Categories of chaos and fractal basin boundaries in forced predator–prey models

Biological communities are affected by perturbations that frequently occur in a more-or-less periodic fashion. In this communication we use the circle map to summarize the dynamics of one such community – the periodically forced Lotka–Volterra predator–prey system. As might be expected, we show that the latter system generates a classic devilʹs staircase and Arnold tongues, similar to that found from a qualitative analysis of the circle map. The circle map has other subtle features that make it useful for explaining the two qualitatively distinct forms of chaos recently noted in numerical studies of the forced Lotka–Volterra system. In the regions of overlapping tongues, coexisting attractors may be found in the Lotka–Volterra system, including at least one example of three alternative attractors, the separatrices of which are fractal and, in one specific case, Wada. The analysis is extended to a periodically forced tritrophic foodweb model that is chaotic. Interestingly, mode-locking Arnold tongue structures are observed in the model’s phase dynamics even though the foodweb equations are chaotic.