Stephen Hubbell, Brant Faircloth, Greg Gilbert, and Travis Glenn

This project integrates taxonomic, genetic, and functional approaches in a combination of observations and experiments to test the Enemy Susceptibility Hypothesis (ESH) for tropical tree diversity and rarity. This hypothesis predicts that rare tree species will have a greater percentage of hollow-trunk trees, harbor more pathogenic fungal species, and also share more of these pathogens with other tree species, than common tree species. At forest plots on Barro Colorado Island, Panama, we used sonic tomography to examine thousands of living trees, detect internal decay, and sample fungi from wood and leaves to determine which fungal species are present on which trees. We will be experimentally testing pathogenicity, as well as looking at expression of lignin-degrading enzymes in the fungi, and how they may be involved in host ranges.

If the Enemy Susceptibility Hypothesis is true, it may provide a general explanation for the high diversity and rarity of tree species in tropical forests worldwide. The project also includes experiential, inquiry-based learning for a number of Panamanian interns. The project has also incorporated two 10-day field courses for high school students, college students, and high school science teachers from California, Georgia, and Panama. The participants conducted research in the rain forest for 10 days, learning about tropical ecology, tree diseases, and molecular approaches to field ecology.

Greg Gilbert & Ingrid Parker

Locally rare species are expected to have a survival advantage over more abundant species, because rarity reduces their risk of damage from pathogens. This "rare-species advantage" may help maintain plant diversity in natural systems and explain how introduced plant species become invasive weeds. Usually, a plant species is considered rare if few individuals of that species are present; this assumes that pathogens are specialized on individual species. In fact, most pathogens are able to infect a variety of different hosts. Which hosts are susceptible is not a random assortment of species, because closely related species are more likely to share a pathogen than are more distantly related species. This means that rarity is not simply a function of species density (numerical rarity), but of the combined density of all species with which it shares a pathogen. A species with no closely related neighbors has the added benefit of "phylogenetic rarity". We are evaluating the relative importance of phylogeny and abundance in the rare-species advantage of plants in four plant communities along the Central Coast of California, measuring disease on different plant species, the host ranges of the responsible pathogens, and the ability of the pathogens to reproduce on different hosts. This is the first attempt to evaluate how the overall evolutionary structure of a plant community affects the amount of disease suffered by each species in the community.

Check out our recent Nature paper highlight our findings.