Size-asymmetric competition

Size-asymmetric competition refers to situations in which larger individuals exploit disproportionately greater amounts of resources when competing with smaller individuals. This type of competition is common among plants but also exists among animals. Size-asymmetric competition usually results from large individuals monopolizing the resource by "pre-emption". i.e. exploiting the resource before smaller individuals are able to obtain it. Size-asymmetric competition has major effects on population structure and diversity within ecological communities. Resource competition can vary from complete symmetric (all individuals receive the same amount of resources, irrespective of their size, known also as scramble competition) to perfectly size symmetric (all individuals exploit the same amount of resource per unit biomass) to absolutely size asymmetric (the largest individuals exploit all the available resource). The degree of size asymmetry can be described by the parameter θ in the following equation focusing on the partition of the resource r among n individuals of sizes Bj. ri refers to the amount of resource consumed by individual i in the neighbourhood of j. When θ =1, competition is perfectly size symmetric, e.g. if a large individual is twice the size of its smaller competitor, the large individual will acquire twice the amount of that resource (i.e. both individuals will exploit the same amount of resource per biomass unit). When θ >1 competition is size-asymmetric, e.g. if large individual is twice the size of its smaller competitor and θ =2, the large individual will acquire four times the amount of that resource (i.e. the large individual will exploit twice the amount of resource per biomass unit). As θ increases, competition becomes more size-asymmetric and larger plants get larger amounts of resource per unit biomass compared with smaller plants. Competition among plants for light is size-asymmetric because of the directionality of its supply. Higher leaves shade lower leaves but not vice versa.