| Studying all members of a community at once becomes daunting in any reasonably sized group of organisms. I propose hypotheses for the underlying mechanisms of a tritrophic interaction, and illustrate the different implications of these hypotheses in a modification of the Lotka-Volterra type predator-prey model.; I examined a tritrophic system involving an antagonistic interaction (herbivory) and a mutualistic mycorrhizal interaction. In a series of experiments involving Plantago lanceolata, Precis coenia, and three species of mycorrhizal fungi (Glomus white, Archaeospora trappei, and Scutelospora calospora), I examined plant defense responses, herbivore response to the presence of mycorrhizae, and mycorrhizal response to herbivory.; I found that the species identity of mycorrhizal fungi a plant associates with alters plant allocation to growth, root to shoot ratio, and constitutive and inducible defenses. Herbivory tended to reduce root to shoot ratio in the presence of Glomus white and A. trappei, and increase the root to shoot ratio of plants associated with S. calospora or no fungi. Constitutive levels of defense increased with S. calospora and A. trappei, but herbivory only leads to induction of defensive chemicals in plants associated with sterile soil.; Herbivory also differentially affects mycorrhizal species. Herbivory reduced the number of resource exchange sites (arbuscules and coils) present in plant roots in two out of three mycorrhizal species. However, in competition, herbivory had only small effects on mycorrhizal colonization. Most surprisingly, in the presence of both competition and herbivory the parasitic mycorrhizal species (S. calospora) produced more resource exchange sites than either beneficial species.; Mycorrhizal species also differentially impacted herbivore development and survival. Larvae survived longer and grew to be older on plants associated with a mixture of mycorrhizal species or Glomus white. |
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