By: Allison E. Drew & George K. Roderick
Patterns of arthropod richness and diversity were studied across six hybrid zones within the silversword alliance (Asteraceae: Heliantheae-Madiinae), an adaptive radiation of endemic Hawaiian plants. Hybrid zones spanned a range of parental plant relatedness from those in which parental types were phenotypically and genetically similar to those in which parental types were different both phenotypically and genetically. This system of hybrid zones is uniquely suited for distinguishing among alternative hypotheses that account for biodiversity of herbivores in plant hybrid zones. In the six hybrid zones studied, arthropod family-level diversity was as high or higher on hybrid plants compared with parental plants. Family-level richness was as high or higher on hybrids plants in four cases and lower on hybrids in two cases. Generally, results were consistent with hypotheses that predict either a breakdown in insect defenses in hybrid plants or an additive increase in host recognition cues in hybrids. Hypotheses that predict an increase in hybrid resistance to herbivory were supported in two cases. Measures of community similarity suggest insect communities are more similar to each other on hybrid–parent pairs than are communities on parent–parent pairs and that insect communities across interspecific hybrid zones are more similar than communities across intergeneric hybrid zones. In four zones, hybrids supported unique arthropod taxa. Because a greater or equal diversity of arthropods is found on hybrids than on parental plant taxa and because hybrid plants may represent unique environments to insects, plant hybrid zones are important targets for conserving Hawaii’s native biota.