Habitat Fragmentation in a Seagrass Landscape: Patch Size and Complexity Control Blue Crab Survival

Habitat fragmentation is increasingly common on land and in the sea, leading to small, isolated habitat patches in which ecological processes may differ substantially from those in larger, continuous habitats. Seagrass is a productive but fragmented subtidal habitat that serves as a refuge from predation for many animals because its structural complexity limits the detection and capture of resident prey. The singular influence of seagrass habitat fragmentation (e.g., patch size) on faunal survival is largely unknown and has been difficult to quantify because seagrass habitat complexity (e.g., shoot density) and patch size are often confounded and vary seasonally. In early summer 1998 we quantified the effect of seagrass habitat fragmentation on juvenile blue crab (Callinectes sapidus) survival in the absence of covarying complexity by exposing tethered crabs to predators in density-controlled, artificial eelgrass (Zostera marina) plots embedded within natural seagrass patches of four broad size classes (<1 m2 to >30 000 m2). We repeated this experiment in late summer 1998 with three different shoot densities, after predictable environmental events (defoliation and bioturbation) had increased seagrass habitat fragmentation and decreased shoot density. In early summer, crab survival was inversely correlated with seagrass patch area; survival of juvenile blue crabs increased as patch size decreased, in contrast to patterns typically observed in terrestrial and marine systems. This pattern appears to have been due to low abundance of adult blue crabs, the chief predator of juvenile conspecifics, in small patches. In late summer, blue crab survival was greater than in early summer, and survival increased with artificial seagrass shoot density but did not vary with patch size. The breakdown of the relationship between crab survival and patch size in late summer may have resulted from influx of cownose rays, which fragmented large, continuous patches of seagrass into smaller patches in midsummer, potentially equalizing fragmentation across the seagrass meadow. These results show that (1) fragmented seagrass landscapes hold significant refuge value for juvenile blue crabs, (2) fragmentation and crab survival vary temporally, and (3) crab survival increases with habitat complexity (shoot density) regardless of patch size. The findings indicate that habitat patch size and complexity jointly drive organismal survival, and that their influence differs temporally in this dynamic landscape. Thus, ecological processes are sensitive to landscape structure, and studies of habitat structure should incorporate multiple scales of space and time, as well as potentially confounding structural variables.