Competitive asymmetry, foraging area size and coexistence of annuals

Individuals allocate resources to the expansion of their foraging area and those resources are no longer available for the traits that determine how well those individuals are able to protect their foraging area against competitors. The resulting trade-off between foraging area size and the traits associated with the ability to compete for the resources within the foraging area applies to ecological scenarios as different as territorial defence by individuals and colonies, and light competition in plants. Whether the trade-off affects species performance in competition for resources at the area of overlap between foraging areas depends on the symmetry of resource division. In symmetric competition resources are divided equally between the competitors, while in asymmetric competition the individual with the smallest foraging area, and consequently the greatest competitive ability, gains all the resources. Competition may also be a combination of the symmetric and asymmetric processes. I studied the effects of competitive asymmetry on population dynamics and coexistence of two annual species with different sized foraging areas using an individual-based spatially explicit simulation model. Symmetric competition favoured the species with the larger foraging area and did not allow coexistence. Competitive asymmetry favoured the species with smaller foraging area and allowed coexistence, which was due to the consequences of losing an asymmetric competition being more severe than losing a symmetric competition. The mechanism of coexistence is the larger foraging area's superiority in low population densities (little competition) and the smaller foraging area's ability to win a large foraging area when competition was intense. Competitive asymmetry and small size of both foraging areas led to population dynamics dominated by long-term fluctuations of small intensity. Symmetric competition and large size of the foraging areas led to large short-term fluctuations, which often resulted in the extinction of one or both of the species due to demographic stochasticity.