Adaptive habitat selection by a single celled alga?

Abstract

Faculty and students in the Department of Biology are bound together by a common interest in explaining the diversity of life, the fit between form and function, and the distribution and abundance of organisms. The research reported here shows how habitat selection and adaptive movement influence population dynamics. I first demonstrate by theory why habitat selection should vary with density. I then describe experiments in which I measured per capita population growth rates of the single-celled alga, Chlamydomonas reinhardtii, in separate shaded and unshaded (light) habitats across a range of densities, and used them to predict the number of cells that one should observe in adjacent habitats. Fitness was higher in the light habitat than in the shade. Using these observations, theory predicts that all individuals should occupy the light habitat at low density, but as density increases individuals should increasingly occupy the shade. More generally, individual cells should move to habitats of higher mean fitness (adaptive movement). My experiments on Chlamydomonas demonstrated a rather novel form of adaptive movement that only partially confirmed the predictions. The fitness of cells released in the light habitat in rich environments was not different from that in shade because density was higher in light. Cells released in shade, and those released in poor environments, moved such that there were no differences in density between habitats, even though fitness was usually higher in light. Adaptive movement thus depends not only on the quality of the occupied habitat, but also on mean environmental quality.