Native oyster species were once vital ecosystem engineers, but their populations have collapsed worldwide because of overfishing and habitat destruction. In 2004, we initiated a vast (35-hectare) field experiment by constructing native oyster reefs of three types (high-relief, low-relief, and unrestored) in nine protected sanctuaries throughout the Great Wicomico River in Virginia, United States. Upon sampling in 2007 and 2009, we found a thriving metapopulation comprising 185 million oysters of various age classes. Oyster density was fourfold greater on high-relief than on low-relief reefs, explaining the failure of past attempts. Juvenile recruitment and reef accretion correlated with oyster density, facilitating reef development and population persistence. This reestablished metapopulation is the largest of any native oyster worldwide and validates ecological restoration of native oyster species.

Many exploited fish and macroinvertebrates that utilize the coastal zone have declined, and the causes of these declines, apart from overfishing, remain largely unresolved. Degradation of essential habitats has resulted in habitats that are no longer adequate to fulfil nursery, feeding, or reproductive functions, yet the degree to which coastal habitats are important for exploited species has not been quantified. Thus, we reviewed and synthesized literature on the ecological value of coastal habitats (i.e. seagrass beds, shallow subtidal and intertidal habitats, kelp beds, shallow open water habitats, saltmarshes, mussel beds, macroalgal beds, rocky bottom, and mariculture beds) as feeding grounds, nursery areas, spawning areas, and migration routes of 59 taxa, for which the International Council for the Exploration of the Sea (ICES) gives management advice, and another 12 commercially or ecologically important species. In addition, we provide detailed information on coastal habitat use for plaice (Pleuronectes platessa), cod (Gadus morhua), brown shrimp (Crangon crangon), and European lobster (Homarus gammarus). Collectively, 44% of all ICES species utilized coastal habitats, and these stocks contributed 77% of the commercial landings of ICES-advice species, indicating that coastal habitats are critical to population persistence and fishery yield of ICES species. These findings will aid in defining key habitats for protection and restoration and provide baseline information needed to define knowledge gaps for quantifying the habitat value for exploited fish and invertebrates.

Adult soft—shelled clams (Mya arenaria) persist at low densities in Chesapeake Bay sandy habitats despite intense predation by blue crabs (Callinectes sapidus). Clam persistence may be a consequence of variation in blue crab foraging rates as a function of clam density and sediment composition. In laboratory aquaria, we measured the functional responses (prey consumption per predator as a function of prey density) of large blue crabs to six densities of adult soft shelled clams buried at natural depths in two sediment types (mud and sand). Functional responses in sand and mud were differentiated statistically by analyses of (1) residuals and residual sums of squares of discrete and continuous—time models, and (2) the exponent b of a general functional response model. Crab predation rates were significantly higher in mud than sand, and functional responses differed significantly between these two substrates. Blue crabs displayed type III (sigmoid) density—dependent functional responses in sand, and type II (decelerating rise to an upper asymptote) inversely density—dependent responses in mud. Risk of mortality for clams decreased sharply in sand at low densities similar to those observed in the field near the end of the annual period of active predation. These observations (1) suggest that variable blue crab functional responses result in microhabitat—specific mortality rates of benthic prey, and (2) indicate that functional responses can differ significantly according to the physical properties of topographically simple habitats.

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.

Summary1. In a population with Allee effects a positive relationship exists between fitness and population size or density. Allee effects may result in extinction thresholds and are therefore crucial in conservation and management. It has been shown theoretically that Allee effects can be driven by predation; however, there are few empirical data. Previous empirical work on Allee effects has emphasized that taxa with life-history characteristics such as co-operative breeding may be prone to such effects. Because predation is a general ecological mechanism, Allee effects may be more widespread than previously thought. 2. We used a series of simple heuristic models to develop a theoretical framework for understanding predation-driven Allee effects as a function of predator functional and aggregative responses. 3. Predators can create an Allee effect if they have a type I (linear) or type II (saturating) functional response without a type III (sigmoid) aggregative response, or vice versa. In addition, predation must be the main driver of prey dynamics, and prey must have little spatial or temporal refuge from predation. 4. We highlighted several, mainly unrecognized, examples of predation-driven Allee effects from the literature, the majority of which came from systems that had been perturbed by exploitation or introduced predators. 5. Synthesis and applications . Allee effects can arise from a general ecological process under a variety of different combinations of functional and aggregative responses. Allee effects may thus be present in a broad spectrum of different taxa with different types of life history, not only those taxa, such as broadcast spawners and co-operative breeders, on which empirical work has focused thus far. Conservation biologists and managers working with heavily exploited or threatened populations, or attempting reintroductions, should be aware of the possibility of a threshold population size or density below which extinction is likely. These thresholds can occur regardless of species life history, if predation is a major source of mortality and spatial and temporal predation refuges are limited.