PARENT SESSION
Oral Session #66: Plant-Animal Interactions: Webs and herbivory.
Presiding: J.N. Holland
Wednesday, August 7. 1:00 PM to 4:45 PM. Apache Meeting Room, TCC.
Top-down, bottom-up, and horizontal control of plant diversity in a rainforest understory.
Dyer, Lee*,1, Vega Chavarria, Gerardo2, Letourneau, Deborah3, 1 Tulane University, New Orleans, LA2 Organization for Tropical Studies, Puerto Viejo, Sarapiqui, Costa Rica3 University of California, Santa Cruz, CA
ABSTRACT- In the understory of tropical forests, there are many abiotic and biotic factors that maintain high alpha diversity. We examined plant resources (available soil nutrients and light) and biotic interactions (competition and insect herbivory) as predictors of understory plant diversity in a lowland wet forest in Costa Rica. Changes in plant diversity over 20 months were monitored in response to direct and indirect (via herbivores) competitive interactions of the common understory shrub Piper cenocladum (Piperaceae). Control (or "direct competition with Piper") plots included naturally high densities of P. cenocladum, all shrubs of this species were removed from "low-competition" plots, and herbivore-laden fragments were added to "indirect-competition" plots. When studies were conducted at poor-soil (ultisol) sites, none of the measured or manipulated variables were significantly associated with understory diversity. For rich-soil (inceptisol) sites, increases in resource availability were correlated with higher diversity, accounting for over half the variation in plant species richness and over two-thirds the variation in abundance. Biotic interactions also had large significant effects on diversity in rich soils; high density Piper shrub plots had the highest diversity and indirect competition plots had the lowest. Results suggested that P. cenocladum shrubs exclude competitive weedy plant species, thus allowing for maintenance of higher species richness. Conversely, herbivore-susceptible P. cenocladum fragments appeared to be important sources of seedling mortality for other species of plants, leading to lower species richness.
KEY WORDS: diversity, tropics, herbivory, nutrients