Tropical forests are extremely diverse. One can find more than 1000 species of trees within a few square miles in many areas. For comparison, the entire U.S. contains about 600 species. It is also common to find many closely-related species in close proximity, despite predictions that close relatives should share similar traits and therefore competitively exclude one another. One phenomenon that may explain this pattern is that species composition can change dramatically with habitat. This idea will be tested by studying five common and diverse tree lineages that occur in three distinct forest types, surveying all of their species across large areas of Peru and French Guiana. Floras of both countries are highly diverse yet share very few species; this allows a powerful, replicated test of habitat specialization. DNA will be used to construct phylogenies of these five lineages to test if close relatives are generally found in different environments. Finally, a reciprocal transplant experiment will help determine whether closely related tree species share traits that confer flood tolerance, nutrient use efficiency, and defenses against natural enemies, and how these traits benefit survival in each habitat.
This project will determine the relationship between environmental heterogeneity, habitat specialization and species coexistence. The results will have importance to policy makers interested in conserving the forests with the highest biodiversity on Earth. The project will provide research opportunities for students within U.S. institutions and will also foster international exchange among the three countries by including Guianan and Peruvian students.
One of the great challenges to modern evolutionary biology is to understand the origins of the extreme diversity that we observe in the flora of the Neotropics. Within this region, there are single hectares (100 × 100 meters) of forest that host up to 600 species of trees, more species than all of temperate North American forests combined, an area many millions of times larger. Neotropical forests often have very high numbers of species observed at the local scale (high alpha diversity), but this is also complemented by high levels of species differences at the regional level (high, between-site species turnover or beta-diversity). A central theme among hypotheses that attempt to explain this diversity is that habitats in the Neotropics are highly variable, even over small spatial scales. Variations can be abiotic (e.g., differences in soil type, light availability, wind regime, rainfall amount and annual distribution) and biotic (e.g., disease prevalence, pollinator occurrence, herbivore type and abundance). Recent studies [1] have examined the interactions between an abiotic factor (soil type) and a biotic factor (insect herbivory). Experiments in which treelets that were specialists on a given soil type were transplanted to the other soil type showed that nutrient availability and insect herbivory combine to promote the evolution of particular growth and chemical defense strategies. In the current project, we asked the question "how might these biotic/abiotic interactions promote speciation?". In one study, for example, we examined populations of a single species, Protium subserratum (Burseraceae), which is comprised of subspecies that grow on either low-nutrient, white-sand soils or relatively nutrient-rich, clay soils. To place the question in phylogenetic context, we first constructed a detailed phylogenetic tree of P. subserratum populations [2]. This showed that clay-soil populations form a clade that is derived from older white-sand clade. We then characterized the assemblages of insect herbivores to which both soil specialist subspecies were exposed, the types of chemical defenses that were deployed and the growth rates of the plants [3, 4]. We found that the two P. subserratum subspecies had significantly different growth rates and that they deployed qualitatively different chemical defenses. In addition, there was very little overlap in the composition of the herbivore communities to which they are exposed. Our results are consistent with a mechanism in which different habitat types promote functional divergence within species, such as P. subserratum, that are habitat generalists. This suggests one possible route to speciation. 1. Fine, P.V.A.; Mesones, I.; Coley, P.D. Herbivores promote habitat specialization by trees in Amazonian forests. Science 2004, 305, 663-665. 2. Fine, P.V.A.; Zapata, F.; Daly, D.C.; Mesones, I.; Misiewicz, T.M.; Cooper, H.F.; Barbosa, C.E.A. The importance of environmental heterogeneity and spatial distance in generating phylogeographic structure in edaphic specialist and generalist tree species of Protium (Burseraceae) across the Amazon Basin. Journal of Biogeography 2012. 3. Fine, P.V.A.; Metz, M.R.; Lokvam, J.; Mesones, I.; Milagros Z. A., J.; Lamarre, G.P.A.; Vásquez P., M.; Baraloto, C. Insect herbivory, chemical innovation and the evolution of habitat specialization in Amazonian trees. (in review) 2012. 4. Lokvam, J.; Fine, P.V.A. An Oxidized Squalene Derivative from Protium subserratum Engl. (Engl.) Growing in Peru. Molecules (Basel, Switzerland) 2012, 17, 7451-7457.
