Efficacy of feral pig removals at Hakalau Forest National Wildlife Refuge

We compiled and analyzed data from 1987–2004 on feral pig (Sus scrofa) management and monitoring activities at Hakalau Forest National Wildlife Refuge, a tropical montane rainforest on the island of Hawai`i. These data included annual surveys of feral pig and cattle (Bos taurus) activity, the number of feral ungulates removed from closed management units, age and reproductive status from necropsies, and vegetation surveys repeated 4 times over a 16 year period. We found an essentially even sex ratio within the feral pig population and within age classes, although males lived to 60 months while females lived to only 48 months. The pregnancy rate was 23.5%, and lactation rate was 8.3%, regardless of season and age, but lactation peaked in April-June. Reproductive rates also increased with age, peaking at 2–4 years in females. We reconstructed the standing population within a closed unit to examine demographic processes. We estimated that annual removal in excess of approximately 41–43% would be necessary to affect a population decline. We examined annual feral pig activity surveys and found a strong and sustained decline in pig sign after 1997 relative to unmanaged areas. We related the standing population to feral pig activity surveys to build a predictive model of feral pig density, and then applied this model to other management units. We evaluated control methods and found snaring to be more efficient than staff or public hunting. Vegetation monitoring revealed a strong temporal increase in cover of native ferns, and marginally non-significant decreases in cover of bryophytes and exposed soil.