NONLINEAR AND LOGISTIC GROWTH IN EXPERIMENTAL POPULATIONS OF GUPPIES

Previous experiments have shown that for a specific experimental system of guppies (Poecilia reticulata), per capita growth rate is linearly related to population biomass. A length-specific population model is developed to examine how the various mechanisms of density dependence (i.e., changes in somatic growth rates, reproductive rates, and cannibalism) act in concert to shape the overall population response to intraspecific competition. Despite many nonlinear components, equilibrium per capita growth in the model is linearly related to population biomass, mimicking the logistic growth seen in the experiment. Using the model, the components of density dependence are varied to determine how each component shapes the overall density-dependent response. Density dependence in somatic growth rates is found to be the most important population-regulating mechanism in this system. In the past, emphasis has been placed on density dependence in birth and death functions. The importance of somatic growth in shaping per capita population growth curves may be underestimated.