An Artificial Life Approach to Host-Parasite Interactions

A spatially-explicit, individual-based model simulating host-parasite interactions between Townsend's ground squirrels (Spermophilus townsendii) and two of its parasites (an eimerian (Protozoa: Apicomplexa) and a helminth) was developed to assess the interactions of multiple factors (host immunity, parasite life-history, weather, and chance) on this system. Most functions in the model are driven by probabilities rather than deterministic equations. The model results corroborate observations from real squirrel populations and suggest that systems may be quite stable at high host densities, but that equilibrium may be unattainable at host densities often found in real systems because the equilibrium values change as conditions change. Altering the start-up parameters affected which mechanisms most strongly influenced the system—chance events were very important, especially at low host densities, suggesting that long-term studies are needed to fully understand year-to-year variation. Parasite life-history strategies had more influence on model outcome than the other parameters.