Structure and function of a non-interactive, reactive insect-plant system.

Rhagoletis alternata is a common tephritid fly in central Europe, whose larvae feed on the hypanthium of rose hips. The resource-consumer system is "non-interactive", i.e. the insect has little or no impact on host plant fitness and therefore is not able to influence the rate at which larval food resources are renewed. The system is "reactive", since fluctuations in the carrying capacity (hip density) of the host plant are important for determining year-to-year fluctuations in the insect's population size. Insect fluctuations exceed those of its carrying capacity. The insect's efficient exploitation strategy, maximizing its fitness at high as well as low resource supply, must be attributed to the variable and unpredictable relationship between resource availability and consumer density. The only regulatory mechanism is contest competition when larval densities exceed the carrying capacity. Due to the low impact of the insect, its exploitation strategy is apparently not opposed by mechanisms selecting for defence in the host plant. This lack of defence and the efficient exploitation strategy may be important factors for the frequently observed high degree of the resource utilization by the insect.