1999). Second, there is value in using indices for both feral swine populations and their damage pre and post management activities (e.g., see Engeman et al, 2007b).…”

“…However, colleagues in Australia found removal rates of feral swine to range from 65 to 97% using aerial shooting (Hone, 1983;Saunders and Bryant, 1988;Hone, 1990;Saunders, 1993), with success depending upon the availability of refuge habitat, population density, and habituation to aerial shooting by feral swine (Choquenot et al, 1999). Nonetheless, in western New South Wales, 1 year following an initial aerial shooting campaign, populations had recovered to 77% of the their original levels, suggesting that annual campaigns may be needed to keep damage within acceptable limits due to increased immigration and other factors (Saunders, 1993).…”

Effects of simulated removal activities on movements and space use of feral swine

“…Hunting with dogs has been used to reduce feral swine populations around the world (Katahira et al 1993;Maillard and Fournier 1995;Parkes et al 2010) and is a popular recreational activity in the USA (Dickson et al 2003). Rapid eradication of feral swine can also be achieved through aerial shooting (Hone 1983;Choquenot et al 1999;Parkes et al 2010), although success is impacted by overstory vegetation types and ability to locate feral swine (West et al 2009;Campbell et al 2010;Massei et al 2011).…”