Introduction to the Modeling and Analysis of Complex Systems

19.4. ECOLOGICAL AND EVOLUTIONARY MODELS 457agents = []rdata = []fdata = []...def observe():global agents, rdata, fdatasubplot(2, 1, 1)cla()rabbits = [ag for ag in agents if ag.type == ’r’]rdata.append(len(rabbits))if len(rabbits) > 0:x = [ag.x for ag in rabbits]y = [ag.y for ag in rabbits]plot(x, y, ’b.’)foxes = [ag for ag in agents if ag.type == ’f’]fdata.append(len(foxes))if len(foxes) > 0:x = [ag.x for ag in foxes]y = [ag.y for ag in foxes]plot(x, y, ’ro’)axis(’image’)axis([0, 1, 0, 1])subplot(2, 1, 2)cla()plot(rdata, label = ’prey’)plot(fdata, label = ’predator’)legend()A typical simulation result with this revised code is shown in Fig. 19.6. You can seethat the populations of the two species are definitely showing oscillatory dynamics, yetthey are nothing like the regular, cyclic ones predicted by equation-based models (e.g.,Fig. 4.8). Instead, there are significant fluctuations and the period of the oscillations isnot regular either. Spatial extension, discreteness of individual agents, and stochasticity