No abstract

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.

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.

Ecological theory predicts that stable populations should yield to large-amplitude cycles in richer environments1±3. This does not occur in nature. The zooplankton Daphnia and its algal prey in lakes throughout the world illustrate the problem4±6. Experiments show that this system its the theory's assumptions7±9, yet it is not destabilized by enrichment 6. We have tested and rejected four of ive proposed explanations 10. Here, we investigate the fifth mechanism: inedible algae in nutrient-rich lakes suppress cycles by reducing nutrients available to edible algae. We found three novel results in nutrient-rich microcosms from which inedible algae were excluded. First, as predicted by theory, some Daphniaedible algal systems now display large-amplitude predator-prey cycles. Second, in the same environment, other populations are stable, showing only small-amplitude demographic cycles. Stability is induced when Daphnia diverts energy from the immediate production of young. Third, the system exhibits coexisting attractors -a stable equilibrium and large-amplitude cycle. We describe a mechanism that flips the system between these two states

Abstract. Recent size-structured cannibalistic models point to the importance of the energy gain by cannibals and also show that this gain may result in the emergence of giant individuals. We use a combination of a 10-year field study of a perch (Perca fluviatilis) population and quantitative within-season modeling of individual and population-level dynamics to investigate which mechanisms are most likely to drive the dynamics of the studied perch population. We focused on three main aspects to explain observed discrepancies between earlier model predictions and data: (1) introduction of more than one shared resource between cannibals and victims, (2) whether or not several victim age cohorts are necessary to allow giant growth, and (3) the intensity of inter-cohort competition between young-of-the-year (YOY) perch and 1-yr-old perch.At the start of the study period, the perch population was dominated by ''stunted'' perch individuals, and recruitment of perch to an age of 1-yr-old was negligible. Following a major death in adult perch, strong recruitments of perch to 1-yr-old were thereafter observed for a number of years. As 1-yr-olds these successful recruiters subsequently starved to death due to competition with the new YOY. The few surviving adult perch accelerated substantially in growth and became ''giants.'' At the end of the study period, the perch population moved back to the situation with stunted individuals. There was a high agreement between observed diets of cannibalistic perch and those predicted by the model for both the stunted and the giant phases. Analyses of growth rates showed that cannibalistic perch could become giants on a diet of YOY perch only, but that a supplement with the second shared resource (macroinvertebrates) was needed to reach the observed sizes. Modeling of growth and diet in the giant phase showed an exploitative competitive effect of YOY perch on 1-yr-old perch, but a restriction in habitat use of 1-yr-old perch had to be assumed to yield the observed growth rate and diet. The resource dynamics of zooplankton and macroinvertebrates were both accurately predicted by the model. Also, YOY perch mortality was accurately predicted and, furthermore, suggested that one of the trawling methods used may underestimate the number of YOY perch when they increase in size.We conclude that the presence of a second shared resource and the restricted habitat use and absence of cannibalistic consumption by 1-yr-old perch individuals are two important mechanisms to explain the discrepancy between model predictions and data. Our results also point to the fact that that the dynamics observed may be explained by complex dynamics not involving the presence of a giant and dwarf cycle.