The impact of herbivore-plant coevolution on plant community structure

Abstract

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. 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 communities are measured. 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.

Fig. 1.

Bursera aptera. The genus Bursera reaches its maximum diversity and abundance in the tropical dry forests of Mexico where, with ≈85 endemic species, it is one of the major elements of the flora.

Fig. 2.

Blepharida pallida. The Blepharida genus includes ≈45 species, which all feed only on Bursera.

Fig. 3.

Number of hosts attacked by Blepharida species. Most Blepharida species are highly specialized and are known to feed on only one (monophagous) or up to four hosts (oligophagous). But two of them are more generalized, feeding on 12 or 14 hosts (here called “polyphagous”).

Fig. 4.

Geographic localization of selected areas of study. Areas 1–5 contain the highest diversity of Bursera and monophagous and oligophagous Blepharida. In areas 6 and 7, polyphagous Blepharida are more frequent. 1, Chilpancingo; 2, Cañón del Zopilote; 3, Tehuacán; 4, Infiernillo; 5, Aguililla; 6, Zihuatanejo; 7, Aquila.

Fig. 5.

Average chemical distance for Bursera communities that develop at the lowest altitudes in each of the seven areas studied (purple bars) compared with the average chemical distance of 10,000 random Bursera communities of equal size (pink bars). Black lines above bars indicate confidence intervals. ∗, P < 0.1; ∗∗, P < 0.05.

Fig. 6.

Standardized average chemical distances for 27 Bursera communities in the seven geographic areas studied vs. percentage of Bursera species that are attacked by monophagous or oligophagous Blepharida. Blue squares indicate communities that develop at altitudes up to 1,000 m. Red squares indicate larger communities that develop at altitudes up to 2,200 m. Yellow dots indicate communities in which chemical defenses are statistically more dissimilar than random. Regression lines for small and large communities were fit simultaneously adjusting for correlated error structure (assuming local, compound symmetric error covariance; see Methods).