Eradication of feral swine from a barrier island in Florida, USA: an examination of effort and multi-method, multi-species population indexing

Abstract

Feral swine were targeted for and successfully eradicated from Saint Vincent Island (SVI), a National Wildlife Refuge (NWR) along the coast of Florida’s panhandle to protect its habitats and uncharacteristically high diversity of wildlife species for barrier islands in the region, including federal and state-listed threatened and endangered species. The eradication effort was initiated in early 2015 and concluded in 2019. A total of 438 feral swine were removed from the Island, 417 by federal control experts and 21 by recreational hunters. In general, the amount of effort needed to eradicate each feral swine slowly increased as the eradication effort progressed; however, effort increased by an order of magnitude in the final six months. The last three feral swine took 77 days of effort to remove. The eradication effort provided an opportunity for evaluating and comparing methods for indexing feral swine population abundance and their abilities to describe population trends and to detect animals at low population abundance. The feral swine population was monitored from 2015–2019 using a passive tracking index (PTI) twice each year and using camera traps. Camera and track plot data were used to calculate abundance indices based on a well-documented indexing paradigm applied to feral swine populations. In addition, we simultaneously monitored relative abundance of other mammalian species crucial to management for the Island. The PTI and camera index both well-tracked population abundance simultaneously for the large ungulates inhabiting the Island (feral swine, white-tailed deer, sambar deer). However, the sensitivity for the PTI to capture animal observations was much greater than for the camera stations. This held true even over 5-day observation sessions by cameras versus 3-day observation sessions for track plots. Additionally, the PTI was sensitive for simultaneously capturing data for smaller animals, raccoons and armadillos, whereas the camera stations were ineffective for the smaller species, likely due to camera positions being optimised to capture feral swine. Our 100-m track plots outperformed the camera stations in many regards, but the camera stations required less labour in the field and were less fragile in the field, especially from weather or access issues. In 2018, Hurricane Michael, a category 5 hurricane, struck SVI. Its habitat damage may have adversely impacted white-tailed deer and sambar deer populations, but not armadillos or raccoons. Both the swine eradication and hurricane impacts provided valuable means for validating indexing procedures.