COTTONWOOD HYBRID ZONES AS CENTERS OF ABUNDANCE FOR GALL APHIDS IN WESTERN NORTH-AMERICA - IMPORTANCE OF RELATIVE HABITAT SIZE

1. In western North America, populations of the leaf-galling aphid Pem phigus berne are concentrated in natural zones of-overlap and hybridiz ation between species of cottonwoods (Populus). Averaged over seven dr ainages in four American states and one Canadian province, P. betae ga lls were 28-fold more abundant in hybrid zones than in adjacent pure z ones of the narrowleaf host species. 2. The strength of this pattern i s impressive in that it occurred in all of the river drainages surveye d and spanned a north-south gradient of 1600 km (15 degrees latitude). Furthermore, this pattern was not restricted to one species-pair of c ottonwoods, but occurred in zones of narrowleaf x Fremont, narrowleaf x Eastern, and narrowleaf x balsam x Eastern cottonwood. 3. The concen tration of galls in the hybrid zone was not influenced by the absolute size of the hybrid zone. However, there was a significant inverse rel ationship (P = 0.02) between relative gall density and the relative si ze of the hybrid zone. All else being equal, small hybrid zones suppor ted higher concentrations of aphids than large hybrid zones. Three hyp otheses, 'hybrid zones as aphid sinks', 'hybrid zones as aphid sources ' and an 'introgression' hypothesis, are proposed which may explain th is regional pattern. 4. Riparian cottonwood forests shelter a rich div ersity 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 plant s and their dependent species, preserving these relatively small areas could have a disproportionately positive role in preserving biodivers ity.