The Pomacea Project - Historical Perspectives

Despite the federal listing of the snail kite as endangered in 1967, very few direct observations of apple snails were published until the mid-1990s. The reasons for this may be four-fold. First, aside from their egg clusters and piles of empty shells left by predators, Pomacea paludosa is generally an inconspicuous species. Second, apple snails live in vegetated wetlands and typically are very difficult to sample in the field. Third, conclusions from early studies on apple snail dry down survival lacked inclusion of a key aspect of snail life history and consequently were misleading. Fourth, in some cases inaccurate assumptions about snail ecology were based on observations of snail kite behavior rather than on direct observations of the snails.

A substantial portion of Phil Darby's research program has emphasized how to sample apple snails. Indirect measures of relative abundance via egg cluster counts, though easy to obtain, have not proven sufficiently reliable for comparing abundances as affected by habitat condition and water levels. Consequently, Darby has developed several approaches for direct assessment of snail density and behavior (see Sampling Methods). The issue of their relative inconspicuousness has largely been resolved.

Early studies reported that Florida apple snails, unlike other Pila and Pomacea species (see Cowie 2002), were unable to tolerate wetland dry down conditions (Kushlan 1975; Little 1968; Turner 1994). These studies concluded that Florida apple snails could not tolerate dry downs, including those lasting less than one month. These findings were questioned by Darby and colleagues based on snail persistence in wetlands that do dry down periodically, and because of their similarity to other Pomacea snails. Evidence was produced that these past reports likely were confounded by an annual spring post-reproductive die-off of adult apple snails (Darby et al. 2003). It has since been found that adult-sized snails can survive several weeks to months of dry down conditions (Darby et al. 2008; see Dry Down Impacts on Survival).

Because the Florida apple snail was known as the critical food of snail kites, water management recommendations historically focused on making snails available for kites. Availability and abundance, however, are two very different things. Kites do not forage for snails in dry down conditions, so the kites abandon dry marsh and have been characterized as nomadic within their range (Sykes et al. 1995). Although unavailable, apple snail populations (as we now know) can persist in wetlands periodically subject to dry down conditions (depending on the duration, timing, etc.). With emphasis on the availability to kites, however, recommendations have been made to avoid wetland dry downs (Stieglitz and Thompson 1967; Beissinger 1988; Beissinger 1995), or to have wetlands “permanently maintained in a flooded condition” (Martin and Doebel 1977). There is accumulating evidence that because apple snails evolved under periodic dry down conditions, as did the habitats in which they live, periodic dry downs may be essential in supporting robust snail populations (i.e., those more natural in terms of timing, frequency, and duration) (Bennetts et al. 2006; Karunaratne et al. 2006; Darby et al. 2008).