Strong and surprising effects of ontogenetic niche shifts on ecosystem functioning

Background/Question/Methods

Many animal species, in particular those that grow over substantial ranges of body size between birth and maturation, occupy different ecological niches in different stages of their life as during ontogeny they change, for example, their use of resources and/or habitats. Such ontogenetic niche shifts are especially important in size-structured populations and communities and contribute significantly to within-species size variation. Although the influence of between-species diversity on ecosystem functioning has been studied extensively, the influence of the within-species diversity arising from variation in body size has been studied to a much lesser extent. We investigated, using population dynamic models as well as population-level experiments, the effects of ontogenetic niche shifts on total population biomass, representing an important ecosystem function, in a consumer-resource system, in which juvenile and adult consumers feed on their own exclusive resource.

Results/Conclusions

Analysis of a stage-structured biomass model for the dynamics of a consumer population, in which juveniles and adults forage on separate resources, predicts that varying the ratio between the productivities of the juvenile and adult resource can substantially change total consumer biomass at equilibrium, even though total resource productivity is constant. Population experiments over multiple generations with the Least Killifish (Heterandria formosa) confirm this prediction, revealing that reallocating a constant system productivity from an equal resource delivery to juveniles and adults to an adult-biased delivery more than doubles consumer biomass. This increase is as high as the biomass increases observed with increasing species diversity. The data also show that total population biomass is higher when juvenile and adult consumers forage on separate resources compared to when they compete for a single, shared resource. The stage-structured biomass model furthermore predicts an increase in total consumer biomass if mortality of all consumers is increased. This counterintuitive, positive effect of mortality is shown to occur in different size-structured consumer-resource models, as long as consumers go through an ontogenetic niche shift during their life history. We therefore argue that ontogenetic niche shifts and within-species diversity are important factors determining ecosystem functioning.