Funded Grants

Plankton community assembly: theory and practice

"[A lake] forms a little world within itself - a microcosm within which all the elemental forces are at work and the play of life goes on in full, but on so small a scale as to bring it easily within the mental grasp." - Stephen A. Forbes (1887), The Lake as a Microcosm

Ecological communities are complex systems. They contain hundreds of species that compete for resources, eat each other, and modify their environment. Organisms can have complicated behaviors, and physical and chemical factors influence communities in predictable and random ways. Most communities have open borders, so they receive fluxes of matter and organisms from outside the system. Not only are ecosystems complex, most are also logistically dicult to study, due to their slow dynamics and large sizes. Yet, ecosystems provide valuable services to humans, from natural products to aesthetic properties, and are increasingly disturbed by human activities, so we must improve our understanding of how ecosystems are assembled (what determines the species present) and operate (what controls how nutrients and energy flow among biotic and abiotic pools).

We focus our research on plankton communities. Plankton are the microscopic plants (phytoplankon) and animals (zooplankton) that form the base of food webs in lakes and the oceans. The oceans' phytoplankton play a major role in the planet's carbon cycle since they perform about half the earth's photosynthesis. Zooplankton graze phytoplankton and serve as a conduit for solar energy to higher animals. Phytoplankton affect water quality: some merely cause unsightly blooms, others are toxic to humans and animals. Humans impact aquatic ecosystems by introducing nonnative species, overharvesting commercially valuable species, changing natural hydrological patterns, and polluting them with excess nutrients. A better understanding of what determines the abundance and composition of plankton communities in space and time will improve our ability to predict how aquatic ecosystems will respond to these human perturbations.

Aside from their direct importance to people, plankton communities are ideal systems for exploring ecological complexity, as Forbes noted over a century ago. Though we can't see most plankton with the naked eye, a microscope reveals a diverse and fascinating community. Plankton grow quickly, so processes that would take decades in forests occur in weeks in lakes. Plankton are easily grown in the lab, allowing us to measure physiological parameters. By studying plankton communities as model systems, we hope to illuminate processes that may also structure other, less tractable ecosystems.

"A predictive understanding of fundamental factors affecting aquatic productivity, controls on ecosystem properties, and long-term vitality of aquatic systems will require new approaches to modeling ... that are immensely more sophisticated than those currently in use." - Naiman et al. (1995), The Freshwater Imperative: A Research Agenda

Our main goal is to develop and test new theoretical approaches to understanding the assembly and operation of plankton communities. We want to know how many and what types of species will be present in a particular lake, given knowledge of its abiotic characteristics such as depth and nutrient content. Traditional models in ecology determine the outcome of interaction between a few predefined species, predicting whether the species will coexist and if so, what their abundances will be. It's like being able to predict the outcome of a boxing match between two particular fighters. The problem is that an ecosystem is much closer to a barroom brawl than an orderly boxing match, with large numbers of species assailing each other simultaneously. The models we will develop determine which species will be left standing in a particular ecosystem after the dust settles. In our approach community structure emerges from the interaction of all possible species rather than being presupposed by the particular species chosen at the outset.

Our approach is based on recent developments in evolutionary game theory. We define a universe of possible species in terms of their ecological strategy: how good they are at acquiring different resources, avoiding predators, and so on. We derive an invasion function that tells us which species can invade an existing community, then look for some species or combination of species that can keep all others out. This set of species represents a stable endpoint of community assembly and is the type of community we'd expect to find in the field. We repeat this process for different types of environments to see how communities depend on the abiotic characteristics of the ecosystem.

These models meet the real world in a few ways. First, we will perform a series of physiological experiments on different species to determine the trade-offs between traits that define the universe of possible species. These physiological studies provide the raw material that the models transform into predictions of how community organization depends on environmental conditions. We will test these predictions against an extensive set of field data collected by the EPA in the 1970's that has been largely forgotten. We expect a continuous interplay between these studies so that new empirical discoveries will also guide model development. Often theoreticians and empiricists work in isolation from each other. Integrating laboratory, field, and theoretical approaches provides a powerful method to understand ecological complexity.