Nonlinearities in Competitive Interactions: Component Variables or System Response

Nonlinear responses for component demographic variables do not necessarily imply nonlinear responses for net population variables. Two examples, one drawn from the laboratory and one from the field, contrast the responses of Daphnia brood size and per capita growth rate during density manipulations. While the response of brood size is markedly nonlinear, the response of the per capita growth rate is approximately linear. These examples caution against inferences about population change based solely on knowledge of single component variables. We also emphasize the importance of checking physiological adjustments over the entire density range.