Large-scale manipulations reveal that top-down and bottom-up controls interact to alter habitat utilization by saltmarsh fauna

We used ecosystem-wide predator removal and nutrient enrichment to examine top-own and bottom-up forces on saltmarsh fauna. We reduced the killifish Fundulus heteroclitus by similar to 60%, which elicited significant increases (2 to 4 x) in hydrobiid snails Hydrobia spp., suggesting top-own control. The talitrid amphipod Uhlorchestia spartinophila was the most abundant species of epifauna in the creek-bank Spartina alterniflora habitat. Our elevated water column nutrients (similar to 70 mu mol 1(-1) , similar to 10x background) and predator reduction treatments alone both yielded nonsignificant increases in amphipod abundance, suggesting weak top-down and bottom-up control over this species. However, treatments interacted antagonistically in this habitat such that amphipod abundances were lower than would be expected by an additive response to both treatments. We found corresponding amphipod abundance increases in the adjoining creek-wall habitat, suggesting that amphipods move under the influence of our treatments from the marsh edge onto the creek wall, possibly as a result of an increased incidence of a parasitic (i.e. trematode Levinseniella sp.) infection. Coincident to increased amphipods on creek walls, semipalmated sandpipers Calidris pusilla, amphipod predators, increasingly foraged in creek channels in combined treatments (similar to 30x higher than controls based on unreplicated creek systems). Our results suggest that top-down and bottom-up effects may at times be difficult to detect in small, single-factor studies because of inter-habitat movements of target species and redundant predation. Because the effects of stressors may not be limited to one part of the landscape even for small, motile fauna, landscape-level studies may generally be needed to capture responses to anthropogenic activities.