Mammal-Exclusion Fencing and the Reproductive Success of an Endangered Native Waterbird

Abstract

Novel relationships in ecological communities are forming faster than historical rates due to globalization and the resulting increase in species introductions. In the Hawaiian Islands, which prior to humans had no terrestrial reptiles or amphibians and only one terrestrial mammal, the introduction of invasive predators dramatically impacted island food webs. Wetlands, as ecosystems where terrestrial, aquatic, and marine species intersect, were particularly impacted by introduced species. The Hawaiian Stilt (Himantopus mexicanus knudseni) or Ae‘o, is one of five Hawaiian waterbirds listed under the Endangered Species Act. Currently estimated to range from ~1,300 to ~1,800 individuals, the Hawaiian Stilt must reach a self-sustaining population of 2,000 birds in order to be delisted. One factor hindering recovery may be the predation of Stilt eggs and chicks by invasive predators. To address this threat, the U.S. Fish and Wildlife Service recently constructed a fence that excludes mammalian predators around a wetland on O‘ahu. Although most island avian communities respond positively to invasive mammalian predator removal, mammal-exclusion fencing is an expensive tool, and only controls for one type of predator – mammals. Avian, aquatic, and amphibious predators may still impact birds inside the conservation fencing. In this study, I compared the reproductive success of Stilts nesting inside and outside the newly built mammal-exclusion fence to test hypotheses regarding the impact of invasive predators and estimate the effectiveness of mammal-exclusion fencing as a management action. The results of the proposed research should help to inform management decisions regarding which predator control tools will be most cost-effective in a given scenario, by identifying the improvement in reproductive success of the Hawaiian stilts nesting inside versus outside of a mammalian predator exclusion fence.