Metabolic rate links density to demography in Tetrahymena pyriformis

Many populations show density-dependent growth rates. We suggest that population growth rate may be connected to density through the density dependence of metabolic rate. If metabolic rate is an index of the total biochemical work being done by an organism, then as populations grow and density suppresses metabolic rate, the rate of reproduction should slow and the ability to avoid death should diminish. To test this idea, we grew axenic populations of a single-celled protist, Tetrahymena pyriformis, in laboratory microcosms, and measured metabolic rate and density. We also estimated division (birth) and death rates. The proposed connection was supported by two observations. First, increasing population density suppressed per-capita metabolic rate in accordance with the predictions of a resource-division model. The same pattern was shown with experimentally altered densities. Second, per-capita metabolic rate was positively related to per-capita division rate and negatively related to per-capita death rate. Thus, the particular pattern of population growth and regulation depends on exactly how individuals respond energetically to the density of conspecifics, and how they allocate metabolism to maintenance and production. The physiological basis of population regulation in this system is based on the constraints imposed on the total metabolic work that individual cells can perform.