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Distribution of mobile animals may reflect decisions on how to balance conflicting demands associated with foraging and avoiding predators. A simple optimality model predicts that mobile animals should respond to changes in mortality risk (μ) and growth rate (g) by shifting habitats in a way that maximizes net benefits. In this study, field caging and tethering experiments quantified habitat‐specific growth rates and mortality risk, respectively, for three different sizes of a coral reef fish, Nassau grouper (Epinephelus striatus), during its juvenile tenure in off‐reef nursery habitats. These sizes bracketed the size at which this species undergoes an ontogenetic habitat shift from the interstices of macroalgal clumps (“algal habitat”) to areas outside, or adjacent to, macroalgae and other physically complex microhabitats (“postalgal habitats”). Experimental results were used in a cost–benefit analysis to test the following alternative (but not mutually exclusive) hypotheses: (1) juvenile grouper shift habitats in a way that maximizes growth rates (g); (2) juveniles shift habitats in a way that minimizes mortality (predation) risk (μ); and (3) if trade‐offs exist between maximizing growth rate and minimizing mortality risk, juveniles shift habitats in a way that minimizes the ratio of mortality risk to growth rate (μ/g). Results suggested that small fish face a trade‐off between living in the relatively safe algal habitat and achieving high growth rates in postalgal habitats. The value of μ/g was significantly lower in the algal than postalgal habitats for small fish, which typically reside in the algal habitat, and significantly lower in postalgal habitats for medium and large fish, which typically reside in postalgal habitats. Thus, habitat use by juvenile Nassau grouper was consistent with the “minimize μ/g hypothesis.” These results highlight how behavioral responses to ecological processes, such as changing predation risk with body size, determine distribution patterns of mobile animals.

The persistence of prey encountering intense predation varies by species, prey density, and habitat type; however, the collective impact of these factors has rarely been tested experimentally in natural marine systems. Using the thin‐shelled clams Mya arenaria and Macoma balthica as prey, and the main epibenthic predator of whole adult clams, the blue crab Callinectes sapidus, we conducted a series of experiments in Chesapeake Bay tributaries that (1) links field abundance and distribution of bivalve prey species with habitat‐specific mortality patterns; (2) represents the first comprehensive field test of species‐specific, habitat‐specific, and density‐dependent mortality for subtidal, soft‐bottom, deep‐burrowing prey; and (3) thereby enables development of a conceptual model to be used as a heuristic tool linking predator–prey dynamics, habitat type, and evolutionary defense tactics for marine benthos. In 15 years of field monitoring, Mya was more common in sand than mud habitats, and Macoma was widely distributed and at higher densities than Mya in mud and sand. In field experiments, mortality of both Mya and Macoma was density dependent in those habitats where the clams are common. The blue crab population in the field exhibited a type III “guild functional response” on Mya in sand, and on Macoma in both mud and sand. Mortality was lower in sand than mud for Mya, and similar in mud and sand for Macoma, correlating with the high abundances of Mya in sand and Macoma in sand and mud. The persistence of large juvenile and adult bivalves when confronted with intense predation derived substantially from a low‐density refuge from predation that varied in a species‐specific manner with habitat type, demonstrating the species‐specific importance of density and habitat to clam survival. We developed a conceptual model detailing the relative importance of behavior, morphology, habitat features, and the basic components of predator–prey interactions to the survival of bivalve molluscs. At one extreme are bivalve molluscs, such as oysters, that emphasize morphological refuges that increase the predator's handling time. At the other extreme are bivalves, such as Mya and Macoma, that reduce predator encounter rates. The model is intended to be used as a heuristic tool to develop testable hypotheses.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Distribution of mobile animals may reflect decisions on how to balance conflicting demands associated with foraging and avoiding predators. A simple optimality model predicts that mobile animals should respond to changes in mortality risk (p.) and growth rate (g) by shifting habitats in a way that maximizes net benefits. In this study, field caging and tethering experiments quantified habitat-specific growth rates and mortality risk, respectively, for three different sizes of a coral reef fish, Nassau grouper (Epinephelus striatus), during its juvenile tenure in off-reef nursery habitats. These sizes bracketed the size at which this species undergoes an ontogenetic habitat shift from the interstices of macroalgal clumps ("algal habitat") to areas outside, or adjacent to, macroalgae and other physically complex microhabitats ("postalgal habitats"). Experimental results were used in a cost-benefit analysis to test the following alternative (but not mutually exclusive) hypotheses: (1) juvenile grouper shift habitats in a way that maximizes growth rates (g);(2) juveniles shift habitats in a way that minimizes mortality (predation) risk (pL); and (3) if trade-offs exist between maximizing growth rate and minimizing mortality risk, juveniles shift habitats in a way that minimizes the ratio of mortality risk to growth rate (pu/g).Results suggested that small fish face a trade-off between living in the relatively safe algal habitat and achieving high growth rates in postalgal habitats. The value of xL/g was significantly lower in the algal than postalgal habitats for small fish, which typically reside in the algal habitat, and significantly lower in postalgal habitats for medium and large fish, which typically reside in postalgal habitats. Thus, habitat use by juvenile Nassau grouper was consistent with the "minimize pL/g hypothesis." These results highlight how behavioral responses to ecological processes, such as changing predation risk with body size, determine distribution patterns of mobile animals. model (substituting foraging rate for growth rate), could be used to predict habitat shifts by stream fishes.Ontogenetic habitat shifts are common for mobile marine species whose postlarvae settle from the pelagic environment to benthic habitats that serve as early juvenile nurseries. For example, in temperate systems, the juveniles of many species use vegetated or other complex benthic habitats as nursery areas before moving into adult habitats (e.g., Orth and von Montfrans 1987, Holbrook et al. 1990, Ross and Moser 1995, Arsenault and Himmelman 1996, Gillanders and Kin...

Adult infaunal clams (Macoma balthica) persist at low densities in sandy and muddy habitats in Chesapeake Bay, USA, despite intense predation by blue crabs Callinectes sapidus; another infaunal soft-shelled clam (Mya arenaria) only persists in sandy habitats. We hypothesized that the persistence of M. balthica and M. arenaria in certain habitats was due to blue crabs exhibiting a type 111 (sigmoid) functional response whereby the risk of mortality is reduced at low clam densities. Laboratory experiments assessed functional responses (prey consumption predator-' as a function of prey density) of large male blue crabs to 6 densities of M. balthica as a function of sediment type (sand and mud) and tank size (54.6 cm and 97.2 cm diameter). These results were compared with previous experiments with M. arenana. Abundances of blue crabs, M. balthica, and M. arenaria were measured 4 to 10 times yr-l from 1979 to 1986 at both sand and mud stations in the mesohaline zone of Chesapeake Bay. Laboratory functional response results were then related to seasonal habitat-specific abundance patterns of M. balthica and M. arenaria in the field. With the exception of M. balthica in mud, abundances of M. arenaria and M. balthica decreased as blue crab abundance increased during the summer. When blue crabs reached their peak abundances in July. M. balthica was predominant in mud whereas M. arenaria numbers dropped to zero in mud and persisted in sand at similar low densities to M. balthica in sand. In the laboratory, blue crabs exhibited density-dependent (type 111) functional responses to M. balthica Irrespective of sediment type and tank size, even though mortality rates of clams were significantly higher in sand than mud. Differences in habitat-specific burial depth probably accounted for the differential survival of M. balthica in sand and mud. Thus, M. balthlca obtained a relative refuge from blue crab predation at low densities similar to those in the field near the end of the seasonal period of active predation. Previous laboratory experiments with blue crabs indicated a type 111 functional response to M. arenaria in sand and an inversely density-dependent type I1 response to M. arenaria in mud. Thus, the collective laboratory and field evidence from this study and others strongly suggests that blue crabs are critical determinants of specles-and habitat-specific prey persistence in marine soft-bottom communities, and that analysis of predator functional and aggregative responses may help to explaln much of the spatial variation of clam abundance patterns in the mesohal~ne zone of Chesapeake Bay.

Abundance of early juvenile Dungeness crab (Cancer magister) is dramatically higher in intertidal shell habitats compared to mud habitats in several coastal estuaries of the Pacific Northwest. To define the mechanisms underlying this habitat—specific pattern in abundance, we concurrently examined four components of recruitment to intertidal shell and mud habitats at two locations within the Grays Harbor estuary (Washington, USA): (1) water column supply of crab megalopae (postlarvae); (2) settlement patterns of crab megalopae 8 h after settlement substrates were deployed; (3) density of first benthic juvenile instars (J1) 48 h after deployment of such substrates; and (4) density of early juvenile crab in shell and mud habitats over a 4—mo period. We also describe the physical processes likely to be influencing postlarval supply within Grays Harbor, and take advantage of natural variation in postlarval supply between two locations, in combination with a predator exclusion experiment, to define the relative importance of postlarval supply vs. post—settlement survival in regulating population size of juvenile crab in certain intertidal habitats. Water column postlarval supply (measured with plankton and neuston nets, and artificial settlement substrates) in terms of both megalopal density (number per cubic metre) and flux (number per hour) was significantly higher in the southern part of the estuary vs. the northern part during a week—long settlement pulse. Our field observations and measurements suggest that spatial variation in postlarval supply was due to local differneces in wind—driven surface currents, since tidal current speeds in the two locations were similar. More—over, there was no correlation between current speed and flux of megalopae over the bottom. There was generally no difference in postlarval supply between shell and mud habitats. Our experimental results further indicate that: (1) the abundance of recently settled crab megalopae in 0.25 m2 settlement trays was significantly higher in shell than in mud habitats, irrespective of whether the trays were placed in 3—5 ha of shell vs. mud; (2) there was a positive and significant correlation between postlarval supply and density of megalopae in shell and mud habitats; and (3) there was a positive and significant correlation between postlarval supply and density of J1 instars only in habitats where specific predators were excluded. Once the number of J1 instars at both geographical locations was reduced to similar levels, equivalent but steadily decreasing densities persisted throughout the summer growing season. The decoupling of settlement patterns and density of J1 instars took place within our 48—h sampling interval. Thus, future attempts to examine the correspondence between larval supply and post—settlement abundance of marine benthic species with planktonic larvae should do so at extremely small temporal scales or a critical life history phase may be overlooked. The results from this study demonstrate that substrate selection can affect distributi...

A large-scale study of early juvenile blue crab Callinectes sapidus recruitment within a shallow, predominantly wind-driven estuarine system demonstrated that distribution and abundance patterns were jointly influenced by location from oceanic sources of postlarvae, time period, habitat type, and post-settlement planktonic dispersal. The Croatan-Albemarle-Pamlico Estuarine System (CAPES) in North Carolina, USA, is a lagoonal body of water that is separated from the Atlantic Ocean by a chain of barrier islands, which are bisected by Oregon, Hatteras, and Ocracoke Inlets. For sampling purposes, the CAPES was divided into 4 regions that differed in distance and orientation from oceanic sources of postlarvae, as well as available complex benthic habitat types. The Eastern region was closest to oceanic waters, contained 3 major inlets, and harbored extensive seagrass beds. The Northern and Western regions were located along the inland boundary of the CAPES, and contained alternative habitat types including the submersed rooted vascular plant Myriophyllum spicatum and shallow detrital habitats. The Southern region was inshore and contained patchy seagrass. During a period that lacked storm events, virtually all juvenile recruitment occurred within seagrass beds at the Eastern region. Conversely, early juvenile blue crabs were distributed widely throughout the CAPES after the passage of tropical cyclones. The Eastern region appears to act as a relatively consistent initial recruitment site, whereas Northern and Western regions of the CAPES may act as episodic recruitment areas after the passage of tropical cyclones. Similar densities of early juveniles were found in different complex benthic habitats (seagrass, shallow detrital habitat, M. spicatum). A comparison of site-specific, settler-recruit densities (which represent distinct cohorts) suggested that post-settlement juveniles dispersed planktonically throughout the CAPES, most likely due to stormdriven transport. Post-settlement, planktonic dispersal altered the settler-recruit relationship, by both masking and potentially enhancing a density-dependent relationship between settlers and recruits. This study illustrates that ecological processes influencing recruitment, such as post-settlement dispersal, may be missed when studied at relatively small spatial scales, and that our interpretation of population regulation can vary depending on the scale of study. Studies conducted over broad spatial scales can provide a more complete understanding of recruitment dynamics and can elucidate the interconnectedness of subpopulations by identifying potential 'source' areas in species with open populations.