A geographic mosaic of tri-trophic interactions: Bottom-up and top-down selection forces change across the landscape

Recently, two theories of ecology and evolution have expanded understanding of how biodiversty may arise and persist.  First, community genetics theory  suggests that the genetics of foundation plant species (a bottom-up process) shapes community composition and  patterns of species diversity.  Second, geographic mosaic theory suggests that selection regimes may change as biotic and abiotic conditions change across the landscape.  Here, we demonstrate principles that are fundamental to both of these ecological/evolutionary frameworks.  Examining a tri-trophic system, consisting of the narrowleaf cottonwood tree (Populus angustifolia), a galling herbivore (Pemphigus betae) and an insectivore (the black-headed grosbeak, Pheucticus melanocephalus), we examined the bottom-up effects of tree genotype on gall traits and aphid fitness and the top-down selection pressure of avian predation on these same herbivore traits.  Finally, we show that both the bottom-up and top-down selection regimes change as a function of geographic locale.  Specifically, we show that tree genotypes that promote high aphid fitness in one locale can promote moderate fitness in another locale and fail to support aphids in a 3^rd locale.  Also, we show that top-down selection on aphid fitness shifts from directional to stabilizing across the landscape.  Our results indicate that individual plant genotypes can exert unique selection pressure on the surrounding community members and selection studies should be viewed as context dependent.