Tobias van Kooten

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.

Invasion success depends on invader body size in a size-structured mixed predation-competition community

1. The size of an individual is an important determinant of its trophic position and the type of interactions it engages in with other heterospecific and conspecific individuals. Consequently an individuals ecological role in a community changes with its body size over ontogeny, leading to that trophic interactions between individuals are a size-dependent and ontogenetically variable mixture of competition and predation. 2. Because differently sized individuals thus experience different biotic environments, invasion success may be determined by the body size of the invaders. Invasion outcome may also depend on the productivity of the system as productivity influences the biotic environment. 3. In a laboratory experiment with two poeciliid fishes the body size of the invading individuals and the daily amount of food supplied were manipulated. 4. Large invaders established persistent populations and drove the resident population to extinction in 10 out of 12 cases, while small invaders failed in 10 out of 12 trials. Stable coexistence was virtually absent. Invasion outcome was independent of productivity. 5. Further analyses suggest that small invaders experienced a competitive recruitment bottleneck imposed on them by the resident population. In contrast, large invaders preyed on the juveniles of the resident population. This predation allowed the large invaders to establish successfully by decreasing the resident population densities and thus breaking the bottleneck. 6. The results strongly suggest that the size distribution of invaders affects their ability to invade, an implication so far neglected in life-history omnivory systems. The findings are further in agreement with predictions of life-history omnivory theory, that size-structured interactions demote coexistence along a productivity gradient.

When does fishing lead to more fish? Community consequences of bottom trawl fisheries in demersal food webs

Bottom trawls are a globally used fishing gear that physically disturb the seabed and kill non-target organisms, including those that are food for the targeted fish species. There are indications that ensuing changes to the benthic invertebrate community may increase the availability of food and promote growth and even fisheries yield of target fish species. If and how this occurs is the subject of ongoing debate, with evidence both in favour and against. We model the effects of trawling on a simple ecosystem of benthivorous fish and two food populations (benthos), susceptible and resistant to trawling. We show that the ecosystem response to trawling depends on whether the abundance of benthos is top-down or bottom-up controlled. Fishing may result in higher fish abundance, higher (maximum sustainable) yield and increased persistence of fish when the benthos which is the best-quality fish food is also more resistant to trawling. These positive effects occur in bottom-up controlled systems and systems with limited impact of fish feeding on benthos, resembling bottom-up control. Fishing leads to lower yields and fish persistence in all configurations where susceptible benthos are more profitable prey. Our results highlight the importance of mechanistic ecosystem knowledge as a requirement for successful management.

Size‐Dependent Mortality Induces Life‐History Changes Mediated through Population Dynamical Feedbacks

The majority of taxa grow significantly during life history, which often leads to individuals of the same species having different ecological roles, depending on their size or life stage. One aspect of life history that changes during ontogeny is mortality. When individual growth and development are resource dependent, changes in mortality can affect the outcome of size‐dependent intraspecific resource competition, in turn affecting both life history and population dynamics. We study the outcome of varying size‐dependent mortality on two life‐history types, one that feeds on the same resource throughout life history and another that can alternatively cannibalize smaller conspecifics. Compensatory responses in the life history dampen the effect of certain types of size‐dependent mortality, while other types of mortality lead to dramatic changes in life history and population dynamics, including population (de‐)stabilization, and the growth of cannibalistic giants. These responses differ strongly among the two life‐history types. Our analysis provides a mechanistic understanding of the population‐level effects that come about through the interaction between individual growth and size‐dependent mortality, mediated by resource dependence in individual vital rates.

Complete compensation in Daphnia fecundity and stage-specific biomass in response to size-independent mortality

1. Recent theory suggests that compensation or even overcompensation in stage-specific biomass can arise in response to increased mortality. Which stage that will show compensation depends on whether maturation or reproduction is the more limiting process in the population. Size-structured theory also provides a strong link between the type of regulation and the expected population dynamics as both depend on size/stage-specific competitive ability. 2. We imposed a size-independent mortality on a consumer-resource system with Daphnia pulex feeding on Scenedesmus obtusiusculus to asses the compensatory responses in Daphnia populations. We also extended an existing stage-structured biomass model by including several juvenile stages to test whether this extension affected the qualitative results of the existing model. 3. We found complete compensation in juvenile biomass and total population fecundity in response to harvesting. The compensation in fecundity was caused by both a higher proportion of fecund females and a larger clutch size under increased mortality. We did not detect any difference in resource levels between treatments. 4. The model results showed that both stages of juveniles have to be superior to adults in terms of resource competition for the compensatory response to take place in juvenile biomass. 5. The results are all in correspondence with that the regulating process within the population was reproduction. From this, we also conclude that juveniles were superior competitors to adults, which has implications for population dynamics and the kind of cohort cycles seen in Daphnia populations. 6. The compensatory responses demonstrated in this experiment have major implications for community dynamics and are potentially present in any organisms with food-dependent growth or development.

Coexistence of two stage-structured intraguild predators

An organism can be defined as omnivorous if it feeds on more than one trophic level. Omnivory is present in many ecosystems and multiple omnivorous species can coexist in the same ecosystem. How coexisting omnivores are able to avoid competitive exclusion is very much an open question. In this paper we analyze a model of a community consisting of two omnivorous predators and a basal resource. The population of both predators is explicitly structured into juveniles and adults, of which juveniles only feed on basal resource and adults feed on a varied proportion of basal resource and juveniles of the other population. We thereby separate the omnivorous roles (competitor for basal resource and predator of competitors) over life history. We show in this study that persistence of multiple omnivorous predators is possible when predators differ in adult diets. In this case, coexistence occurs because community dynamics force one of the model species to act as a predator and the other to act as a consumer. We conclude that separation of omnivorous roles over life history not only offers an explanation on why systems with omnivory can persist, but also how multiple omnivores can coexist at the same trophic levels of those systems.

LOCAL FORAGING AND LIMITED MOBILITY: DYNAMICS OF A SIZE-STRUCTURED CONSUMER POPULATION

Size-structured population models often exhibit single generation cycles, which are driven by scramble competition within a generation and size-based competitive asymmetry among generations. These cycles are characterized by the dominance of a single cohort and thus by a high degree of synchronization of the individual life histories. The models, however, do not generally allow for divergence in size among individuals born at the same time. Size divergence may, for example, result from the stochasticity that arises due to local interactions between individuals and their environment and has been shown to affect the population dynamics within generations. We studied the effect of the size divergence that develops as a result of stochasticity over many generations, considering the full population dynamical feedback, including resource dynamics. The stochastic var- iation in our model was generated by local interactions of individuals with the environment. We varied the mobility of individuals, which regulated the strength of the local resource feedback on the consumers. We found that at very high mobility our model provided a good correspondence to similar but fully deterministic models, showing the single gener- ation cycles typical for a size-structured consumer-resource interaction. Intermediate levels of mobility had no notable effect on the dynamics of our model population. At very low mobility, the dynamics appeared to be strongly influenced by stochasticity. We showed that by superposition of the underlying deterministic dynamics and the stochasticity induced by local interactions we could fully understand the dynamics of the model. This finding led us to conclude that, while individual variability may have an impact on population structure and dynamics, it does not necessarily change the deterministic interactions that determine global population dynamics. More specifically, our study highlights the robust- ness of single generation cycles, showing that even at high levels of individual variability the population dynamics will intermittently exhibit patterns resembling these cycles.

Size-based species interactions shape herring and cod population dynamics in the face of exploitation

Size-specific competition and predation interactions often link the population dynamics of fish species in their response to exploitation. The effects of harvesting on interacting fish species is of increasing relevance as more and more fish populations worldwide are reduced by fishing. When stocks are harvested, effects of harvesting may percolate to populations of other species with which it interacts through competition, predation, etcetera. When multiple species are exploited, this can lead to interactions between fisheries, mediated by ecological interactions. Nevertheless, most fish stocks are managed using a single-species framework. We studied how single-species explanations of historical population dynamics work out when size-based interactions between harvested species are taken into account. We have taken as a case study the dynamics of cod (Gadus morhua) and herring (Clupea harengus) in the North Sea. These dynamics are generally considered to be shaped by fishing pressure on and food availability to single species. Our results indicate that the explanatory power of these factors is maintained with the inclusion of species interactions, but the processes leading to the observed patterns are altered as the fates of the species are interdependent. The sign and magnitude of the interaction between the species depends on the state of the populations, their exploitation history and environmental factors such as resource productivity. This context-dependent response to changing fishery intensity has important ramifications for management. We show that management plans for the exploitation of either one of these species, or for the recovery of North Sea cod, which do not account for these subtle interactions, may fail or backfire. Hence, such interactions link the fate of these species in complex ways, which must be taken into consideration for successful management of their exploitation, including harvesting at maximum sustainable yield, as we move towards an ecosystem-based management of marine fisheries.

Using marine reserves to manage impact of bottom trawl fisheries requires consideration of benthic food-web interactions

Marine protected areas (MPAs) are widely used to protect exploited fish species as well as to conserve marine habitats and their biodiversity. They have also become a popular management tool for bottom trawl fisheries, a common fishing technique on continental shelves worldwide. The effects of bottom trawling go far beyond the impact on target species, as trawls also affect other components of the benthic ecosystem and the seabed itself. This means that for bottom trawl fisheries, MPAs can potentially be used not only to conserve target species but also to reduce impact of these side effects of the fishery. However, predicting the protective effects of MPAs is complicated because the side effects of trawling potentially alter the food-web interactions between target and non-target species. These changes in predatory and competitive interactions among fish and benthic invertebrates may have important ramifications for MPAs as tools to manage or mitigate the effects of bottom trawling. Yet, in current theory regarding the functioning of MPAs in relation to bottom trawl fisheries, such predatory and competitive interactions between species are generally not taken into account. In this study, we discuss how food-web interactions that are potentially affected by bottom trawling may alter the effectiveness of MPAs to protect (1) biodiversity and marine habitats, (2) fish populations, (3) fisheries yield, and (4) trophic structure of the community. We make the case that in order to be applicable for bottom trawl fisheries, guidelines for the implementation of MPAs must consider their potential food-web effects, at the risk of failing management.

Interspecific resource competition effects on fisheries revenue.

In many fisheries multiple species are simultaneously caught while stock assessments and fishing quota are defined at species level. Yet species caught together often share habitat and resources, resulting in interspecific resource competition. The consequences of resource competition on population dynamics and revenue of simultaneously harvested species has received little attention due to the historical single stock approach in fisheries management. Here we present the results of a modelling study on the interaction between resource competition of sole (Solea solea) and slaice (Pleuronectus platessa) and simultaneous harvesting of these species, using a stage-structured population model. Three resources were included of which one is shared with a varied competition intensity. We find that plaice is the better competitor of the two species and adult plaice are more abundant than adult sole. When competition is high sole population biomass increases with increasing fishing effort prior to plaice extinction. As a result of this increase in the sole population, the revenue of the stocks combined as function of effort becomes bimodal with increasing resource competition. When considering a single stock quota for sole, its recovery with increasing effort may result in even more fishing effort that would drive the plaice population to extinction. When sole and plaice compete for resources the highest revenue is obtained at effort levels at which plaice is extinct. Ignoring resource competition promotes overfishing due to increasing stock of one species prior to extinction of the other species. Consequently, efforts to mitigate the decline in one species will not be effective if increased stock in the other species leads to increased quota. If a species is to be protected against extinction, management should not only be directed at this one species, but all species that compete with it for resource as well.