Relative strengths of competition for space and food in a sessile filter feeder

Previous workers have demonstrated that sessile filter feeders compete for food and space, but little is known about the relative strengths of these two processes. To determine this, the density and position of barnacles (Balanus improvisus) in a unidirectional current were manipulated to alter the amount of competition for space and food, respectively. Results indicated that competition for space significantly reduced growth, and marginally reduced survivorship. Competition for food was also detected, but only among uncrowded individuals; thus, it appears to be the weaker of the two interactions. However, under crowded conditions, downstream individuals actually grew more than those upstream. The most likely explanation for this result is that downstream individuals fed more efficiently because they were not exposed to the full force of the current. The results also suggest that since natural densities started high but continually decreased throughout the study, barnacles undergo an ontogenetic shift in the relative importance of these processes.