Bottom-up and top-down influences in the trophic system of a willow, a galling sawfly, parasitoids and inquilines

Strong bottom-up effects dominated the trophic system of the host plant, Salix pentandra, the specialist galling sawfly, Euura amerinae, inquilines in the genus Eurytoma and the most common parasitoid attacking both genera of herbivores, Pteromalus capreae. A young willow population provided the basic resources for the second and third trophic levels, and the inquilines and parasitoid species showed strong positive, but density-independent, responses to food supply. Colonized trees ranged from 3 to 7 years old in 1989, and the Euura population had increased by 1991, declined significantly by 1992, and went almost extinct in 1994. The sex ratio declined steadily from 1989 to 1992. Populations of Eurytoma and Pteromalus tracked their resources closely, without influencing the major aspects of Euura population change dictated by availability of young trees and the rapid development of host resistance correlated with tree age. Trees continuously colonized by Euura from before 1989 to 1992 were much more favorable to both higher trophic levels than trees colonized for the first time between 1989 and 1992, indicating the importance of host heterogeneity in the system. Overall, effects from the host plant up the trophic system were very much stronger than the effects of carnivores down the food web.