The Effect of Plant Community Structure on Apple Snail Abundance in the Everglades

Phil Darby and Laksiri Karunaratne University of West Florida, Pensacola, FL, USA

Robert E. Bennetts US Geological Survey, Center for Aquatic Resources Studies, Gainesville, FL, USA

As the exclusive food of the endangered snail kite and prey to a variety of other wetland fauna, apple snails are generally recognized as a critical resource warranting monitoring in the context of the Greater Everglades ecosystem restoration. Snail kite abandonment of wetlands in dry down conditions have led to unsubstantiated conclusions that drying events (presumably of any timing and duration) devastate snail populations, thus forcing kites to leave. Researchers and natural resource managers have subsequently recommended nearly continuous inundation of wetlands deemed critical habitat to kites. However, suppressing drying events would be contrary to critical aspects of snail autecology that have been documented: 1) longer hydroperiods decrease the abundance of emergent wet prairie macrophytes needed for snail oviposition and aerial respiration and 2) snails do tolerate dry down conditions (the majority surviving up to 4 months once they reach a threshold size) and that this species has a life history well adapted to periodic dry downs. Not only are snails adapted to dry downs, we believe that drying events are essential to supporting suitable snail habitat. Elucidating the relationship between apple snail abundance and habitat structure also relates directly to habitat management for snail kites. As visual hunters, snail kites cannot forage in densely vegetated habitats such as sawgrass and cattail, but selection among more structurally 'open' habitats (slough or prairie) have not been documented. If they have a preference, it could be because some habitats support more snails than others, or because the structure of some habitats may render snails more available to foraging kites. This study tests the hypothesis that snail abundance is greater in prairie versus slough habitats, and contributes to an understanding of habitat selection by foraging kites.

Two concurrently funded projects were initiated in Spring 2002 in two different Everglades wetland units: WCA-1 (funded by US Fish and Wildlife Service) and WCA-3A (funded by US Geological Survey), which taken together will function as a single more comprehensive study. A total of 6 sites were selected to test hypotheses about snail abundance and habitat structure. Each site consisted of prairie habitat (dominated by emergent species of Eleocharis, Rhynchospora or

1 Panicum) juxtaposed and/or interspersed with slough habitat (dominated by Nymphaea odorata). In most sites, the prairie (especially) or slough was not contiguous; i.e., the prairie habitat sampled within a site may have been partitioned into 3 or more patches. Patches of habitat judged as transitional or intermediate between prairie and slough (i.e., patches with both Nymphaea and emergent macrophytes) were avoided.

Snail density was determined using 1-m2 throw traps (n50-70 throw traps in a 50 x 50 m area) extracted with a dip net, with explicit estimation of capture probabilities to assess sampling efficiency across different habitat types. The 1-m2 throw trap method has also proven effective in estimating crayfish (Procambarus alleni) and freshwater prawn (Palaemonetes paludosus) densities; therefore, we recorded numbers of these important prey items found during our snail sampling effort. Throw trap sampling was conducted from approximately February through early May in order to avoid the annual post-reproductive die-off. Egg cluster production was monitored in transects established in each sampling site in order to document that throw trapping was completed prior to the die-off.

At this point we have not completed our analyses; thus, any conclusions should be considered preliminary. Data were analyzed within a generalized linear modeling framework. Preliminary results indicate consistently higher snail densities in the prairie habitat relative to the Nymphaea-dominated slough. A similar trend was found for crayfish. Habitat effect on freshwater prawn density varied among sites, but no overall habitat effect was indicated. The data appear to support the hypothesis that snail abundance in prairie exceeds that in slough habitats.

Qualitative assessment of the data thus far suggests that within the wet prairie communities, there may be specific associations among snails and some plant species. Such associations will be a primary focus of this study over the next two years, and we expect to refine the hypothesis regarding apple snail-plant community- snail kite interactions accordingly. Given the inextricable link between hydrology, vegetation and apple snails, this research effort will support the development of ecologically significant performance measures that respond to the Greater Everglades restoration activity.

Phil Darby, University of West Florida, Biology Department, 11000 University Parkway, Pensacola, FL, 32514, Phone: 850-474-2647, Fax: 850-474-2749, [email protected].

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