Intraspecific competition is often asymmetric such that some individuals have a high negative effect on others, but suffer relatively little from competition themselves. Individual body size is the main determinant of this competitive asymmetry because of its effect on foraging capacity, metabolic costs and the ability to withstand resource scarcity. The strength and direction of competitive asymmetry is known to be a main driver of population dynamics and has a large impact on community structure. However, it is not understood whether selection favors this asymmetry in intraspecific competition. We address this question using a size-structured consumer-resource model to study the evolution of the scaling of competitive ability with body size. Competitive ability is measured by an individual-level R*–value and competition is asymmetric when this R*–value changes with body size. In addition, biomass production asymmetry occurs when the mass-specific production of new biomass in terms of growth and reproduction changes with body size. This type of asymmetry can occur even when the competitive ability does not change with size. We use adaptive dynamics to identify the direction of selection and possible evolutionary endpoints.
Results/Conclusions
The main results are twofold. First, selection minimizes competitive asymmetry, such that differently sized individuals are competitively equal. This leads to stable population dynamics and precludes to occurrence of starvation. Second, the scaling of biomass production with body size responds adaptively to changes in mortality and other life-history parameters. This adaptive response increases the growth and/or reproduction of the life stage with increased mortality, but does not violate the evolved competitive symmetry. Also, species that have a low ratio of size at birth to size at maturation invest mainly in juvenile growth, while species in which this ratio is high invest mainly in post-maturation growth and reproduction. These results provide a possible explanation for the variation in intraspecific scaling exponents of metabolic rate with body size as observed in nature. We present data on metabolic scaling exponents with body size that support these predictions.