Abiotic Cycles Mediate the Strength of Cross-Boundary Consumption Within Coastal Food Webs

Abstract

Understanding the effect of habitat edges on species interactions (e.g. predation) is critical for determining landscape-scale patterns in productivity and the structuring of communities in an ever-changing environment. Both abiotic cycles and habitat structure can mediate faunal movements across habitat edges and determine predators’ ability to access prey across both space and time. To quantify the effects of cyclical abiotic factors and habitat structure on consumer-resource dynamics across habitat boundaries at the land-sea interface, four complementary studies were conducted. Marsh periwinkles Littoraria irrorata were tethered within salt marshes of varying tidal amplitude, at 3 distances from the marsh edge, and assessed for predation after 24 h. Nekton catch rate was assessed with fyke net sampling as a proxy for predator utilization of the marsh platform. Consumption rates were positively correlated with tidal amplitude and proximity to the seaward marsh edge, and there was also a slight positive relationship between tidal amplitude and nekton access to the marsh. Tidal amplitude was positively correlated with Spartina alterniflora shoot density and negatively correlated with shoot height. Therefore, to separate the effects of habitat structure from tidal forcing, independent manipulations of shoot density and shoot height were conducted. We found that the signal of local habitat structure on consumption rates appears to be secondary to the effects of abiotic cycles on consumption. Disentangling the interactions between abiotic cycles and biotic structure of ecosystems across ecological boundaries is key to understanding both the strengths of species interactions and the mediation of cross-boundary energy flow.