Rough Around the Edges: Lessons Learned and Future Directions in Marine Edge Effects Studies

Abstract

After several decades of research on edge effects in marine habitats, we still have little understanding of how organisms respond to marine ecotones, and methodological gaps appear to be limiting our progress. Using recent literature (2010–2018), we synthesized responses and processes of organisms across several marine habitats. Specifically, we examined the uniformity of studies across biogenic habitats, the scales selected for exploring edge effects, the experimental approaches used, and the confounding influences that muddle our interpretation of results. The majority of edge effect studies are still conducted in seagrass systems and focused on response patterns. We found that the majority of studies were equally likely to report an increase, decrease, neutral, or equivocal effect depending on the context of the organism or habitat. Additionally, only a single measure, or a few related responses, is assessed and causal mechanisms are rarely tested. We note that most studies quantitatively defined an edge habitat as a linear distance from a habitat boundary (e.g., < 1 m, < 5 m), but the distances were not usually scaled to the size, trophic level, or mobility of focal organisms. We provide a conceptual diagram as a roadmap for researchers for navigating the myriad influences that affect floral and faunal responses to marine habitat edges. Future efforts should seek to move beyond mensurative searches, explicitly incorporate potentially confounding variables, and more consistently test putative causal factors when known or hypothesized. Additionally, we advise expanding research on habitat types other than seagrasses (e.g., mangroves, shellfish, corals) and adjusting observational scales to more appropriately match mechanisms. Ultimately, we should move beyond pattern description, repeated in a limited subset of nearshore habitats, and toward a quantitative understanding of the processes acting in these unique and potentially impactful marine ecotones.