Removal of corallivorous snails as a proactive conservation tool

Abstract

PrePrints IntroductionPredator control is most commonly considered as a management strategy for invasive predators (Baxter et al. 2008;Barbour et al. 2011;Morris Jr et al. 2011) or outbreaks of endemic predators (Yamaguchi 1986;Sanz-Aguilar et al. 2009). Previous attempts to cull corallivores, specifically Acanthaster planci, have largely been aimed at localized outbreaks with the goal of preserving coral tissue over a large area (Yamaguchi 1986). These efforts have been deemed ineffective due to the large numbers and migrating aggregations of these predators (Yamaguchi 1986;Johnson et al. 1990). However, removal of a relatively sedentary predator from targeted populations of a threatened coral species has not been evaluated.Ecological theory on predator-prey dynamics can provide insight on situations when predator removal may be effective in protecting prey. Sinclair et al. (1998) present a framework whereby controlling natural predators may improve the outcome for management of declining or reintroduced populations of threatened species. In this framework, the appropriate scale of intervention depends on the functional and numerical response of predators to changing prey abundance. In cases where the effects of predation are depensatory and prey abundance is so low that they are vulnerable to stochastic events, predator control could provide benefit to prey populations (Sinclair et al. 1998). Rotjan and Lewis (2008), in a review of corallivory, suggest that the rapid pace of coral decline over the past two decades, largely from factors other than predation, may have indeed reached such a depensatory threshold such that predation is exerting undue influence, potentially compromising coral reef resilience.On reefs in the western Atlantic, the dominant framework builder, Acropora palmata, is preyed upon by the corallivorous snail Coralliophila abbreviata. Although disease, storms, and bleaching have largely driven the range-wide decline of A. palmata populations, snail predation is PrePrints recognized as one of the top three threats to the persistence and recovery of these populations (Bruckner 2002;Williams and Miller 2012). In the upper Florida Keys, there has been a 50% decline in A. palmata tissue abundance since 2004. Although the main culprit has been disease, feeding by C. abbreviata accounted for an estimated one-quarter of the observed tissue loss (Williams and Miller 2012). It is unknown whether or not the C. abbreviata population is increasing; however, typical predators of shelled gastropods are grunts, wrasses, trunkfish, triggerfish, and pufferfish (Randall 1967). Therefore it is possible that reduced predation on snails due to overfishing may have increased snail abundance (Burkepile and Hay 2007), though specific data to support this hypothesis are lacking. Regardless, as A. palmata populations decline, snails have been observed to become more concentrated on the remaining A. palmata (Bruckner et al. 1997;Bruckner 2000;Baums et al. 2003a;Williams and Miller 2012) rather than declining themselves,...