Does Ecological Stoichiometry and Defense Theory Predict Patterns of Resource and Predator Limitation in a Tropical Litter Food Web?

University Of Oklahoma Norman Campus, Norman OK

Investigators

Abstract

A basic goal of ecology is to understand the processes that generate the vast variation in abundance and biomass among taxa in time and space. Food webs summarize the flow of elements and energy in ecological systems. One basic type of web groups taxa into trophic levels based on shared prey and predators. A significant and attainable goal for Ecology is to predict the biomass across these trophic levels in time and space. Three ecological theories, knit together, should help us achieve this goal: Are resources limiting? OFAN How strong is selection for defense? Defense theory How do prey allocate between growth and defense? Ecological stoichiometry Which elements are potential resources? 1. OFAN (an acronym for its architects) proposes that the balance of resource and predator imitation varies with the amount of resources available at the base of the web. 2. Ecological stoichiometry posits that resources limiting biomass reflect the balance of chemical elements needed for growth and defense relative to their availability in the environment. What available elements can build defenses? How do defenses shape elemental composition? 3. Defense theory posits that the optimal allocation between growth and defense is determined by the intensity of predation and the time it takes for the prey to grow to reproductive age. These theories will be used to explore the dynamics of tropical litter food webs in the Republic of Panama's Barro Colorado National Monument (BCNM). Most of what we know about the structure and function of food webs comes from plant-based, or green webs. However, over 80% of the carbon fixed in photosynthesis in the tropical canopy falls as litter where it is decomposed in the brown food web of microbes, microbivores, and their predators. The taxa in these webs may represent 60% of the forest's invertebrates and their abundance can vary 100-fold at small spatial scales. Yet, despite its importance in releasing carbon into the atmosphere, recycling ecosystem nutrients, and sustaining much of the forest's biodiversity, we are profoundly ignorant of the factors shaping abundance in these webs. Like the green forest canopy above it, the tropical litter represents one of the last ecological frontiers on the planet. BCNM, with its extensive infrastructure, its century-long stewardship by Smithsonian Tropical Research Institute, and its ongoing large-scale experimental and demographic studies, is the best place to undertake this project. Ecological stoichiometry will be used to identify which elements (among C, N, Ca, P, S, Mg, K, Cu, Fe, Zn) limit biomass in the trophic levels and common taxa of BCNM's brown food webs. Its 50 ha tree plot will be used to map this biomass onto the chemical signatures of 10 common tree species at three stages of litter decomposition. These patterns will be further tested experimentally with long-term 40 x 40 m fertilization plots, and pulses of nutrients in 1-m 2 plots. OFAN's prediction of a stairstep accumulation of biomass along nutrient gradients will be evaluated with the above data. Its mechanism--that the impact of predator limitation varies along resource gradients--will be tested using fences that keep out predacious ants and by supplementing plots with litter ant nests. Defense' theory's life history tradeoffs will be evaluated in microcosms by measuring growth rates of common litter taxa and evaluating their palatability to predators. If defenses by fungi and microbivores significantly curtail predation then life history tradeoffs that play out along resource gradients are a key phenomenon shaping the balance of predation and resource limitation in this system.