Cannibals and their victims often share common resources and thus potentially compete. Smaller individuals are often competitively superior to larger ones because of size-dependent scaling of foraging and metabolic rates, while larger ones may use cannibalism to counter this competition. We study the interplay between cannibalism and competition using a size-structured population model in which all individuals consume a shared resource but in which larger ones may cannibalize smaller conspecifics. In this model, intercohort competition causes single-cohort cycles when cannibalism is absent. Moderate levels of cannibalism reduce intercohort competition, enabling coexistence of many cohorts. More voracious cannibalism, in combination with competition, produces large-amplitude cycles and a bimodal population size distribution with many small and few giant individuals. These coexisting ''dwarfs'' and ''giants'' have very different life histories, resulting from a reversal in importance of cannibalism and competition. The population structure at time of birth determines whether individuals suffer severe cannibalism, with the few survivors reaching giant sizes, or whether they suffer intense intracohort competition, with all individuals remaining small. These model results agree remarkably well with empirical data on perch population dynamics. We argue that the induction of cannibalistic giants in piscivorous fish is a populationdynamic emergent phenomenon that requires a combination of sizedependent cannibalism and competition.

We analyze a stage-structured biomass model for size-structured consumer-resource interactions. Maturation of juvenile consumers is modeled with a food-dependent function that consistently translates individual-level assumptions about growth in body size to the population level. Furthermore, the model accounts for stage-specific differences in resource use and mortality between juvenile and adult consumers. Without such differences, the model reduces to the Yodzis and Innes (1992) bioenergetics model, for which we show that model equilibria are characterized by a symmetry property that reproduction and maturation are equally limited by food density. As a consequence, biomass production rate exactly equals loss rate through maintenance and mortality in each consumer stage. Stage-specific differences break up this symmetry and turn specific stages into net producers and others into net losers of biomass. As a consequence, the population in equilibrium can be regulated in two distinct ways: either through total population reproduction or through total population maturation as limiting process. In the case of reproduction regulation, increases in mortality may lead to an increase of juvenile biomass. In the case of maturation regulation, increases in mortality may increase adult biomass. This overcompensation in biomass occurs with increases in both stage-independent and stage-specific mortality, even when the latter targets the stage exhibiting overcompensation.

Cannibalism is characterized by four aspects: killing victims, gaining energy from victims, size-dependent interactions and intraspecific competition. In this review of mathematical models of cannibalistic populations, we relate the predicted population dynamic consequences of cannibalism to its four defining aspects. We distinguish five classes of effects of cannibalism: (i) regulation of population size; (ii) destabilization resulting in population cycles or chaos; (iii) stabilization by damping population cycles caused by other interactions; (iv) bistability such that, depending on the initial conditions, the population converges to one of two possible stable states; and (v) modification of the population size structure. The same effects of cannibalism may be caused by different combinations of aspects of cannibalism. By contrast, the same combination of aspects may lead to different effects. For particular cannibalistic species, the consequences of cannibalism will depend on the presence and details of the four defining aspects. Empirical evidence for the emerged theory of cannibalism is discussed briefly. The implications of the described dynamic effects of cannibalism are discussed in the context of community structure, making a comparison with the community effects of intraguild predation.

Trophic cascades have been a central paradigm in explaining the structure of ecological communities but have been demonstrated mainly through comparative studies or experimental manipulations. In contrast, evidence for shifts in trophic cascades caused by intrinsically driven population dynamics is meager. By using empirical data of a cannibalistic fish population covering a 10-year period and a size-structured population model, we show the occurrence of a dynamic trophic cascade in a lake ecosystem, in which the community over time alternates between two different configurations. The intrinsically driven change in the size structure of the fish population from a dominance of stunted individuals to a dominance of gigantic cannibals among adult individuals is the driving force behind distinct abundance switches observed in zooplankton and phytoplankton. The presence of the phase with gigantic cannibals depends critically on the energy they extract from their victims, allowing strong reproduction for a number of years. C ommunity-wide trophic cascades, the propagation of indirect mutualism between nonadjacent trophic levels in food webs, have been suggested to occur more frequently in aquatic than in terrestrial systems (1-4). This suggestion is based on the arguments that terrestrial systems have a higher heterogeneity, a higher overall species diversity, and more chemical defenses among primary producers (higher plants vs. algae; ref. 1). Although the validity of all of these arguments has been questioned (2, 4), undoubtedly the empirical evidence for community-wide trophic cascades is, at present, substantially stronger for aquatic than for terrestrial systems. The empirical evidence largely stems from two sources: comparative studies of different systems in which the trophic structure, such as food chain length, differs (5-6) and experimental manipulations of top predators, either intentional or unintentional (species invasions; refs. 2, 3, and 7-9). In contrast, there is hardly any evidence for dynamic trophic cascades, in which major shifts in overall food-web structure are intrinsically driven by population dynamics. Only a few studies on recruitment variation have considered this aspect (10-12).Cannibalism has been shown to have a number of diverse effects on population dynamics and persistence (13-17). These effects, among others, include a potential for alternative stable states (18,19) and chaotic dynamics (20). Although many cannibalistic models ignore the energy that cannibals gain from cannibalism and, thus, are essentially ''infanticide'' models (13,15,20), some theoretical studies have shown that the energy extracted by the cannibal may have substantial impact on population persistence and individual life history (14, 16). Empirical evidence also suggests such an effect of energy extraction on population dynamics because of increased growth and thereby increased per-capita fecundity of cannibals (17).Here we present strong evidence for a whole-lake trophic cascade that is dynamic and intrinsically ...