Feeding Preferences of Two Predatory Snails Introduced to Hawaii and Their Conservation Implications

Abstract

Two introduced predatory land snails, Euglandina rosea and Oxychilus alliarius, have been implicated in the decline of native Hawaiian and Pacific island land snails. We examined the feeding ecology of E. rosea and O. alliarius, focusing first on prey size and species preferences, and second on quantifying consumption rates, which is necessary to address the possible levels of impact on natural populations. In prey-size preference experiments, E. rosea always consumed more snails in the smaller size class. In prey-species preference experiments, E. rosea preferred snails to slugs but showed no preference among snail species. Size preference experiments indicated that O. alliarius will only consume prey less than 3 mm in shell length. In prey species preference experiments O. alliarius consumed every snail species offered but did not consume either of the slug species. Consumption rates were positively related to the size of the predator, with the larger predator, E. rosea, having a much higher consumption rate than O. alliarius, and with larger E. rosea consuming more than smaller E. rosea. These results have unfortunate conservation ramifications, because since they indicate that both E. rosea and O. alliarius are generalist snail predators feeding preferentially on the smallest snails, although they rarely eat slugs. Since many extant Hawaiian and Pacific island snail species are small, they are at risk of predation by these two introduced predators, while invasive slugs — there are no native slugs in Hawaii — may not be impacted by them.