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

点击翻译按钮获取中文摘要

A basic goal of ecology is to understand the processes that generate the vast variation inabundance and biomass among taxa in time and space. Food webs summarize the flow of elementsand energy in ecological systems. One basic type of web groups taxa into trophic levels based onshared prey and predators. A significant and attainable goal for Ecology is to predict the biomassacross these trophic levels in time and space. Three ecological theories, knit together, should help us achieve this goal:Are resources limiting? OFANHow strong is selection for defense? Defense theoryHow do prey allocate between growth and defense? Ecological stoichiometryWhich 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 bythe 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 ofPanama's Barro Colorado National Monument (BCNM). Most of what we know about the structureand function of food webs comes from plant-based, or green webs. However, over 80% of thecarbon fixed in photosynthesis in the tropical canopy falls as litter where it is decomposed in thebrown food web of microbes, microbivores, and their predators. The taxa in these webs mayrepresent 60% of the forest's invertebrates and their abundance can vary 100-fold at small spatialscales. Yet, despite its importance in releasing carbon into the atmosphere, recycling ecosystemnutrients, 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 TropicalResearch Institute, and its ongoing large-scale experimental and demographic studies, is the bestplace 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 ofBCNM's brown food webs. Its 50 ha tree plot will be used to map this biomass onto the chemicalsignatures of 10 common tree species at three stages of litter decomposition. These patterns will befurther tested experimentally with long-term 40 x 40 m fertilization plots, and pulses of nutrients in1-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.