Rapid plastic shifts in body size precede and determine population growth

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, how body size influences, and is influenced by, population growth and density is less clear. Specifically, 1) whether body size, or population dynamics, more strongly influences the other, and, 2) whether observed changes in body size are due to plasticity or rapid evolutionary change, are not well understood.Here, we address these two issues by experimentally tracking population density and mean body size in the protist Tetrahymena pyriformis as it grows from low density to carrying capacity. We then use state-of-the-art time series analyses to infer the direction, magnitude, and causality of the link between body size and ecological dynamics. Last, we fit two alternative dynamical models to our empirical time series to assess whether plasticity or rapid evolution better explains changes in mean body size.Our results clearly indicate that changes in body size precede and determine changes in population density, not the other way around. We also show that a model assuming that size changes via plasticity more parsimoniously explains these observed coupled phenotypic and ecological dynamics than one that assumes rapid evolution drives changes in size.Together these results suggest that rapid, plastic phenotypic change not only occurs well within ecological timescales but may even precede—and causally influence—ecological dynamics. Furthermore, large individuals may be favored and fuel high population growth rates when population density is low, but smaller individuals may be favored once populations reach carrying capacity and resources become scarcer. Thus, rapid plastic changes in functional traits may play a fundamental and currently unrecognized role in familiar ecological processes like logistic population growth.