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 onset of competition are magnified into large differences in seed-set by asymmetric competition. We formulate a novel neighborhood model, which reflects this seedling size effect as modified by the type of competitive symmetry. In the model, competition type is represented by a single, biologically meaningful parameter. We implement the model in a population growth model for two species, one at low density (the invader), and one at high density (the resident). The species are the same, except for their seedling biomass distributions. Under these conditions, we find that asymmetric competition always favors invasion by the species with lager average seedling size, but impairs invasion by the other species. Based on this invasibility criterion, we conclude that asymmetric competition always favors competitive exclusion in our model. However, by modifying some of the model assumptions, we suggest scenarios in which asymmetric competition may promote coexistence