A trophic cascade occurs when a predator eats enough herbivores (plant-eating animals) to improve plant growth or to alter the structure of the plant community. Cascades have been documented in oceans and in places where wolves have been reintroduced. Although it is widely believed that forest birds benefit trees by eating insect pests of these trees, there are several reasons why birds might not drive a trophic cascade. For instance, plants might compensate for eaten leaves by rapidly producing new leaves, or birds might eat the same amount of insects as would have died due to other causes (weather, insect predators, starvation). In this research, investigators will test the hypothesis that predation of insects by birds benefits plants under some circumstances and will identify the conditions that affect the strength of the trophic cascade. The investigators will study the effects of insectivorous birds on cottonwood trees in the Southwestern United States. Cages will be built around whole trees to exclude birds from trees with high and low genetic susceptibility to insect pests. This will be the first study of its kind to measure growth responses of adult trees, and the first to measure seed production. By excluding birds from trees during different seasons, the study will determine how trees would respond to declines in resident birds (present year-round) or migratory birds (spring and summer only).
By comparing the effect of birds to that of insecticides, the study will document the relative effectiveness of birds in insect control. The project will provide training for one PhD student and at least two undergraduates.
