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
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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,
pdarby@uwf.edu.
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