A damage function for wild pig rooting in temperate rainforest

Abstract

Wild pigs (Sus scrofa) cause significant damage to ecosystems by rooting up the ground in search of food. Pig rooting can damage habitat used by native species, alter environmental processes and facilitate the spread of invasive weeds and pathogens. In order to effectively control wild pig populations to limit pig rooting, managers need to understand the damage function – the relationship between pig density and pig rooting. This research calculated a damage function for wild pig rooting in temperate rainforests of New Zealand. Grids of passive infra-red monitoring cameras were used to calculate a trapping rate/1000 nights for pigs in the Hunua Ranges, Auckland, while line transects measured the extent of pig rooting (damage). These data were then used to calculate a damage function to test the hypothesis that there was high threshold relationship between pig abundance and pig rooting. A Zero-altered General Linear Model found no meaningful relationship between camera trap rate and recent pig rooting at the scale of individual cameras and transects. However, regression analysis at the scale of camera grids (1 km2) found evidence of a high threshold relationship, with a significant quadratic term (p <0.001) and 94% of the variability in rooting explained by variation in trap rate. A Zero-altered general linear model was constructed to predict the extent of rooting damage using environmental variables within each plot. The model provided some evidence that rooting was positively related to poorly drained soils, terraced and gully environments under canopies of taraire and tawa-podocarp forest, but had limited explanatory power (R2 = 0.11). The results of this study suggest that pig rooting is relatively minimal at low pig abundances, but accelerates rapidly at higher abundances. This study may be useful in optimising pig management strategies in forested environments, and a discussion of the implications for managers is included. In particular, it is recommended that managers monitoring the outcome of pig control programmes in New Zealand temperate rainforest, aim to keep pig trap rates below 20 pigs / 1000 camera nights / per camera grid.