Feral pigs in a temperate rainforest ecosystem: ecological impacts and management

Abstract

Feral pigs (Sus scrofa) have negative impacts on ecosystems globally and are increasingly perceived as a problem in New Zealand. However, there is a lack of scientific evidence of the impacts of pigs in New Zealand, and no evidence-based management strategies to mitigate these impacts. This thesis determines the impacts of feral pigs in a temperate rainforest, describes the relationship between disturbance and pig density, and explores disturbance thresholds. The negative impacts of pigs on ecosystem processes, vegetation composition and structure were investigated using exclosures during a 21-month study in a podocarp-broadleaf forest in the Waitakere Ranges, Auckland. The negative impact of plant disease transmission was also assessed, by testing soil collected from feral pigs for Phytophthora Taxon Agathis (PTA), a disease attacking kauri (Agathis australis). The relationship between ground disturbance and pig density was explored using empirical data from ground disturbance monitoring transects and pig culls conducted in the Waitakere Ranges, which was then used in a model to simulate management scenarios and explore impact mitigation through the use of disturbance thresholds. Feral pigs were found to significantly increase litter decomposition rates and soil nutrient concentration, reduce seedling density and change plant species richness and composition. Feral pigs also vectored a large number of plant pathogens. No PTA was found in the soil associated with pigs, although this is likely due to detectability issues and pigs may still be implicated in the spread of the disease. Pig culling in the Waitakere Ranges failed to reduce pig numbers below maximum productivity, although a reduction in ground disturbance was still observed. Model simulations demonstrated the use of disturbance thresholds in maintaining disturbance at an acceptable level, although at a higher cost than fixed frequency culling regimes. The overall conclusion of this research is that feral pigs should be managed as an invasive species in New Zealand. Repeated disturbance by pigs could increase the risk of plant disease spread and may have long-term impacts on seedling recruitment and composition. This research demonstrates the capability to reduce pig ground disturbance without large reductions in pig populations and provides management recommendations advocating disturbance thresholds.