Cottonwood Hybrid Zones as Centres of Abundance for Gall Aphids in Western North America: Importance of Relative Habitat Size

Abstract

1. In western North America, populations of the leaf-galling aphid Pemphigus betae are concentrated in natural zones of overlap and hybridization between species of cottonwoods (Populus). Averaged over seven drainages in four American states and one Canadian province, P. betae galls were 28-fold more abundant in hybrid zones than in adjacent pure zones of the narrowleaf host species. 2. The strength of this pattern is impressive in that it occurred in all of the river drainages surveyed and spanned a north-south gradient of 1600 km (15° latitude). Furthermore, this pattern was not restricted to one species-pair of cottonwoods, but occurred in zones of narrowleaf x Fremont, narrowleaf x Eastern, and narrowleaf x balsam x Eastern cottonwood. 3. The concentration of galls in the hybrid zone was not influenced by the absolute size of the hybrid zone. However, there was a significant inverse relationship (P = 0.02) between relative gall density and the relative size of the hybrid zone. All else being equal, small hybrid zones supported higher concentrations of aphids than large hybrid zones. Three hypotheses, 'hybrid zones as aphid sinks', 'hybrid zones as aphid sources' and an 'introgression' hypothesis, are proposed which may explain this regional pattern. 4. Riparian cottonwood forests shelter a rich diversity of animal and plant species that is threatened by habitat loss through anthropogenic activities. If the concentration of P. betae in cottonwood hybrid zones is a pattern common to other hybridizing plants and their dependent species, preserving these relatively small areas could have a disproportionately positive role in preserving biodiversity.