David Claessen

Gigantic cannibals driving a whole-lake trophic cascade

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.

Food‐Dependent Growth Leads to Overcompensation in Stage‐Specific Biomass When Mortality Increases: The Influence of Maturation versus Reproduction Regulation

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.

Temperature-Driven Regime Shifts in the Dynamics of Size-Structured Populations

Global warming impacts virtually all biota and ecosystems. Many of these impacts are mediated through direct effects of temperature on individual vital rates. Yet how this translates from the individual to the population level is still poorly understood, hampering the assessment of global warming impacts on population structure and dynamics. Here, we study the effects of temperature on intraspecific competition and cannibalism and the population dynamical consequences in a size-structured fish population. We use a physiologically structured consumer-resource model in which we explicitly model the temperature dependencies of the consumer vital rates and the resource population growth rate. Our model predicts that increased temperature decreases resource density despite higher resource growth rates, reflecting stronger intraspecific competition among consumers. At a critical temperature, the consumer population dynamics destabilize and shift from a stable equilibrium to competition-driven generation cycles that are dominated by recruits. As a consequence, maximum age decreases and the proportion of younger and smaller-sized fish increases. These model predictions support the hypothesis of decreasing mean body sizes due to increased temperatures. We conclude that in size-structured fish populations, global warming may increase competition, favor smaller size classes, and induce regime shifts that destabilize population and community dynamics.

Evolution of virulence in ahost-pathogen system with local pathogen transmission

If hosts can be effectively infected by more than a single pathogen and pathogen transmission is spatially homogeneous, natural selection favors higher virulence levels, since pathogens run the risk to share their host with other co-exploiters or lose it entirely to a stronger competitor. In a host-pathogen system where pathogen transmission is localized between neighboring hosts, infected hosts either occur in relatively small patches, isolated from each other by susceptible hosts, or the entire spatial domain is more or less evenly covered with infected hosts with only few susceptible hosts in between. The patchy distribution arises when multiple pathogen infections have a large negative effect on pathogen productivity and leads to less virulent pathogens being favored. The contiguous distribution of infected hosts occurs when multiple infections are less deleterious and leads to more virulent pathogens being favored. In contrast, when multiple host infections are not possible, selection favors the pathogen with a maximal basic reproduction ratio irrespective of the mode of pathogen transmission. The spatially heterogeneous distributions that emerge because of local pathogen transmission have additional consequences for pathogen invasion and population dynamics.

Stage‐specific biomass overcompensation by juveniles in response to increased adult mortality in a wild fish population

Recently developed theoretical models of stage-structured consumer-resource systems have shown that stage-specific biomass overcompensation can arise in response to increased mortality rates. We parameterized a stage-structured population model to simulate the effects of increased adult mortality caused by a pathogen outbreak in the perch (Perca fluviatilis) population of Windermere (UK) in 1976. The model predicts biomass overcompensation by juveniles in response to increased adult mortality due to a shift in food-dependent growth and reproduction rates. Considering cannibalism between life stages in the model reinforces this compensatory response due to the release from predation on juveniles at high mortality rates. These model predictions are matched by our analysis of a 60-year time series of scientific monitoring of Windermere perch, which shows that the pathogen outbreak induced a strong decrease in adult biomass and a corresponding increase in juvenile biomass. Age-specific adult fecundity and size at age were higher after than before the disease outbreak, suggesting that the pathogen-induced mortality released adult perch from competition, thereby increasing somatic and reproductive growth. Higher juvenile survival after the pathogen outbreak due to a release from cannibalism likely contributed to the observed biomass overcompensation. Our findings have general implications for predicting population- and community-level responses to increased size-selective mortality caused by exploitation or disease outbreaks.

Stabilization of population fluctuations due to cannibalism promotes resource polymorphism in fish

Resource polymorphism is a well‐known phenomenon in many taxa, assumed to be a consequence of strong competition for resources and to be facilitated by stable environments and the presence of several profitable resources on which to specialize. In fish, resource polymorphism, in the form of planktivore‐benthivore pairs, is found in a number of species. We gathered literature data on life‐history characteristics and population dynamics for 15 fish species and investigated factors related to the presence of such resource polymorphism. This investigation indicated that early cannibalism and low overall population variability are typically associated with the presence of resource polymorphism. These findings match previously reported patterns of population dynamics for size‐structured fish populations, whereby early cannibalism has been shown to decrease temporal variation in population dynamics and to equalize the profitability of the zooplankton and macroinvertebrate resources. Our study suggests that competition alone is not a sufficient condition for the development of resource polymorphism because overly strong competition is typically associated with increased temporal variation (environmental instability). We conclude that although resource competition is an important factor regulating the development of resource polymorphism, cannibalism may also play a fundamental role by dampening population oscillations and possibly by equalizing the profitability of different resources.

ADAPTIVE RADIATION DRIVEN BY THE INTERPLAY OF ECO‐EVOLUTIONARY AND LANDSCAPE DYNAMICS

We investigate an individual‐based model of adaptive radiation based on the biogeographical changes of the Great African Lakes where cichlid fishes radiated. In our model, the landscape consists of a mosaic of three habitat types which may or may not be separated by geographic barriers. We study the effect of the alternation between allopatry and sympatry called landscape dynamics. We show that landscape dynamics can generate a significantly higher diversity than allopatric or sympatric speciation alone. Diversification is mainly due to the joint action of allopatric, ecological divergence, and of disruptive selection increasing assortative mating and allowing for the coexistence in sympatry of species following reinforcement or character displacement. Landscape dynamics possibly increase diversity at each landscape change. The characteristics of the radiation depend on the speed of landscape dynamics and of the number of geographically isolated regions at steady state. Under fast dynamics of a landscape with many fragments, the model predicts a high diversity, possibly subject to the temporary collapse of all species into a hybrid swarm. When fast landscape dynamics induce the recurrent fusion of several sites, diversity is moderate but very stable over time. Under slow landscape dynamics, diversification proceeds similarly, although at a slower pace.

Body downsizing caused by non-consumptive social stress severely depresses population growth rate

Chronic social stress diverts energy away from growth, reproduction and immunity, and is thus a potential driver of population dynamics. However, the effects of social stress on demographic density dependence remain largely overlooked in ecological theory. Here we combine behavioural experiments, physiology and population modelling to show in a top predator (pike Esox lucius) that social stress alone may be a primary driver of demographic density dependence. Doubling pike density in experimental ponds under controlled prey availability did not significantly change prey intake by pike (i.e. did not significantly change interference or exploitative competition), but induced a neuroendocrine stress response reflecting a size-dependent dominance hierarchy, depressed pike energetic status and lowered pike body growth rate by 23 per cent. Assuming fixed size-dependent survival and fecundity functions parameterized for the Windermere (UK) pike population, stress-induced smaller body size shifts age-specific survival rates and lowers age-specific fecundity, which in Leslie matrices projects into reduced population rate of increase (λ) by 37–56%. Our models also predict that social stress flattens elasticity profiles of λ to age-specific survival and fecundity, thus making population persistence more dependent on old individuals. Our results suggest that accounting for non-consumptive social stress from competitors and predators is necessary to accurately understand, predict and manage food-web dynamics.

WITHIN- AND BETWEEN-POPULATION VARIATION FOR WOLBACHIA-INDUCED REPRODUCTIVE INCOMPATIBILITY IN A HAPLODIPLOID MITE

Abstract Wolbachiapipientis is a bacterium that induces cytoplasmic incompatibility (CI), the phenomenon in which infected males are reproductively incompatible with uninfected females. CI spreads in a population of hosts because it reduces the fitness of uninfected females relative to infected females. CI encompasses two steps: modification (mod) of sperm of infected males and rescuing (resc) of these chromosomes by Wolbachia in the egg. Infections associated with CI have mod+resc+ phenotypes. However, mod∼resc+ phenotypes also exist; these do not result in CI. Assuming mod/resc phenotypes are properties of the symbiont, theory predicts that mod∼resc+ infections can only spread in a host population where a mod+resc+ infection already occurs. A mod∼resc+ infection spreads if the cost it imposes on the infected females is lower than the cost inflicted by the resident (mod+resc+) infection. Furthermore, introduction of a mod‐Wolbachia eventually drives infection to extinction. The uninfected population that results can be recolonized by a CI‐causing Wolbachia. Here, we investigated whether variability for induction of CI was present in two Tetranychus urticae populations. In one population all isofemale lines tested were mod‐. In the other, mod+resc+ and mod∼resc+ isofemale lines coexisted. We found no evidence for a cost difference to females expressing either type (mod+/∼). Infections in the two populations could not be distinguished based on sequences of two Wolbachia genes. We consider the possibility that mod‐ is a host effect through a population dynamics model. A mod‐ host allele leads to infection extinction in the absence of fecundity differences. Furthermore, the uninfected population that results is immune to reestablishment of the (same) CI‐causing Wolbachia.

Ecological speciation in dynamic landscapes

Although verbal theories of speciation consider landscape changes, ecological speciation is usually modelled in a fixed geographical arrangement. Yet landscape changes occur, at different spatio‐temporal scales, due to geological, climatic or ecological processes, and these changes result in repeated divisions and reconnections of populations. We examine the effect of such landscape dynamics on speciation. We use a stochastic, sexual population model with polygenic inheritance, embedded in a landscape dynamics model (allopatry–sympatry oscillations). We show that, under stabilizing selection, allopatry easily generates diversity, but species coexistence is evolutionarily unsustainable. Allopatry produces refuges whose persistence depends on the characteristic time scales of the landscape dynamics. Under disruptive selection, assuming that sympatric speciation is impossible due to Mendelian inheritance, allopatry is necessary for ecological differentiation. The completion of reproductive isolation, by reinforcement, then requires several sympatric phases. These results demonstrate that the succession of past, current and future geographical arrangements considerably influence the speciation process.