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 plan t, Salix pentandra, the specialist galling sawfly, Euura amerinae, inq uilines in the genus Eurytoma and the most common parasitoid attacking both genera of herbivores, Pteromalus capreae. A young willow populat ion provided the basic resources for the second and third trophic leve ls, and the inquilines and parasitoid species showed strong positive, but density-independent, responses to food supply. Colonized trees ran ged from 3 to 7 years old in 1989, and the Euura population had increa sed by 1991, declined significantly by 1992, and went almost extinct i n 1994. The sex ratio declined steadily from 1989 to 1992. Populations of Eurytoma and Pteromalus tracked their resources closely, without i nfluencing the major aspects of Euura population change dictated by av ailability 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 le vels than trees colonized for the first time between 1989 and 1992, in dicating the importance of host heterogeneity in the system. Overall, effects from the host plant up the trophic system were very much stron ger than the effects of carnivores down the food web.