Gigantic cannibals driving a whole-lake trophic cascade

Abstract

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 ...

Population structure determines functional differences among species and ecosystem processes

“…Importantly, this implies that changes in the population structure of species can alter the functional composition of communities and lead to concordant changes in ecosystem functioning long before any species are extirpated. This prediction is consistent with observational data on whole ecosystems, where longterm changes in the size/stage structure of predatory fish were correlated with relative changes in the biomasses of lower trophic levels, even when the total biomass of predatory fish remained unchanged 3,32 . It also suggests that natural ecosystems are much more sensitive to human disturbances (for example, size-selective harvesting) than previously thought and emphasizes the importance of the demographic structure of populations for conserving ecosystem processes.…”

Causes of decoupling between larval supply and settlement and consequences for understanding recruitment and population connectivity

“…Models of population dynamics that include cannibalism, especially in size and 1 age-structured populations, predict a series of effects, from destabilization of populations, to 2 regulation, chaos, bistability (through the creation of local states that are driven by the final size 3 of cannibals), and size-dependent effects (Costantino et al, 1997;Claessen and De Roos, 2003;4 Claessen et al, 2004;Shurin et al, 2004). These effects of cannibalism on populations depend on 5 which factors are included in the models: (1) mortality of prey, (2) feeding rates of predators 6 (Fox, 1975), (3) energy gain at the individual and at the population level (Persson et al, 2003), 7 (4) size-dependent interactions, and (5) intraspecific competition (Polis, 1981;Claessen and De 8 Roos, 2003). To date, the majority of models that include cannibalism in aquatic systems address 9 fish community structure (Persson et al, 2003;Olson et al, 2005).…”

“…These effects of cannibalism on populations depend on 5 which factors are included in the models: (1) mortality of prey, (2) feeding rates of predators 6 (Fox, 1975), (3) energy gain at the individual and at the population level (Persson et al, 2003), 7 (4) size-dependent interactions, and (5) intraspecific competition (Polis, 1981;Claessen and De 8 Roos, 2003). To date, the majority of models that include cannibalism in aquatic systems address 9 fish community structure (Persson et al, 2003;Olson et al, 2005). 10…”

Physiologically Structured Population Models: Towards a General Mathematical Theory

“…The p-phenomenon is that a population may persist at low renewal rates for adult food, simply since juvenile food becomes indirectly available to adults via cannibalism (the most extreme example is found in some lakes in which a predatory fish, such as pike or perch, occurs but no other fish whatsoever, cf. Persson et al 2000Persson et al , 2003. So reproduction becomes similar to farming, gaining a harvest from prior sowing (Getto, Diekmann and de Roos, submitted).…”