Ecological and evolutionary responses to changing environments in a laboratory species assemblage

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Ecosystems face changing environments over a wide range of time-scales, from yearly changes to those occurring over millions of years. Moreover, the rate and magnitude of environmental change has been intensified by human activities. Therefore, a key task for understanding the dynamics of living systems, and for predicting the impacts of human activity on those systems, is to understand how ecosystems respond to environmental change. Two types of response can be considered. 1) Ecological. This entails changes in species abundances and distribution, leading to a different community structure before and after the environmental change. 2) Evolutionary. This entails genotypic change within species leading to their adaptation to the new conditions present after the environmental change. Despite considerable work on both types of response, typically they have been treated separately. Ecosystem studies have focused on ecological changes, whereas evolutionary studies have focused on single species or pairs of interacting species such as pathogens and hosts. Yet, most organisms live in diverse assemblages: inferences on evolutionary dynamics from such small systems may not apply in realistic settings. This project will address the fundamental question of whether species diversity tends to increase, decrease or not affect the amount of evolution within species. Our approach is to study evolution in the laboratory, using species of bacteria isolated from tree-holes of beech (Fagus sylvatica). These bacteria can be cultured in mixtures of different numbers of species in the lab, and have been used previously to explore the relationship between diversity and ecosystem function. By virtue of rapid generation times and very large population sizes, they also evolve fast enough to study evolution in species mixtures. We will assemble random mixtures of up to 24 species and expose them to environmental change, in terms of a shift in pH and resource type in turn. The evolutionary response of each species will be quantified using phenotypic assays and compared among treatments that differ in species richness and composition. Competition experiments between isolates from the same and different cultures will test for the occurrence of coevolutionary interactions. Bacteria have been widely used to test evolutionary hypotheses in vitro, but no previous studies have looked evolution in more than a pair of interacting species. The results will generate new conceptual understanding of the interaction between ecological and evolutionary processes in ecosystems facing environmental change.