Arthropod Communities on Hybrid Cottonwood Hosts Show Unique and Persistent Phylogenetic Patterns

We examined the hypothesis that insect and other arthropod communities would differ in their phylogenetic (branching evolutionary) patterns on hybrid cottonwood hosts relative to parental species. Hybrids between narrowleaf and Fremont cottonwoods have been shown to have differing arthropod communities than either narrowleaf or Fremont cottonwoods, so we postulated that the arthropod communities on hybrids would also exhibit phylogenetic patterns that differ from those found on narrowleaf and Fremont cottonwoods. We collected community composition and abundance data of arthropods on narrowleaf, Fremont, and hybrid cottonwood hosts growing in a common garden over four years. We then assembled a phylogeny of the arthropods we found, based on previous phylogenetic studies. We performed analyses of phylogenetic patterns of the arthropod communities on each individual host tree, as well as on communities pooled by year, host type, and host type within years. We found two main results: 1) Phylogenetic diversity and species richness is higher in individual hybrid hosts than in individual parental species hosts, but communities pooled by host type indicated no such increase in phylogenetic diversity in arthropod communities on hybrids. 2) Arthropod communities on Fremont and narrowleaf hosts are phylogenetically clustered, but arthropod communities on hybrids are either less clustered than other host types or phylogenetically overdispersed. This indicates that hybrids between Fremont and narrowleaf cottonwoods host a more diverse array of arthropod species, and that hybrids may act as a fundamentally different force in structuring arthropod communities.