Drought-induced shifts in a wetland crayfish assemblage.
Dorn, Nathan 1, Trexler, Joel 1, 1 Department of Biological Sciences, Miami, FL, USA
ABSTRACT- Conceptual models based on habitat permanence and predator transitions can successfully explain variation in community composition (and species' traits) in ponds and lakes. We are currently examining assemblage dynamics and traits of 2 crayfish (Procambarus alleni and P. fallax) in a large subtropical wetland with spatial and temporal variation in water depth and drought intensity. Over a 9-year period we examined the density and composition of the assemblage in 4 sites (2 deep and 2 shallow), each comprised of replicate 1 ha plots in Taylor Slough, Everglades National Park. P. fallax dominated the assemblage at deeper sites during 3 years of continuously flooded conditions while nearby shallow sites had a mixture of both species. Following drier conditions in 1999, 2001, and 2002, P. alleni invaded the deep sites and P. fallax density declined at the shallow sites, suggesting P. alleni is a more drought tolerant species. We tested drought tolerance of these species with a 31-day mesocosm experiment that simulated drought. Both P. alleni and P. fallax survived well in flooded controls. P. alleni had greater survival in dry conditions (86%) than P. fallax (53%), and several P. fallax died without initiating burrows. Survivors of the two species were found in burrows of similar depths, and burrow depth was positively correlated with crayfish size (length or mass). P. alleni adults reach larger sizes and are heavier than P. fallax (per unit length). Furthermore, juvenile P. alleni grew faster than juvenile P. fallax when fed artificial diets in the lab. Faster growth and larger sizes, coupled with size-dependent burrow depths, should give P. alleni an even greater advantage during more intense droughts. The results of our experimental drought and comparisons of trait values are consistent with our hypothesis and field observations; the direct mortality effects of drought fall more heavily on P. fallax, favoring dominance by P. alleni.
Key words: environmental gradient, drought, crayfish, community structure
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