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Trophic cascades are often a potent force in ecological communities, but abiotic and biotic heterogeneity can diffuse their influence. For example, inducible defenses in many species create variation in prey edibility, and size-structured interactions, such as cannibalism, can shift predator diets away from heterospecific prey. Although both factors diffuse cascade strength by adding heterogeneity to trophic interactions, the consequences of their interaction remain poorly understood. We show that inducible defenses in tadpole prey greatly intensify cannibalism in predatory larval salamanders. The likelihood of cannibalism was also strongly influenced by asymmetries in salamander size that appear to be most important in the presence of defended prey. Hence, variation in prey edibility and the size structure of the predator may synergistically affect predator--prey population dynamics by reducing prey mortality and increasing predator mortality via cannibalism. We also suggest that the indirect effects of prey defenses may shape the evolution of predator traits that determine diet breadth and how trophic dynamics unfold in natural systems. Key words: behavior; cannibalism; diet; food webs; Hynobius retardatus (Dunn); inducible defense; predation risk; Rana pirica (Matsui); salamander; trait-mediated indirect effects; trophic cascade.

Antagonistic phenotypic plasticity may strongly influence trait evolution in tightly interacting predator--prey pairs as well as the role that trait plasticity plays in community dynamics. Most work on trait plasticity has focused on single predator--prey pairs, but prey must often contend with multiple predators in natural environments. Hence, a better understanding of the evolutionary and ecological significance of phenotypic plasticity requires experiments that examine how multiple predators shape prey trait plasticity. Here, using a simple food chain consisting of a top predator (dragonfly larvae, Aeshna nigroflava), an intermediate predator (salamander larvae, Hynobius retardatus), and frog (Rana pirica) tadpoles as prey, we show that the presence of dragonfly risk cues substantially modifies the intensity of antagonistic morphological plasticity in both amphibians. In the absence of dragonflies, tadpoles produced bulgier bodies in response to salamanders, and salamanders responded to this defense by enlarging their gape size. However, in the presence of dragonfly risk cues, the expression of both antagonistic traits was significantly reduced because tadpoles and salamanders produced phenotypes that are more effective against dragonfly predators. Thus, the reduced antagonism likely emerged, in part, because the benefits of antagonistic trait expression were outweighed by the potential cost of increased vulnerability to dragonfly predation. In addition, our results suggest that when all three species were present, salamander activity levels, which influence the amount of signals required to induce antagonistic traits, were more strongly affected by dragonfly risk cues than were tadpole activity levels. This species-specific difference in activity levels was likely responsible for the reduced tadpole mortality caused by salamanders in the presence vs. absence of dragonfly risk cues. Hence, dragonflies had a positive trait-mediated indirect effect on tadpoles by modifying both the morphological and behavioral traits of salamanders. Key words: arms race; coevolution; gape-limited predation; intraguild predation, morphological change; multiple predators; phenotypic plasticity; predation risk; top predator; trait-mediated indirect interactions; trophic polyphenism.

The role of native species diversity in providing biotic resistance to invasion remains controversial, with evidence supporting both negative and positive relationships that are often scale dependent. Across larger spatial scales, positive relationships suggest that exotic and native species respond similarly to factors other than diversity. In the case of island habitats, such factors may include island size and isolation from the mainland. However, previous island studies exploring this issue examined only a few islands or islands separated by extreme distances. In this study, we surveyed exotic and native plant diversity on 25 islands separated by < 15 km in Boston Harbor. Exotic and native species richness were positively correlated. Consistent with island biogeography theory, species richness of both groups was positively related to area and negatively related to isolation. However, the isolation effect was significantly stronger for native species. This differential effect of isolation on native species translated into exotic species representing a higher proportion of all plant species on more distant islands. The community similarity of inner harbor islands vs. outer harbor islands was greater for exotic species, indicating that isolation had a weaker influence on individual exotic species. These results contrast with recent work focusing on similarities between exotic and native species and highlight the importance of studies that use an island biogeographic approach to better understand those factors influencing the ecology of invasive species. Key words: biotic resistance; Boston Harbor Islands, Massachusetts, USA; dispersal; invasibility; invasive species; island biogeography; isolation; plant diversity; species richness; vascular flora.

The North Atlantic intertidal community provides a rich set of organismal and environmental material for the study of ecological genetics. Clearly defined environmental gradients exist at multiple spatial scales: there are broad latitudinal trends in temperature, meso-scale changes in salinity along estuaries, and smaller scale gradients in desiccation and temperature spanning the intertidal range. The geology and geography of the American and European coasts provide natural replication of these gradients, allowing for population genetic analyses of parallel adaptation to environmental stress and heterogeneity. Statistical methods have been developed that provide genomic neutrality tests of population differentiation and aid in the process of candidate gene identification. In this paper, we review studies of marine organisms that illustrate associations between an environmental gradient and specific genetic markers. Such highly differentiated markers become candidate genes for adaptation to the environmental factors in question, but the functional significance of genetic variants must be comprehensively evaluated. We present a set of predictions about locus-specific selection across latitudinal, estuarine, and intertidal gradients that are likely to exist in the North Atlantic. We further present new data and analyses that support and contradict these simple selection models. Some taxa show pronounced clinal variation at certain loci against a background of mild clinal variation at many loci. These cases illustrate the procedures necessary for distinguishing selection driven by internal genomic vs. external environmental factors. We suggest that the North Atlantic intertidal community provides a model system for identifying genes that matter in ecology due to the clarity of the environmental stresses and an extensive experimental literature on ecological function. While these organisms are typically poor genetic and genomic models, advances in comparative genomics have provided access to molecular tools that can now be applied to taxa with well-defined ecologies. As many of the organisms we discuss have tight physiological limits driven by climatic factors, this synthesis of molecular population genetics with marine ecology could provide a sensitive means of assessing evolutionary responses to climate change. Key words: adaptation; climate; cline; endogenous selection; hybrid zone; intertidal; latitude; polymorphism.

It is well established that predators can scare as well as consume their prey. In many systems, the fear of being eaten causes trait-mediated cascades whose strength can rival or exceed that of more widely recognized density-mediated cascades transmitted by predators that consume their prey. Despite this progress it is only beginning to be understood how the influence of predation risk is shaped by environmental context and whether it can exert an important influence on ecosystem-level processes. This study used a factorial mesocosm experiment that manipulated basal-resource identity (either barnacles, Semibalanus balanoides, or mussels, Mytilus edulis) to determine how resources modify the influence of predation risk, cascade strength, and the efficiency of energy transfer in two, tritrophic, rocky-shore food chains containing the predatory green crab (Carcinus maenas) and an intermediate consumer (the snail, Nucella lapillus). The effect of predation risk and the strength of trait-mediated cascades (both in absolute and relative terms) were much stronger in the barnacle than in the mussel food chain. Moreover, predation risk strongly diminished the efficiency of energy transfer in the barnacle food chain but had no significant effect in the mussel food chain. The influence of resource identity on indirect-effect strength and energy transfer was likely caused by differences in how each resource shapes the degree of risk perceived by prey. We suggest that our understanding of the connection between trophic dynamics and ecosystem functioning will improve considerably once the effects of predation risk on individual behavior and physiology are considered. Key words: barnacle cf. mussel food chain; consumptive and nonconsumptive effects; density-mediated effects; ecosystem function; energy transfer; mesocosm experiment; predation risk perception; resource identity; trait-mediated effects; tritrophic rocky-shore food chains; trophic cascade.

An important goal in ecology is developing general theory on how the species composition of ecosystems is related to ecosystem properties and functions. Progress on this front is limited partly because of the need to identify mechanisms controlling functions that are common to a wide range of ecosystem types. We propose that one general mechanism, rooted in the evolutionary ecology of all species, is adaptive foraging behavior in response to predation risk. To support our claim, we present two kinds of empirical evidence from plantbased and detritus-based food chains of terrestrial and aquatic ecosystems. The first kind comes from experiments that explicitly trace how adaptive foraging influences ecosystem properties and functions. The second kind comes from a synthesis of studies that individually examine complementary components of particular ecosystems that together provide an integrated perspective on the link between adaptive foraging and ecosystem function. We show that the indirect effects of predators on plant diversity, plant productivity, nutrient cycling, trophic transfer efficiencies, and energy flux caused by consumer foraging shifts in response to risk are qualitatively different from effects caused by reductions in prey density due to direct predation. We argue that a perspective of ecosystem function that considers effects of consumer behavior in response to predation risk will broaden our capacity to explain the range of outcomes and contingencies in trophic control of ecosystems. This perspective also provides an operational way to integrate evolutionary and ecosystem ecology, which is an important challenge in ecology. Key words: adaptive forming; ecosystem function; nonconsumptive effects; nutrient cycling; predation risk; top-down control; trophic efficiencies; trophic interactions.

Predator effects on prey dynamics are conventionally studied by measuring changes in prey abundance attributed to consumption by predators. We revisit four classic examples of predator-prey systems often cited in textbooks and incorporate subsequent studies of nonconsumptive effects of predators (NCE), defined as changes in prey traits (e.g., behavior, growth, development) measured on an ecological time scale. Our review revealed that NCE were integral to explaining lynx hare population dynamics in boreal forests, cascading effects of top predators in Wisconsin lakes, and cascading effects of killer whales and sea otters on kelp forests in nearshore marine habitats. The relative roles of consumption and NCE of wolves on moose and consequent indirect effects on plant communities of Isle Royale depended on climate oscillations. Nonconsumptive effects have not been explicitly tested to explain the link between planktonic alewives and the size structure of the zooplankton, nor have they been invoked to attribute keystone predator status in intertidal communities or elsewhere. We argue that both consumption and intimidation contribute to the total effects of keystone predators, and that characteristics of keystone consumers may differ from those of predators having predominantly NCE. Nonconsumptive effects are often considered as an afterthought to explain observations inconsistent with consumption-based theory. Consequently, NCE with the same sign as consumptive effects may be overlooked, even though they can affect the magnitude, rate, or scale of a prey response to predation and can have important management or conservation implications. Nonconsumptive effects may underlie other classic paradigms in ecology, such as delayed density dependence and predatormediated prey coexistence. Revisiting classic studies enriches our understanding of predatorprey dynamics and provides compelling rationale for ramping up efforts to consider how NCE affect traditional predator-prey models based on consumption, and to compare the relative magnitude of consumptive and NCE of predators. Key words: behavior; consumptive effects; keystone predators; nonconsumptive effects; predator prey cycles and interactions; trait-mediated indirect effects; trophic cascades.

Predator-induced morphological defenses are a well-known form of phenotypic plasticity, but we continue to have a limited understanding of geographic variation in these responses and its genetic basis. Here we examine genetic variation and geographic differentiation in the inducible defenses of tadpoles (Rana pirica) in response to predatory salamander larvae (Hynobius retardatus). To do so, we crossed male and female frogs from a 'mainland' Japanese island having predaceous salamanders and a more isolated island not having predaceous salamanders and raised resulting offspring in the presence and absence of H. retardatus. Mainland tadpoles exhibited a higher capacity to express the inducible morphology (a more bulgy body) than those from the predator-free island, and expression of the bulgy morph in mainland island hybrids produced phenotypes that were intermediate to those produced by pure crosses. In addition, parental sex had no effect on expression of the bulgy morph. Our results support the hypothesis that geographic variation in inducible defenses is linked to the additive effects of autosomal alleles that are shaped by differences in historical exposure to the inducing predator. Key words: arms race; bulgy; coevolution; costs; genetic variation; inducible defense; inducible offense; local adaptation; phenotypic plasticity; predation risk; salamander; tadpole.

Food chain length is an important property of ecosystems, but the mechanisms maintaining it remain elusive. Classical views suggest that energetic inefficiencies (the 'energy-flow hypothesis') limit food chain length, but others have argued that better explanations reside in more complex scenarios that consider the stability of food webs or the combined effects of productivity and ecosystem size. We argue that abandonment of the energy-flow hypothesis is premature. For a simple tritrophic rocky intertidal food chain, we show that the efficiency of energy transfer is strongly influenced by predation risk and consumer density. Effects tied to predation risk were particularly strong, explaining 32% of the variation in growth efficiency (compared to 15% for density effects) and reducing it by 44-76%. Hence, the stress (fear of being eaten) that predators impose on prey may be instrumental in limiting energy transfer up the food chain and thus contribute to the shortening of food chains. Key words: Carcinus maenas; ecological growth efficiency; ecosystem size; energy flow hypothesis; food chain length; foraging; Nucella lapillus; predation risk; productivity; risk cues; species interactions; trophic structure.

Mussel beds and algal canopies on Gulf of Maine (USA) rocky shores have been hypothesized to represent stochastically determined community states, but we recently found that they were highly deterministic, consumer-controlled states in a tidal river. Mussel beds dominated high-flow habitats with high larval delivery and low consumer pressure, whereas algal canopies dominated low-flow habitats with high consumer pressure. In this paper we examine the alternative-community-state hypothesis on rocky shores on the open coast of Maine. We hypothesized that community states would be less deterministic due to reduced consumer pressure and more stochastic flow patterns. At 10 mussel-bed and 10 seaweed-canopy sites on the open coast of central Maine, we created 9-[m.sup.2] and 1-[m.sup.2] clearings and an unmanipulated control area, each containing control (no caging), caged, and cage-control quadrats. Independent of disturbance patch size, mussel beds and seaweed canopies rapidly began to return to their original community type in the absence of consumers (crabs and snails). With consumers present, recovery of the original communities did occur, but at a much slower pace. Our results suggest that mussel beds and algal canopies in the Gulf of Maine are deterministic consumer-controlled states, rather than being stochastically determined. If stochastic alternative community states exist in this system, they occur in a very restricted subset of habitats where the strong consumer control and determinism we have found in both tidal river and open coast habitats are relaxed. Key words: alternative community states; Ascophyllum nodosum; community ecology; consumer control of community pattern; disturbance theory,; Gulf of Maine (USA) rocky shores; Mytilus edulis; predation; rocky intertidal; secondary succession; tidal-river cf open-coast habitats.

The influence of predation on rocky intertidal community structure has long emphasized the importance of indirect interactions. Most efforts in this area have focused on the density-mediated, or lethal effects, of predators on prey density. Recently, there has been growing interest in trait-mediated indirect interactions (TMIIs): the presence of a predator in the environment influences the interaction between two other species (prey and their resource) by altering a trait of the prey species. For example, waterborne cues released by predators can cause changes in prey species behavior, such as feeding rates, thereby altering the impact of the prey species on their resources. Thus, TMIIs represent the non-lethal effects of predators that contrast with the more traditional emphasis on lethal indirect effects. Marine ecologists are just beginning to explore the role of TMIIs in their systems. We examined whether risk cues released by a ubiquitous crab predator (Carcinus maenas) influence the abundance of two dominant species in the rocky intertidal zone (barnacles [Semibalanus balanoides] and fucoid algae [Ascophyllum nodosum]) by altering the behavior of two of its snail prey (Nucella lapillus and Littorina littorea). We found that the presence of green crab risk cues can have strong cascading indirect effects on the abundance of barnacles and fucoid algae. N. lapillus exposed to risk cues consumed up to 29% fewer barnacles compared to conspecifics feeding in the absence of risk cues, whereas L. littorea exposed to risk cues consumed 459% fewer fucoids compared to conspecifics feeding in the absence of risk cues. These cascading interactions appear to reflect suppression of snail feeding by predator risk cues. In both food chains, snails exhibited more refuge-seeking behavior and grew less in the presence of risk cues. Our experiments suggest that TMIIs may have an important and underappreciated influence on species interactions that shape community dynamics on rocky intertidal shores. Key words: barnacles; community structure; crab predator; food chains; fucoid algae; indirect effects; plasticity; prey species behavior; rocky intertidal zone; snails; trait-mediated indirect interactions.

It has recently been hypothesized that intertidal mussel beds and seaweed canopies in the Gulf of Maine are alternate community stable states or disturbance patch mosaics dominated by either seaweed or mussel communities. The community that occurs in a given site is proposed to be stochastic and dependent on the size of the original disturbance and subsequent recruit availability. Large disturbances are postulated to be dominated by mussel beds and barnacles with widely dispersed larvae, whereas smaller disturbances are dominated by seaweeds, with limited dispersal. Positive feedbacks are proposed to maintain these two communities. We tested this hypothesis in a tidal estuary in central Maine. At eight mussel bed and eight seaweed canopy sites, we created 9-[m.sup.2] and 1-[m.sup.2] clearings and an unmanipulated control area, and in each plot established control, caged, and cage control quadrats. After three years of monitoring, our results do not support the alternate stable state hypothesis. Instead, they suggest that the occurrence of mussel beds and seaweed canopies is highly deterministic. Seaweed canopies dominate habitats with relatively little water movement, whereas mussel beds dominate habitats with high flows; and largely independent of disturbance size, mussel beds and seaweed canopies rapidly returned to their original community type, but only in the absence of consumers (crabs and snails). With consumers present, neither community showed significant signs of recovery, even after three years. In the presence of consumers, community recovery appears to be dependent on cracks and crevices providing refuges from consumers to seaweed and mussel recruits. The idea that natural communities may represent stochastically determined alternate stable states has important implications for understanding and managing natural ecosystems, but the very existence of alternate stable states in nature has been difficult to establish. Our results suggest that intertidal seaweed canopies and mussel beds in tidal rivers in the Gulf of Maine are highly deterministic alternative community states under consumer control. More generally, since all proposed examples of alternate community stable states are based on indirect, inferential evidence, our results imply that stochastically determined alternate community stable states might be an interesting theoretical idea without a definitive empirical example. Key words: alternative community stable state; Ascophyllum nodosum; Carcinus maenus; consumer control of community pattern; disturbance theory; Fucus spp.; Littorina littorea; predation; Mytilus edulis; rocky intertidal; secondary succession; Semibalanus balanoides.

Despite considerable progress in the study of inducible defenses, our understanding of the role of cues and costs to their ecology and evolution remains limited. This study focused on inducible defenses in the intertidal snail, Littorina obtusata, in response to the invasive green crab, Carcinus maenas, in the Gulf of Maine. We used widely separated (~400 km) snail populations from northern Maine and Massachusetts because they differ in their extent of historical and present-day contact with C. maenas. Crabs are presently more abundant in the southern Gulf and have been there for twice as long (~100 yr). We examined whether these geographic differences in contact history and abundance may contribute to geographic differences in risk cue sensitivity, the amount of induced defense, and the scaling of trade-offs accompanying the induced defense. Risk cues from both crabs and damaged conspecifics induced the greatest increases in snail shell thickness, whereas exposure to just crab cues induced intermediate levels of thickening. Interestingly, our results suggest that the effects of risk cues are additive and that geographic differentiation in cue sensitivity only occurs in response to crab cues. While northern snails showed greater shell thickness plasticity in response to crab cues, there was no geographic differentiation in sensitivity to cues from damaged conspecifics. We also found that the scaling of trade-offs in body size vs. shell thickness was similar for both regions. This result contrasts with theory because one would expect the more intense selection regime caused by increased crab predation in the southern Gulf to diminish the impact of trade-offs on southern snails. However, this result is consistent with the hypotheses that (1) trade-offs in body size are likely mediated by architectural constraints imposed by the shell and (2) circumventing these constraints via changes in shell shape would likely result in a shell that is more vulnerable to crab predation. Analysis of aperture area and spire height for field populations revealed no geographic differentiation in aperture area, but taller spires in northern snails. Hence, more intense selection via crab predation in the southern Gulf may prevent changes in shell shape that reduce the impact of trade-offs. Finally, despite the large trade-offs accompanying increases in shell thickness, our study suggests that the functional value of increased shell strength in reducing vulnerability to crab predation outweighs these costs. Key words: Carcinus maenas; crab predation; evolution; geographic variation; Gulf of Maine; inducible defense; intertidal snail; Littorina obtusata; plasticity; risk cues; shell shape and thickness; trade-offs.

Phenotypic clines, plasticity , and morphological trade-offs have been studied in an intertidal snail, Littorina obtusata. The results suggest that potential life-history trade-offs related to increased shell production exist. Increased water temperature and crab effluent affect shell form in like ways, thus more experiments are needed to see which has how much of an effect. Phenotypic plasticity clearly does have an important role in producing geographic variation in L. obtusata shell form.

Prey species may reduce their risk of predation in many ways (for reviews, see Edmunds 1974; Vermeij 1978, 1987). Although avoidance behaviors and chemical defenses can act as deterrents to [...]

The expression of defensive morphologies in prey often is correlated with predator abundance or diversity over a range of temporal and spatial scales. These patterns are assumed to reflect natural selection via differential predation on genetically determined, fixed phenotypes. Phenotypic variation, however, also can reflect within-generation developmental responses to environmental cues (phenotypic plasticity). For example, water-borne effluents from predators can induce the production of defensive morphologies in many prey taxa. This phenomenon, however, has been examined only on narrow scales. Here, we demonstrate adaptive phenotypic plasticity in prey from geographically separated populations that were reared in the presence of an introduced predator. Marine snails exposed to predatory crab effluent in the field increased shell thickness rapidly compared with controls. Induced changes were comparable to (i) historical transitions in thickness previously attributed to selection by the invading predator and (ii) present-day clinal variation predicted from water temperature differences. Thus, predator-induced phenotypic plasticity may explain broad-scale geographic and temporal phenotypic variation. If inducible defenses are heritable, then selection on the reaction norm may influence coevolution between predator and prey. Trade-offs may explain why inducible rather than constitutive defenses have evolved in several gastropod species.