Habitat complexity disrupts predator-prey interactions but not the trophic cascade on oyster reefs

Despite recognition of the significance of both food web interactions and habitat complexity in community dynamics, current ecological theory rarely couples these two processes. Experimental manipulations of the abundance of the two predators in an oyster-reef trophic cascade, and the structural complexity provided by reefs of living oysters, demonstrated that enhanced habitat complexity weakened the strengths of trophic interactions. The system of tri-trophic interactions included oyster toadfish (Opsanus tau) as the top predator that consumed the mud crab (Panopeus herbstii), which preys upon juvenile oysters (Crassostrea virginica). On reefs of low complexity, toadfish controlled mud crab abundances and indirectly determined the level of mortality of juvenile oysters. The indirect effects of toadfish on oysters emerged through their influence on how intensely mud crabs preyed on oysters. Augmentation of habitat complexity by substituting vertically oriented, living oysters for the flat shells of dead oysters disrupted both of the direct trophic linkages but did not alter the magnitude of the indirect effect of toadfish on juvenile oysters. This paradox can be understood by partitioning the mechanisms by which toadfish influence mud crabs and ultimately juvenile oysters. Trait-mediated indirect interactions (TMIIs; i.e., predator-avoidance behavior in mud crabs) accounted for 95.6-98.2% of toadfish indirect benefits to oyster survival and, consequently, were a much greater contributor than density-mediated indirect interactions (DMIIs; i.e., the reduction in crab abundance by toadfish). Avoidance behavior was unaffected by modification in habitat complexity. Complex reefs increased total oyster survival because added habitat complexity reduced mud crab predation on oysters. Additionally, the magnitude of this effect was much greater than the increase in oyster mortality as a result of complex reefs disrupting toadfish predation on mud crabs. This experimental demonstration of how habitat complexity modifies trophic interactions in a temperate reef community has fundamental implications for our understanding of species interaction webs and community structure. The influence of habitat complexity on the strength of a trophic cascade generally may depend upon whether physical complexity provides actual and perceived refuges for component predator-prey pairs.