A geographic mosaic of trophic interactions and selection: trees, aphids and birds.

Genetic variation in plants is known to influence arthropod assemblages and species interactions. However, these influences may be contingent upon local environmental conditions. Here, we examine how plant genotype-based trophic interactions and patterns of natural selection change across environments. Studying the cottonwood tree, Populus angustifolia, the galling aphid, Pemphigus betae and its avian predators, we used three common gardens across an environmental gradient to examine the effects of plant genotype on gall abundance, gall size, aphid fecundity and predation rate on galls. Three patterns emerged: (i) plant genotype explained variation in gall abundance and predation, (ii) G×E explained variation in aphid fecundity, and environment explained variation in gall abundance and gall size, (iii) natural selection on gall size changed from directional to stabilizing across environments.