PARENT SESSION
Oral Session # 53: Herbivory IV: Communities, Populations, and Genetics.
Presiding: JA Rudgers
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 103.

Herbivory enhances the evolution and maintenance of habitat specialization in Amazonian white sand and clay forests.

Fine, Paul^*,1, Coley, Phyllis¹, Mesones A., Ytalo², ¹ University of Utah, Salt Lake City, Utah² Universidad Nacional de la Amazonia Peruana, Iquitos, Peru, Peru

ABSTRACT- Tropical forests include a diversity of habitats, which has lead to specialization in plants. Near Iquitos, in the Peruvian Amazon, nutrient-poor white sand forests are found immediately adjacent to nutrient-rich clay forests, each harboring a unique composition of habitat specialist trees. Why does habitat specialization occur? Our hypothesis is that the combination of impoverished soils and herbivory creates strong selective pressure for plant defenses in white sand forests. Species that have not evolved high levels of antiherbivore defense are therefore at a disadvantage in white sand forests and excluded by herbivores. In clay forests, due to the tradeoff between growth and defense, heavily defended white sand species should grow more slowly than clay species and be out-competed. To test to what degree herbivores maintain the low overlap in species composition between the two forest types, we conducted a reciprocal transplant experiment and manipulated the presence of herbivores. We transplanted 880 seedlings from 20 species from seven genera that included phylogenetically independent white sand and clay specialist species into 22 herbivore exclosures and 22 controls in both white sand and clay forest. After 20 months, clay species exhibited significantly higher growth rates (both height and leaf area) in white sand forest than white sand species when protected against herbivores. When left unprotected, clay species' average leaf area and height was less than white sand species' averages. In clay forest, white sand species had significantly higher growth rates than they did in their home forest, but always grew significantly less than clay species. These results are the first experimental evidence of the impact herbivores have on plant species distribution in tropical forests and suggest a new way that herbivores influence plant evolution: by sharpening habitat boundaries due to abiotic factors and thereby increasing the potential for habitat specialization and speciation.

Key words: tropical trees, plant defense, amazon, beta diversity