Document: GIN-3-52-43

Keystone species and community structure: Interactions among herbivores determine patterns of arthropod biodiversity.

WIMP, G.M.* and T.G.WHITHAM

Northern Arizona University, Flagstaff, AZ 86011, U.S.A. ¹

Abstract:
We sought to examine how different keystone herbivores, and interactions among these herbivores, contribute to arthropod diversity on juvenile cottonwood ramets. We first examined the effects of the beaver, Castor canadensis, on the surrounding arthropod community. Beaver felling of trees resulted in vigorously growing juvenile tissue on the remaining stump, which is also nutrient-rich and may be attractive to arthropods. When we experimentally simulated beaver felling, we found a 65% increase in arthropod species richness and a 75% increase in abundance on felled trees that were resprouting, compared to control trees. Beavers also affected keystone insect herbivores, which in turn affected the distribution of another 140 species of arthropods. For example, beavers determined the distribution of the leaf-feeding beetle, Chrysomela confluens, which is more commonly found on beaver resprout compared to nearby ramets that have not been attacked by beavers. To experimentally address the effect of leaf-feeding beetles on arthropod community structure, we paired cottonwood ramets and removed beetles from one set of trees. We found a 37% increase in arthropod species richness and a 68% increase in abundance on beetle-removal trees compared to control trees. Not only were beavers responsible for the distribution of beetles on juvenile cottonwoods, but a free-feeding aphid (Chaitophorus populicola) also determined beetle distribution. Free-feeding aphids excluded beetles from trees where they were present through their aggressive, tending ants, and these aphids negatively affected biodiversity. In addition to traditional analyses, we also present NMDS ordinations to illustrate how the interactions among keystone species result in a mosaic community that enhances biodiviersity. These results demonstrate the importance of keystone herbivores, and interactions among these herbivores, in structuring arthropod communities.

Keywords: Castor canadensis, Chrysomela confluens, Chaitophorus populicola, Populus, herbivore, keystone, community, NMDS, biodiversity, arthropod, insect.

This abstract is being presented at: 10:30 AM in session:
Oral Session #44: Terrestrial Invertebrates: Foodwebs and Plant Responses.