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

Abstract

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 (Ͻ1 m 2 to Ͼ30 000 m 2 ). 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.

Musical chairs mortality functions: density-dependent deaths caused by competition for unguarded refuges

“…Other marine species, including blue and dungeness crabs, shelter from predators in stands of eelgrass or within other structurally complex substrata (Fernandez et al 1993;Hovel and Lipcius 2001). Crab survival increases gradually with eelgrass blade density or substratum structural complexity, but also depends on landscape configuration, tidal inundation, and conspecific density (Fernandez et al 1993;Hovel and Lipcius 2001;Hovel and Fonseca 2005). In freshwater environments, amphibian tadpoles seek cover in pond vegetation to avoid predation by predatory insects, and tadpole survival is correlated with the amount of cover available (Babbitt and Tanner 1998).…”

“…Like the bridled gobies of Forrester and Steele (2004), many species of marine reef fishes evade larger piscivorous fishes by retreating to shared structural refuges formed by rocks, coral, or macrophytes, and they typically do so only when threatened or attacked (e.g., Ebeling and Laur 1985;Shulman 1985;Hixon and Beets 1993;Caley and St. John 1996;Anderson 2001;Johnson 2006;Caddy 2008). Other marine species, including blue and dungeness crabs, shelter from predators in stands of eelgrass or within other structurally complex substrata (Fernandez et al 1993;Hovel and Lipcius 2001). Crab survival increases gradually with eelgrass blade density or substratum structural complexity, but also depends on landscape configuration, tidal inundation, and conspecific density (Fernandez et al 1993;Hovel and Lipcius 2001;Hovel and Fonseca 2005).…”

“…Structural features of the habitat that provide shelter are perhaps the most common type of refuge in nature (Berryman and Hawkins 2006). Examples abound of terrestrial, aquatic, and marine prey species that use structural refuges to reduce predation risk, including sparrows (Pulliam and Mills 1977;Lima 1990), rodents (Koivunen et al 2003), insects (Ame et al 2006;Riihimaki et al 2006), amphibians (Sih et al 1988;Babbitt and Tanner 1998), crabs (Fernandez et al 1993;Hovel and Lipcius 2001), lobsters (Wahle and Steneck 1991;Eggleston and Lipcius 1992), and marine fishes (Ebeling and Laur 1985;Shulman 1985;Rangeley and Kramer 1998;Anderson 2001;Holbrook and Schmitt 2002;Forrester and Steele 2004;Hixon and Jones 2005;Johnson 2006;Caddy 2008).…”

Interactive effects of pesticide exposure and habitat structure on behavior and predation of a marine larval fish

“…Treatments without habitat (''none'') consisted solely of the Vexar base covered with gravel, ''low'' habitat consisted of a density of 50 seagrass shoots (500 shoots per m 2 ), and ''high'' habitat consisted of a density of 100 seagrass shoots (1,000 shoots per m 2 ). Densities were chosen to simulate a range of natural densities comparable to previous studies (Orth et al 1984;Hovel and Lipcius 2001;Jones et al 2013) and to allow behavioral observations on small larval fish (*10 mm standard length) that are difficult to track visually. Vegetative shoots were either placed uniformly throughout the tank or in patches of five shoots to simulate different distributions of habitat while controlling for density.…”

“…Such potential interactive effects are environmentally relevant because many urbanized coastal environments are often compromised by chemical, physical, and biological stressors, including both pollution and habitat degradation (Lotze et al 2006). Many degraded estuarine systems display a range of structural habitat complexity, from a lack of vegetation in highly disturbed areas to highly fragmented or dense stands of aquatic vegetation (Hovel and Lipcius 2001). Yet the interactive effects of habitat structure and pesticides in coastal and estuarine systems are unclear, along with the likely consequences for aquatic fauna (Davis et al 2012).…”