The impact of plant-herbivore coevolution on plant community structure

Background/Question/Methods   Coevolutionary theory proposes that the diversity of chemical structures found in plants is, in large part, the result of selection by herbivores. Because herbivores often feed on chemically similar plants, they should impose selective pressures on plants to diverge chemically or bias community assembly toward chemical divergence. Using a coevolved interaction between a group of chrysomelid beetles and their host plants I tested whether coexisting plants of the Mexican tropical dry forest tend to be chemically more dissimilar than random. I analyzed the chemistry of 57 species belonging to the genus Bursera using gas-chromatography mass-spectrometry and constructed a matrix of chemical dissimilarity among species. I quantified the average chemical distance among species in 27 Bursera communities and also estimated the degree of specialization in the interaction between Bursera and Blepharida species in these communities. I tested for chemical overdispersion with randomization tests. I examined whether the particular configurations of species found in these real communities are more chemically diverse than communities randomly chosen from the same overall set of species. The average chemical distance for a given community was compared to the distribution of average chemical distances of many random communities constructed by choosing the same number of species randomly from the full list of species for all communities Results/Conclusions   Results show that some of the communities are chemically overdispersed and that overdispersion is related to the tightness of the interaction between plants and herbivores and the spatial scale at which these organisms interact. As coevolutionary specialization increases and spatial scale decreases, communities tend to be more chemically dissimilar. At fairly local scales and where herbivores have tight, one-to-one interactions with plants, communities have a strong pattern of chemical disparity.