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J. A. Estes et al. chronicle the alarming consequences of apex predator losses through trophic cascades across systems ("Trophic downgrading of Planet Earth," Review, 15 July 2011, p. 301). Two features of the case studies are that (i) indirect predator effects are driven by the numerical consequences of consumption (predators eating prey, prey eating resources), and (ii) that these indirect effects on the abundance an...

J. A. Estes et al. chronicle the alarming consequences of apex predator losses through trophic cascades across systems ("Trophic downgrading of Planet Earth," Review, 15 July 2011, p. 301). Two features of the case studies are that (i) indirect predator effects are driven by the numerical consequences of consumption (predators eating prey, prey eating resources), and (ii) that these indirect effects on the abundance and composition of resources are large and predictable.

We suggest that increased attention to the significance of predation risk will produce a more comprehensive picture of the ecological consequences of predator losses. Predators can initiate trophic cascades by altering the foraging behavior or habitat use of prey (1). The strength of nonconsumptive effects and emergent indirect impacts can rival or exceed those caused through consumption, and produce outcomes that are complex and unpredictable. For example, exclusive focus on consumptive pathways suggests that losses of dolphin (Delphinus delphis) should benefit gannets (Monts capensisi) because they both feed on sardines (Sardinops sagax). However, gannets depend on dolphins to herd sardines near the surface, where they are only then accessible to diving gannets. The "fear of being eaten" can elevate prey metabolism (2), induce heat shock proteins (3), antioxidant enzymes (4), and stress hormones (5), all of which can profoundly affect prey energy budgets and foraging decisions (6).

Risk can have immediate effects on entire prey populations, whereas more time is often required for predators to hunt and consume prey (1). Indeed, risk can strongly influence population dynamics (7, 8), energy flow (9), and ecosystem function (10). An increased focus on risk effects will enhance our understanding of the ecosystem-role of predators. Moreover, attention to risk effects provides a framework that links the evolutionary ecology of individual traits with community and ecosystem processes. We urge wildlife managers to also consider risk effects when evaluating the ecosystem effects of predator declines.