Edge Effects Are Not Linked to Key Ecological Processes in a Fragmented Biogenic Reef

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Reef-forming species play a key role creating, modifying and maintaining important habitat and their associated communities. Globally, many of these habitats are extensively fragmented but our understanding of key ecological processes is drawn from older studies performed in less degraded environments with extensive reef habitats. We tested whether proximity to habitat edges affected the persistence and functioning of fragmented intertidal mussel reefs, Mytilus edulis, at three sites in sedimentary habitats in Northern Ireland. Specifically, we quantified and compared key ecological properties in mid-habitat and edges and hypothesised that at the edges, there would be (i) larger individual mussels, (ii) lower recruitment, (iii) lower associated taxon diversity, (iv) lower biodeposition, (v) greater predation intensity and (vi) greater food availability. Additionally, we tested whether rates of functioning (e.g. organic matter accumulation) were greater in mussel patches (edge and mid-habitat) compared to sediment without mussels (vii). We did find larger mussels at the edges of patches, where there was also less organic enrichment of the sediment compared to mid-habitat (but more than areas without mussels). None of the other variables quanitifed differed consistently between the edge and mid-habitat. The lack of strong edge effects identified may be explained by the nature of the mussel habitat in this study, i.e. intrinsically greater proportion of edges compared to more extensive reefs. Our results suggest that at the scale of the reef patches in this study, edge effects were not as strong as previously thought in terms of regulating key ecological processes.