On detecting interactions between species in population dynamics

If we are to progress out of our current state of uncertainty about the role of interspecific competition in community structure, it is essential that we can distinguish competition from other types of population interaction, and from lack of interaction, in particular case studies. To make such distinctions, it is necessary to quantify the effect of species on each other. One way to do this is to calculate interaction coefficients, and another (only readily applicable in experimental systems) is to plot graphs of N against time and to contrast monocultures with mixed cultures. We show (1) that these methods often appear to give contradictory results, and (2) that the problems are most pronounced when one species has a low equilibrium population size in mixed culture. To resolve the question of whether an interspecific interaction is taking place (and if so, what kind of interaction), it is necessary to apply tests of significance which overcome the problems of serial correlation inherent in all long-term population experiments. We illustrate the use of such tests in the analysis of the results from an experimental Drosophiia system. In the past, this kind of test has generally not been applied, and this raises the question of whether some of the 'classic cases of competition' in the experimental literature were really competition at all.