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, leadin g to small, isolated habitat patches in which ecological processes may diff er substantially from those in larger, continuous habitats. Seagrass is a p roductive but fragmented subtidal habitat that serves as a refuge from pred ation for many animals because its structural complexity limits the detecti on 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 fragmentatio n on juvenile blue crab (Callinectes sapidus) survival in the absence of co varying complexity by exposing tethered crabs to predators in density-contr olled, artificial eelgrass (Zostera marina) plots embedded within natural s eagrass patches of four broad size classes (<1 m(2) to > 30000 m(2)). We re peated this experiment in late summer 1998 with three different shoot densi ties, 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. i n contrast to patterns typically observed in terrestrial and marine systems . This pattern appears to have been due to low abundance of adult blue crab s, the chief predator of juvenile conspecifics, in small patches. In late s ummer, blue crab survival was greater than in early summer, and survival in creased with artificial seagrass shoot density but did not vary with patch size. The breakdown of the relationship between crab survival and patch siz e in late summer may have resulted from influx of cownose rays, which fragm ented large, continuous patches of seagrass into smaller patches in midsumm er, potentially equalizing fragmentation across the seagrass meadow. These results show that (1) fragmented seagrass landscapes hold significant refug e value for juvenile blue crabs, (2) fragmentation and crab survival vary t emporally. and (3) crab survival increases with habitat complexity (shoot d ensity) regardless of patch size. The findings indicate that habitat patch size and complexity jointly drive organismal survival, and that their influ ence differs temporally in this dynamic landscape. Thus, ecological process es are sensitive to landscape structure, and studies of habitat structure s hould incorporate multiple scales of space and time, as well as potentially confounding structural variables.