Evaluation of feral pig removal in Hawaiian preserves

The Nature Conservancy of Hawaii (TNC) recently embarked on an ambitious ungulate control programme throughout their preserves on the islands of Maui and Molokai. The aim of the programme was local eradication of feral pigs (Sus scrofa) and they wanted some way of evaluating their progress. Catch-effort models have previously been applied to cumulative pig dispatches during an island eradication programme (Ramsey et al. 2009; Conservation Biology 23: 449-459). These models simultaneously estimate the parameters describing the initial population size and the probability of detecting an individual per unit of hunting effort, which can then be used to evaluate the likelihood of eradication. However these models rely on a number of assumptions including that the system is closed except for removals and that the relationship between hunting effort and the probability of detection is constant throughout the experiment. As the TNC control programme progressed it became clear the both these assumptions were violated and more pigs were often caught per unit of effort on the later compared with the earlier hunts. There was ongoing immigration into the preserves through breaks in the fence and via unfenced boundaries. Also, later hunts seemed to be more successful per unit of effort than earlier ones, presumably because hunters learnt the best way to cover the area and where the sites most likely to contain pigs were. We described how we incorporated this learning process into a catch-effort model using Bayesian updating in order to evaluate the efficacy of the control programme.