Balancing the Edge Effects Budget: Bay Scallop Settlement and Recruitment Along a Seagrass Edge

Understanding the impact of landscape on species is becoming increasingly important as anthropogenic habitat fragmentation is accelerating. Habitat edge effects have received considerable attention in this area. However, organism responses are highly variable along habitat edges, and the perception of whether edges are beneficial or detrimental varies throughout the literature. Edge effects are often predicted to be either positive (enhancement at the edge) or negative (enhancement within the interior), and neutral effects are often considered to be incorrect results. In seagrasses, edges, while enhancing settlement and increasing organism abundance, can also increase interspecific interactions, particularly predator-prey interactions. Bay scallops have a life cycle that includes a passively drifting larval stage, rapid growth rate, and a strong seagrass association, making the scallop an ideal model organism for investigating edge effects. By simultaneously collecting settlers (those viable larvae available to settle and metamorphose) and recruits (those settlers which survive some period of time, in this case, 6 weeks) on the same collectors, we are able to demonstrate a balance between positive and negative edge effects, resulting in a net neutral effect. In addition, post-settlement loss, most likely due to predation, appears to be the dominant mechanism structuring scallop abundance, not patterns in settlement. Since predation is highest along the seagrass edge, it is possible that edges may represent an ‘ecological trap’ for bay scallops.