Ecologists have long known that complex habitats often provide prey with refuges from predation. This is true for a wide variety of habitat types in terrestrial, freshwater and marine systems. Despite the recognized importance of structural habitat complexity, ecologists have defined and measured complexity in many different ways. We propose 2 new indices of structural habitat complexity that are dimensionless, that can be applied across various habitat types and scales, and that directly measure how structural complexity interferes with a predator's foraging ability. These indices are: the total area of cover within a habitat divided by the total area of the habitat (C-t/A(t)) and the average inter-structural space size divided by the size of the predator (Sp/Pr). C-t/A(t) measures the amount of cover available within a habitat that interferes with a predator's ability to see, or otherwise sense,prey within the habitat. Sp/Pr measures the extent to which the structure interferes with a predator's ability to move through the habitat in search of, or while pursuing, prey. We predicted that prey survivorship should increase hyperbolically with increasing C-t/A(t), and that survivorship should decrease sigmoidally with increasing Sp/Pr. We also predict that both C-t/A(t) and Sp/Pr can influence survivorship independently, and that they form a survivorship plane. We tested our model in 3 laboratory experiments with the fish Fundulus heteroclitus as predator and amphipods as prey, and in 1 field experiment. The results of our laboratory experiments support our model for Sp/Pr, but are only suggestive for C-t/A(t). The results of the field experiment are consistent with our laboratory results, and our model.

We quantified nekton and estimated trophic export at salt marshes with both erosional and depositional edges at the Goodwin Islands (York River, Virginia, USA). At depositional-edge marshes, we examined trophic flows through quantitative sampling with 1.75 m(2) drop rings, and through gut content analyses of captured nekton. Six habitats were sampled on a transect from the marsh surface to the unvegetated subtidal. Consumption of animal prey by nekton, and export of trophic energy via transient nekton, was estimated for each habitat and for the entire marsh transect. The marsh edge was the habitat where we estimated the greatest contribution to export per m2. An estimated 28.0 g dry weight of animal tissue was removed as prey per m2 of depositional marsh edge into the open estuary over 150 days, primarily by blue crabs Callinectes sapidus. When the entire marsh was examined, however, marsh interior areas provided most of the trophic support for resident and transient species due to the greater area of the interior. When we considered the entire tidal cycle, the unvegetated intertidal area was also very productive, and contributed substantially to these trophic pathways. The blue crab Callinectes sapidus was the biomass dominant and probably the most important predator in all habitats. In a separate study, erosional-edge marshes facing open bays were examined with an 80 m(2) enclosure net that sampled the marsh edge and the adjacent unvegetated area. For these marshes we report a high biomass of larger transient species. Blue crabs were the biomass dominant at every sampling date (mean from June to September 1996, 0.31 inds m(-2) and 2.81 g dry weight m(-2)). A high biomass of transient fish species was also seen (mean from June - September, 0.90 inds m(-2) and 1.25 g dry weight m(-2)). We suggest this high biomass implies that these species receive an appreciable benefit from this habitat. The high biomass of predators also suggests the potential for export from erosional-edge marsh areas. Although the gear used to examine erosional and depositional marsh edges was clearly different, both types of edge saw considerable use by transient species. We therefore conclude that marshes with both erosional and depositional edges can export significant biomass to deeper water ecosystems as the consumption of marsh secondary production by transient nekton.