Habitat fragmentation in marine landscapes: relative effects of habitat cover and configuration on juvenile crab survival in California and North Carolina seagrass beds

Processes that dictate faunal abundance (e.g. predation) may be linked primarily to the configuration of habitat (e.g. patch size, patch isolation, proximity to edge) or simply to the amount of habitat (i.e. habitat cover) in marine and terrestrial systems. I asked whether juvenile crab survival in marine seagrass habitats is linked to habitat cover or to habitat configuration, and whether relationships between crab survival and habitat features were similar in two widely separated seagrass landscapes (Bodega Harbor, California and Back Sound, North Carolina, USA). I also evaluated the independent effects of seagrass structural complexity (shoot density and shoot biomass per unit area) and habitat fragmentation on crab survival. Juvenile red rock crabs (Cancer productus: 1+year class) were tethered in Bodega Harbor and juvenile blue crabs (Callinectes sapidus: 0+year class) were tethered in Back Sound. Seagrass cover in both landscapes ranged from <10 to 100% over 10×10 m areas. Crab survival was correlated with elements of landscape structure and structural complexity in both landscapes, but relationships between crab survival and specific habitat features differed between the two landscapes. In California, juvenile red rock crab survival was negatively correlated with seagrass cover and was positively correlated with seagrass shoot density, and was marginally (P=0.06) negatively correlated with seagrass shoot biomass. In North Carolina, juvenile blue crab survival was positively correlated with log₁₀ patch area and was negatively correlated with seagrass shoot biomass. The results indicate that (1) both seagrass cover and configuration may influence crab survival; (2) seagrass structural complexity influences crab survival independently of landscape structure; (3) the relative influence of cover, configuration and structural complexity on survival likely differ among seagrass landscapes; and (4) increasing structural complexity (e.g. shoot biomass) may not always lead to increased faunal survival. The results correspond to those of recent studies in marine and terrestrial landscapes showing widely variable faunal responses to landscape structure among species, geographic location and through time, and imply that seagrass restoration and conservation strategies should incorporate species-specific responses to habitat structure at multiple scales.