POPULATION-DYNAMIC CONSEQUENCES OF COMPETITIVE SYMMETRY IN ANNUAL PLANTS

Asymmetric competition is a form of resource division among plants, in which large plants greatly suppress the growth of smaller neighbors. In annual plants, small size differences between seedlings at the onse t of competition are magnified into large differences in seed-set by a symmetric competition. We formulate a novel neighborhood model, which reflects this seedling size effect as modified by the type of competit ive symmetry. In the model, competition type is represented by a singl e, biologically meaningful parameter. We implement the model in a popu lation growth model for two species, one at low density (the invader), and one at high density (the resident). The species are the same, exc ept for their seedling biomass distributions. Under these conditions, we find that asymmetric competition always favors invasion by the spec ies with lager average seedling size, but impairs invasion by the othe r species. Based on this invasibility criterion, we conclude that asym metric competition always favors competitive exclusion in our model. H owever, by modifying some of the model assumptions, we suggest scenari os in which asymmetric competition may promote coexistence.