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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...