Feral Pig Abundance at Hakalau Forest National Wildlife Refuge

Science Center Objects

Feral pigs (Sus scrofa) have been identified as a significant problem in 120 U.S. National Wildlife Refuges. Pigs cause substantial degradation to natural ecosystems through rooting, digging, and browsing, but they are particularly destructive in Hawai‘I, which has no native terrestrial large mammals.

Project Overview:

Feral pigs (Sus scrofa) have been identified as a significant problem in 120 U.S. National Wildlife Refuges. Pigs cause substantial degradation to natural ecosystems through rooting, digging, and browsing, but they are particularly destructive in Hawai‘I, which has no native terrestrial large mammals. Feral ungulates have been identified as the greatest immediate threat to Hawaiian forest birds at Hakalau Forest National Wildlife Refuge, and the removal of these grazing and browsing animals has been identified as the highest priority management action in a summary report from a workshop on Implementing Recovery for Endangered Forest Birds in Hawai‘I (2008). A Feral Ungulate Management Plan (U.S. Fish and Wildlife Service 1996) for Hakalau was drafted to prioritize and expediently reduce ungulate populations, and a Comprehensive Conservation Plan (U.S. Fish and Wildlife Service 2010) ranked the construction and maintenance of ungulate-proof fences and the removal of ungulates as the highest priority actions to protect and restore habitat for endangered forests birds.    

Photo of a lactating feral pig, Sus scrofa, on Hawai‘i Island

Lactating feral pig, Sus scrofa, on Hawai‘i Island
(Credit: Steven Hess, USGS-PIERC. Public domain.)

Control of feral ungulates is the single most expensive management activity at Big Island National Wildlife Refuge Complex (BINWRC) on the Island of Hawai‘I, requiring construction, continuous maintenance, and cyclical replacement of fences, as well as a major concurrent effort in removing pigs through hunting, trapping, and snaring (Anderson and Stone 1993). It is difficult, however, to determine in advance how many pigs need to be removed to achieve substantial long-term population reductions. Fencing and eradication of ungulates from ecologically sensitive areas have been important steps in conservation and restoration; however, invasive plant species sometimes become problematic after ungulate removal by inhibiting the regeneration of native forest trees and understory plants (Loope and Scowcroft 1985). Biologists and managers at BINWRC have expressed a need to understand and interpret existing data collected from feral ungulate management actions, use models of ungulate abundance to strategically guide control efforts, and monitor the long-term recovery of native vegetation and potential proliferation of invasive plants in managed natural areas of the refuge complex.  

Field surveys coupled with population models of real-time pig abundance would enable Refuge managers to quickly respond with reliably-informed decisions about the appropriate level of strategic effort necessary to manage areas where pigs will be removed, have recovered, or have re-invaded protected natural areas. Many disturbed areas require continual monitoring and specific alien plant management strategies after ungulates have been eliminated. Vegetation monitoring is therefore essential for informing Refuge managers if and where supplemental plantings of important native species may be necessary to recover wildlife habitat. The results of ungulate control and vegetation monitoring will be used to evaluate ongoing management and restoration efforts at BINWRC, and will be beneficial to the Refuge’s natural resources through application of information into adaptive management strategies, thereby providing the Refuge with input and information needed to protect endangered species (U.S. Fish and Wildlife Service 2010).

Project Objectives:

We will automate data entry and analysis procedures of feral ungulate surveys at Hakalau Forest National Wildlife Refuge for USFWS staff by acquiring data from recent feral ungulate surveys at HFNWR, standardize a computer data entry interface; assuring quality control, proof and error-check data such that the error rate is < 1%; Creating a computer Geographic Information System (GIS) shapefile with spatial coordinates of all transects and stations linked to appropriate management units; Setting up an MS Excel spreadsheet to join data fields and generate pivot table summaries of ungulate and weed data which will output the proportion of survey plots with ungulate sign and weed presence within appropriate management units; creating procedures for importing data into GIS, map templates for ungulate and weed distribution, and color-coded legend graphics; applying an existing regression model in MS Excel to determine feral pig population estimates with appropriate statistical uncertainty; and working with USFWS staff to refine procedures and instructions and transfer data management and analytical responsibilities. The outcome will automate estimates of ungulate abundance and distribution.