After nearly a century of recovery from overhunting, sea otter populations are in abrupt decline over large areas of western Alaska. Increased killer whale predation is the likely cause of these declines. Elevated sea urchin density and the consequent deforestation of kelp beds in the nearshore community demonstrate that the otter's keystone role has been reduced or eliminated. This chain of interactions was probably initiated by anthropogenic changes in the offshore oceanic ecosystem.

Dietary diversity often varies inversely with prey resource abundance. This pattern, although typically measured at the population level, is usually assumed to also characterize the behavior of individual animals within the population. However, the pattern might also be produced by changes in the degree of variation among individuals. Here we report on dietary and associated behavioral changes that occurred with the experimental translocation of sea otters from a food-poor to a food-rich environment. Although the diets of all individuals were broadly similar in the food-rich environment, a behaviorally based dietary polymorphism existed in the food-poor environment. Higher dietary diversity under low resource abundance was largely driven by greater variation among individuals. We further show that the dietary polymorphism in the food-poor environment included a broad suite of correlated behavioral variables and that the individuals that comprised specific behavioral clusters benefited from improved foraging efficiency on their individually preferred prey. Our findings add to the growing list of examples of extreme individuality in behavior and prey choice within populations and suggest that this phenomenon can emerge as a behavioral manifestation of increased population density. Individuality in foraging behavior adds complexity to both the fitness consequences of prey selection and food web dynamics, and it may figure prominently as a diversifying process over evolutionary timescales.foraging efficiency ͉ niche width ͉ polymorphism ͉ prey handling A large and long-standing body of theoretical and empirical research has led to the well established view that the dietary diversity of consumers increases as food becomes limiting (1-4). The implicit assumption underlying both the theory and observation is that individuals within populations are responding to changing prey availability in broadly similar ways. However, the commonly observed population-level pattern of increased dietary diversity with reduced prey abundance could also occur via individual diversification (5, 6), in which case the dietary breadth of any given individual in response to prey limitation might change very little. We subsequently refer to the former process as the within-individual diversity hypothesis (WIDH) and the latter as the among-individual diversity hypothesis (AIDH), recognizing that the two processes are not mutually exclusive. The WIDH and AIDH involve fundamentally different mechanisms that have very different implications for population, community, and evolutionary ecology (7), yet the relative importance of these two processes remains largely unevaluated for most free-living consumers, particularly large vertebrates. This lack of attention likely reflects the fact that long-term dietary records from specific individuals are difficult to obtain, and comparable samples of individual behavior and diet under conditions of high and low resource abundance are seldom available.California sea otters (Enhydra lutris nereis) provide a u...

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