Judges’ Queries and Presenter’s Replies

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Presentation Discussion

  • Icon for: Brian Drayton

    Brian Drayton

    May 21, 2012 | 08:52 p.m.

    Very nice work. I am not clear from the poster, though: Do the hybrids host arthropods not found on either “pure” species? And are there some arthropod taxa that show this pattern more than others? Are we mostly talking about herbivorous arthropods, or also (say) arachnids or other groups?

  • Icon for: Karl Jarvis

    Karl Jarvis

    May 21, 2012 | 11:48 p.m.

    Great questions. The answer to the first one is no, strictly speaking. However, the abundance of a species is almost always higher in one host type than another. There are a few cases where hybrids host a species in much greater numbers than in either “pure” species, but the general pattern is trickier to pin down. More often abundances on hybrids for a particular species are somewhere in between the parentals, but that’s also not a general rule.

    A step we’d like to take next is to create a set of simulated hybrid communities based on the composition of parental species, and compare them to the observed hybrid communities. We’d like to see if our observed hybrids have truly unique communities or if they are consistent with a random blend of the communities on the parental species.

    Our analyses include herbivores, predators, parasites, and any other observed arthropods on branches of the host tree. However, we expected that herbivores would have stronger host-specific phylogenetic patterns than predators because of their closer association to the chemicals the trees produce. So we weighted phylogenetic indices by abundance – for example, 250 mites on one host would be much more influential in our analyses than 3 spiders. We also excluded species that we only detected on one host tree, and we excluded species with a total abundance across all years of one or two individuals. When we include the full dataset, we still see the same patterns in the phylogenetic indices, but much less extreme. That makes sense, because we’d expect that species that don’t show up very often probably don’t have as specific a relationship with the host tree.

  • Further posting is closed as the competition has ended.

  1. Karl Jarvis
  2. http://www.igert.org/profiles/4704
  3. Graduate Student
  4. Presenter’s IGERT
  5. Northern Arizona University
  1. Rebecca Beresic-Perrins
  2. http://www.igert.org/profiles/4768
  3. Graduate Student
  4. Presenter’s IGERT
  5. Northern Arizona University
  1. A. Craig
  2. http://www.igert.org/profiles/3831
  3. Graduate Student
  4. Presenter’s IGERT
  5. Northern Arizona University

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.