How recruitment, intraspecific interactions, and predation control speciesborders in a tidal estuary

We examined the relative contribution of recruitment, intraspecific species interactions, and predation in controlling the upper intertidal border of the northern acorn barnacle, Semibalanus balanoides, in a tidal estuary in Maine. We hypothesized that the contracted border at sites that experienced low tidal currents was due to flow-medisted recruitment that resulted in r educed survival due to the absence of neighbor buffering of thermal stress (i.e., positive intraspecific interactions). We tested this hypothesis by m anipulating the density of recently settled barnacles and their thermal env ironment in a field experiment. Counter to our original hypothesis, barnacl es with neighbors suffered severe mortality at low-flow sites. When density -dependent predation by the green crab (Carcinus maenus) was experimentally eliminated, however, we did detect evidence for positive interactions at t he low-flow sites but not at the high-how sites. In spite of the close prox imity of the sites, maximum daily rock temperatures at the low-flow sites w ere slightly, but consistently, greater than those at high-flow sites. Our findings suggest that the upper intertidal border of S. balanoides in the D amariscotta River is limited at low-flow sites by a combination of reduced recruitment, elevated mortality from thermal stress and enhanced predation by green crabs. More generally, oar findings highlight how physical stress and predation interact to alter the nature of density-dependent species int eractions in natural assemblages.