Asymmetric competition between plant species

1. Asymmetric competition is an unequal division of resources amongst competing plants. Thus, competition may be asymmetric in the sense that some individuals remove a disproportionately large amount of resource. Alternatively, competition may be asymmetric in that one species removes a disproportionately large amount of resource. The mechanisms determining the two forms of asymmetry may be similar, for example through initial size advantage or over-topping. 2. We explore the consequences of these two forms of asymmetry for competition models that predict mean performance as a function of the density of interacting species. We do so using neighbourhood models that explicitly consider the allocation of resources to individuals within an interacting mixture. 3. Asymmetric individual competition is modelled by assuming that individuals are formed into a competitive hierarchy such that individuals at the top of the hierarchy are able to remove more resources than those at the bottom. Mean performance declines exponentially, moving from top to bottom of the hierarchy. Asymmetric species-level competition is modelled by assuming that one species occupies all of the upper positions in the competitive hierarchy and hence dominates the resource. 4. When competition is asymmetric at the species level, yield-density responses follow an exponential decline. Otherwise, arithmetic mean performance follows a classic hyperbolic response. 5. Using this approach, we explore the asymmetry of competition between wheat and three species of weeds.