Mud Matters! The Importance of Unstructured Intertidal Habitats to a Mobile Benthic Predator, Cancer magister
Kirstin Holsman, People for Puget Sound
Abstract
Autogenic ecosystem engineers physically alter systems they inhabit and produce cascading impacts on various organisms. Complex habitats created by native, naturalized, and exotic engineering species are often credited with supporting more numerous and diverse species assemblages than unstructured habitats, yet their facilitative effects depend on landscape patterns and the size of associated biota. Positive effects of increased physical structure often benefit small resident invertebrates while inhibiting larger transitory species. In this study, we examined how physical structure along a gradient of habitat complexity influences patterns of migration and habitat use by a transient benthic predator, subadult Dungeness crab (Cancer magister). In particular, we compared habitat use by crabs among unstructured littoral habitats (ULH), native eelgrass beds (Zostera marina), naturalized Pacific oyster beds (Crassostrea gigas), and invasive Spartina alterniflora patches in Willapa Bay, WA. Baited trap surveys on ULH yielded catches of C. magister 30-95% higher than catches from structured habitats. Ultrasonic telemetry observations suggest subadult C. magister making nighttime foraging incursions prefer ULH to other littoral habitats, and underwater video observations show that migrations are influenced by tidal rhythms since movements are correlated with the direction and velocity of current flow. ULH may be the primary foraging habitat for migrating crabs. The risk of stranding during low tide coupled with reduced foraging efficiency in complex habitats likely deter most subadult C. magister from using structured littoral areas, despite the high density of potential prey species they support. Since a spatially explicit bioenergetics model for C. magister showed that daily intertidal forays largely subsidize crab populations in coastal estuaries, areas of ULH are particularly critical to crab production in these systems. ULH are often overlooked in management and conservation decisions but these unstructured areas are essential to subadult C. magister and may be important for other migratory species as well. Understanding how species, particularly migratory predators such as C. magister, utilize ULH should be a focus of future research. Assessing the ecological value of ULH will help clarify the significance of these areas to individual species and elucidate additional estuarine ecosystem functions and services they provide.