Ecological, geographic and phylogenetic dimensions of food webs and their response to global change

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The world is undergoing rapid change in terms of climate, urbanization and habitat fragmentation. These changes will affect species not only through direct effects on their survival and distribution but also via the web of feeding interactions ("food webs") that knit together species. However, because food webs contain mobile animals and often occupy large amounts of space, they are difficult to experimentally manipulate to test theory about global change impacts. My solution is to use naturally contained aquatic food webs, such as those found in bromeliads or tree holes. These systems have the additional advantage of occurring in much of the New World, allowing us to understand which processes are universal, which depend on geographic context, and why. In this proposal, my students and I will combine experiments, theoretical models and syntheses of global databases that we have developed to understand the processes underlying food webs. In order to disentangle different climatic influences on food web structure, we will examine food webs along elevational gradients in the Ecuadorian Andes and a latitudinal gradient from Argentina to Mexico. We will also examine the ratio of nutrients in food resources to understand how energy flows through `channels' in food webs. Organisms require certain ratios of nutrients to support growth, and such individual needs can scale up to affect the entire ecosystem. Species also have traits that determine which prey or resources they can consume (and whether they can avoid being eaten themselves), whether they are physiologically resistant to environmental conditions, and how they use information to avoid predators or risky habitats. Some traits and nutrient requirements of species may be strongly conserved through evolutionary time, restricting the potential of food webs to exploit all combinations (i.e. multivariate "space") of traits and nutrients. In other cases, species representing missing combinations exist elsewhere in the world, but have not yet dispersed to the study location. Understanding these evolutionary and dispersal constraints allow us to understand why food webs are structured differently in different places. These constraints are also important to understanding how a changed environment - whether through climate, urbanization or fragmentation - can affect food webs. For example, fragmented rainforest is composed of young, fast-growing trees that produce leaf litter richer in nutrients than intact forest. This could create landscape-scale changes in litter-based food webs- a mechanism that we can test by experimentally changing the ratio of nutrients within leaf litter. In cities, food web structure may be limited by the availability of colonists, and this colonist pool may be greater in affluent neighbourhoods with many gardens. In sum, I propose a multifaceted research program to understand the fundamental constraints of food webs in order to predict their responses to a changing world.