Environment-organism feedbacks drive changes in ecological interactions

Abstract

Ecological interactions, the impact of one organism on the growth and death of another, underpin our understanding of the long-term composition and the functional properties of communities. In recent years, the context-dependency of interactions - their tendency to change values in different environments, locations and at different times - has become an increasingly important theme in ecological research. However, an overarching theoretical assumption has been that external environmental factors are responsible for driving these changes. Here, we derive a theoretical interaction framework which teases apart the separate roles played by these extrinsic environmental inputs and the intrinsic environmental changes driven by organisms within the environment itself. At the heart of our theory is the instantaneous interaction, a quantity that captures the feedback between environmental composition and the growth of organisms within it. In the limit that intrinsic, organismdriven environmental change dominates over external drivers, we find that this feedback can give rise to temporal and spatial context-dependencies as organisms modify the environment over time and/or space. We use small synthetic microbial communities as model ecosystems to demonstrate the power of this framework, using it to predict time-dependent intra-specific interactions in a toxin degradation system and to relate time and spatial dependencies in crossfeeding communities. Our framework helps to explain the ubiquity of interaction context-dependencies in systems where population changes are driven by environmental changes - such as microbial communities - by placing the environment on an equal theoretical footing as the organisms that inhabit it.