Keeping the Rosy Wolves Away: Reducing Time Costs of Predator Exclusion Fences

Abstract

Habitat loss and predation by introduced species have contributed to the decline and extinction of land snail species across the globe. Predation of Hawaiian land snails, kāhuli in the Hawaiian language, by rodents (Rattus rattus, Rattus norvegicus, Rattus exulans, and Mus musculus), Jackson’s Chameleons (Trioceros jacksonii), and Rosy Wolf snails (Euglandina rosea) have intensified in recent decades. Dramatic declines in recent years can be attributed to the unrelenting spread of the Rosy Wolf snail, a carnivorous species introduced to Hawai‘i as a biocontrol measure for other non-native snail species. Without predator-exclusion fences (exclosures) to surround and shelter kāhuli populations, the density of predators on the landscape does not allow for the persistence of vulnerable populations. The Snail Extinction Prevention Program maintains a growing number of exclosures across the islands, but the person-hours required to ensure sites remain predator-free divert resources from high-priority kāhuli species yet to be protected. Hiding decoy Euglandina at random within an existing exclosure, we designed a study that compared the time costs and efficacy of three search strategies: quadrant, random-quadrat, and random-transect. We identified random-transect searches to be the most efficient method, requiring the least amount of search effort to successfully find decoys. Our findings directly impact SEPP’s ability to reverse kāhuli population decline, allowing the program to better budget its resources by factoring in the hidden costs of maintaining predator-free areas. Knowing an exclosure’s resource requirements upfront enables project managers to accurately calculate the number of exclosures their programs are able to maintain given their available staff resources.