| Recent studies have highlighted plant hybrid zones as focal points of insect biodiversity and centers of insect abundance. To expand knowledge of insect/hybrid plant interactions, insect communities were examined in pure zones of Fremont cottonwood, P. fremontii, narrowleaf cottonwood, P. angustifolia, and in an intervening zone where these two species hybridize. Three sets of results are discussed.;First, because hybrid plants span the morphological and genetic gaps between species, plant hybrid intermediates may facilitate host shifting by herbivores between the parental plant species. This hypothesis is supported by the distributions of galling herbivores on hybrid cottonwoods.;Second, expanded host phenology explains chronically high populations of the leaf-feeding beetle, C. confluens, in the hybrid zone. Narrowleaf and hybrid trees in the hybrid zone leaf out 3-4 wks before Fremont trees and 1-6 wks before trees in the narrowleaf zone. Thus, the hybrid zone supplies the earliest source of food. Further, Fremont and narrowleaf cottonwood overlap only in the hybrid zone which provides beetles with a unique opportunity to shift onto newly-flushed Fremont hosts when the older foliage of sympatric hybrid and narrowleaf trees declines in quality. This shift can increase fecundity by more than 6-fold.;Third, in a survey of insect taxa across pure and hybrid zones of cottonwood, five conclusions resulted: (1) patterns of distribution for individual taxa indicated that responses to host hybridization were idiosyncratic; (2) taxa were shown to differentiate between degrees of host plant hybridization; (3) unique assemblages of insect taxa were observed in the hybrid zone; (4) the hybrid zone supported 13 and 40 percent more taxa than Fremont and narrowleaf zones, respectively; (5) the cumulative relative abundance of taxa was not significantly different across zones nor between different host categories within the hybrid zone.;This study represents the most comprehensive examination to date of insects in plant hybrid zones. The results of this study will prove valuable in directing future research towards new lines of inquiry. |
