Experimental Ecology of Food Webs: Complex Systems in Temporary Ponds

Abstract

A food web graphically represents the paths of nutrients and energy through the living components of an ecosystem and the context in which individuals exploit their prey and avoid their enemies. Temporary ponds are excellent arenas for the study of food webs because they are discrete communities that can be mimicked in containers that approach the realism of natural habitats. Artificial ponds permit repeatable initial conditions and sufficient replication of independent experimental units in complex experiments to test hypotheses about the control of structure and function in natural communities. I used a combination of observations of natural ponds, “experimental natural history” of artificial ponds in my study area, and controlled experiments in an array of 144 replicate ponds to develop, then test, hypotheses about how the structures of food webs are regulated. Understanding food webs begins with population biology. Amphibians use the aquatic larval stage of their biphasic life cycle to exploit ephemeral opportunities for growth and development in temporary ponds. The regulation of population density and the fitness of individuals are determined by complex interactions among competition, predation, and uncertainty in the length of the time ponds retain water. Exponential models of density-dependent recruitment relate the number of metamorphs to the input of eggs into ponds without predators. Extensions of these models include interspecific competition and predation. The addition of predation to these systems has three effects. (1) Predators can reduce, even eliminate, prey. There are species-specific differences among co-occurring prey in their risks of predation, and these risks change with relative body sizes of predator and prey. (2) Individuals may evoke an inducible defense that reduces their risk by either decreasing active foraging or developing morphological adaptations, such as changes in the coloration and shape of tails. These defenses may entail costs in body size and timing of metamorphosis. (3) Finally, mortality due to predators may reduce competition, thereby benefiting the population of the prey by permitting individuals that escape predation to grow rapidly enough to escape drying ponds. The order of arrival of species at breeding ponds has an impact on their own success as well as having a lasting impact on the success of species that arrive later. Such priority effects may result from size-specific changes in trophic connections or indirect effects between speciesmediated through the food web. Anurans can have strong effects on the partitioning of the flow of nutrients through the phytoplankton vs. the periphyton. This effect on partitioning of production can then have strong effects on zooplankton and insects. Salamanders can play the role of keystone, or critical, predators by affecting the structure of the assemblages of zooplankton and anurans that determine much of the dynamics of nutrient flows within food webs in temporary ponds.