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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. P henotypic plasticity, the capacity of an organism to produce different phenotypes in response to environmental cues, can be an important adaptive strategy in variable or changing environments (1, 2). Inducible defenses are a ubiquitous form of plasticity that involve the production of chemicals, morphologies, or behaviors by prey species in response to predator cues (3). These changes reduce prey vulnerability to inducing predators or herbivores. Inducible defenses occur in diverse taxa and examples include: production of chemical defenses in plants (4, 5), formation of spines in rotifers (6) and marine bryozoans (7) and neck teeth and helmets in cladocerans (8), diel vertical migration in marine (9) and freshwater zooplankton (10), and changes in body shape in fish (11) and shell shape and thickness in mollusks and barnacles (12-15).Despite improved understanding of the cues inducing these defenses and their immediate adaptive value (3, 16), our understanding of how this phenomenon contributes to broader temporal and spatial patterns of phenotypic variation remains poor. To date, most studies have examined inducible defenses and their costs on very localized spatial scales (3-15). In doing so, there is limited consideration of environmental complexity and the interactive inf luences of others cues that are likely to occur across a broader scale. For example, many plant and animal species have wide altitudinal and latitudinal distributions where dramatic temperature gradients occur. Because temperature can profoundly inf luence developmental and metabolic rates (17, 18) and phen...

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The observed rates and deleterious impacts of biological invasions have caused significant alarm in recent years, driving efforts to reduce the risk (establishment) of new introductions. Characterizing the supply of propagules is key to understanding invasion risk and developing effective management strategies. In coastal ecosystems, ships' ballast water is an important transfer mechanism (vector) for marine and freshwater species. Commercial ships exhibit a high degree of variation in ballast water operations that affect both the quantity and quality of propagule supply, and thereby invasion risk. The per-ship inoculation size from ballast water depends upon both the volume discharged and the organism density. Moreover, propagule quality will vary among source regions (ports) and voyage routes, due to differences in species composition and transport conditions, respectively. We show that significant differences exist in (i) the frequency and volume of ballast water discharge among vessel types, (ii) the frequency of vessel types and routes (source regions) among recipient ports, and (iii) the transit success (survivorship) of zooplankton in ballast tanks among voyage routes. Thus, propagule supply is not a simple function of total ship arrivals. For ships, as well as other vectors, variation in propagule quantity and quality must be explicitly considered to estimate invasion risk and advance predictive ability.