Feedbacks between size and density determine rapid eco‐phenotypic dynamics

Abstract

AbstractBody size is a fundamental trait linked to many ecological processes—from individuals to ecosystems. Although the effects of body size on metabolism are well‐known, the potential reciprocal effects of body size and density are less clear. Specifically, (a) whether changes in body size or density more strongly influence the other and (b) whether coupled rapid changes in body size and density are due to plasticity, rapid evolutionary change or a combination of both.Here, we address these two issues by experimentally tracking population density and mean body size in the protistTetrahymena pyriformisas it grows from low density to carrying capacity. We then use Convergent Cross Mapping time series analyses to infer the direction, magnitude and causality of the link between body size and population dynamics. We confirm the results of our analysis by experimentally manipulating body size and density while keeping the other constant. Last, we fit mathematical models to our experimental time series that account for purely plastic change in body size, rapid evolution in size, or a combination of both, to gain insights into the processes that most likely explain the observed dynamics.Our results indicate that changes in body size more strongly influence changes in density than the other way around, but also show that there is reciprocity in this effect (i.e., a feedback). We show that a model that only accounts for purely plastic change in size most parsimoniously explains observed, coupled phenotypic and ecological dynamics.Together, these results suggest (a) that body size can shift dramatically through plasticity, well within ecological timescales, (b) that rapid changes in body size may have a larger effect on population dynamics than the reverse, but (c) phenotypic and population dynamics influence each other as populations grow. Overall, we show that rapid plastic changes in functional traits like body size may play a fundamental—but currently unrecognized—role in familiar ecological processes such as logistic population growth.Read the freePlain Language Summaryfor this article on the Journal blog.