Document: DAV-3-56-28

Predation gradients in intertidal communities: The interactive roles of habitat and elevation.

BRUCE LEWIS, D.* ¹ and L.A.EBY ²

University of Wisconsin-Madison, Madison, WI 53706, USA ¹
Duke University Marine Laboratory, Beaufort, NC USA ²

Abstract:
Gradients of predation intensity may be generated by spatial variability in factors that regulate predator-prey interactions. We investigated whether gradients of blue crab predation on marsh periwinkle snails were evident in North Carolina Spartina salt marshes. Factors controlling predator access to prey in marshes include marsh elevation and vegetation height. Field surveys and modeling exercises generated three predictions. First, predation intensity should increase from high elevation portions of the marsh to the low marsh near tidal creeks and bays. Second, predation will be more intense in areas with short marsh grass (Spartina) and less intense where taller grass provides refuge for snails. Third, these predation gradients ordered by elevation and vegetation height may interact. Where marsh grass is short, the disparity between high marsh and low marsh in snail density, crab size, and predation rate should be negligible. Where grass is tall, however, predation intensity in the low marsh will be greater than in the high marsh. To test these predictions, we cut several experimental channels from low to high marsh through Spartina. Each channel was paired with an uncut reference transect, and we monitored abundances of crabs and predation rates on tagged snails. The experiment supported the hypothesis that vegetation height and marsh elevation both order gradients of predation intensity by blue crabs on snails. Support was evident, but equivocal, for the hypothesis that the gradients interact. This study emphasizes the indirect consequences of habitat modification, such as that imposed by feral horses that heavily graze Spartina marshes on several east coast barrier islands.

Keywords: predation gradients, salt marsh, blue crabs, periwinkles, Spartina

This abstract is being presented at: 2:00 PM in session:
Oral Session #65: Wetlands, Estuaries and Salt Marshes.