Effects of Cannibalism on the Dynamics of Populations and the Structure of Communities

Population dynamic models have predicted numerous consequences of cannibalism. In populations regulated by density-dependent (D-D) factors, a fundamental outcome of cannibalism is the influence on population size. In the absence of other D-D interactions, cannibalism may destabilize populations by inducing cycles; this destabilizing effect may be exacerbated by an interaction between cannibalism and environmental perturbations. In contrast, in systems where population cycles are induced by D-D effects other than cannibalism, population models indicate that cannibalism may stabilize these cycles. In such cases, this stabilizing effect is attributed to the weakening effect that cannibalism may have on the factors responsible for the population cycles.

Other models predict that alternative stable states (bistability) may result from the interplay between the positive and negative effects of cannibalism. The negative effect of cannibalism is mortality. Positive effects encompass the direct benefit of energy gained from consuming conspecifics, as well as the indirect positive effect of release from competition (through the elimination of competitors) for shared resources. An example of bistabil-ity resulting from the direct, energetic gain derived from cannibalism has been termed the 'lifeboat mechanism'. This effect may occur when the costs of cannibalism (additional mortality) are smaller than the benefits (extraction of energy that enables additional reproduction). In this scenario, cannibalistic individuals in a population may persist under conditions of food limitation whereas a noncannibalistic but otherwise identical population would go extinct.

Both size-structured models and empirical studies demonstrate the potentially strong interplay between cannibalism and the size distribution of individuals within populations. For instance, a 10-year study of a whole lake food web revealed that 'gigantic' cannibals drive the population dynamics of perch (Perca fluviatilis). Over time, the size structure of the fish population alternated between a dominance of stunted, cannibalistic individuals and a dominance of gigantic cannibals. When the population was dominated by many, but stunted cannibalistic adults, the result was low young of the year (YOY) survival. Cannibalism within this age class resulted in low energetic gain per individual. In contrast, when the population was dominated by a few, but gigantic cannibalistic adults, YOY survival was high due to high energetic gain per individual. The differential energy gain from cannibalism resulted in larger numbers of offspring being produced from the population when it was dominated by fewer gigantic cannibals, compared to low offspring production when dominated by stunted cannibals. These data demonstrate the importance of individual life histories to the dynamics of a population and, moreover, to the community as a whole.

Strong implications for the influence of cannibalism on overall community structure may be derived from empirical studies. For instance, in the 10-year study of perch described above, the effects of the shift between a population dominated by stunted, versus gigantic, individuals reverberated throughout the community to impact lower trophic levels (zooplankton and phytoplankton communities), a phenomenon known as a 'trophic cascade'. In years dominated by a few gigantic cannibals, zooplankton biomasses were lower and phytoplankton biomasses were higher than in years in which the population was dominated by many, stunted cannibals. Thus, the dramatic shifts in the structure of the entire biological community that resulted from the dynamics of the cannibal population affected the entire lake system down to the lowest level of phytoplankton.

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