Dynamics of a food chain model from an arthropod-dominated lotic community

A simulation model, based on a series of Holling type II differential equations, was used to assess the effects of omnivory on the dynamics of individual populations at different trophic levels, as well as on the stability and persistence of food chains. Initial population densities were euqal to actual field estimates, whereas predation rates and population growth rates were obtained from the literature for the top predator (Corydalus cornutus), secondary consumer (Ambrysus circumcinctus), and primary consumer (Petrophila sp.). Fluctuations in population density depended on the trophic level at which each population feeds. Demographic trajectories of primary and secondary consumers was similar to those produced by Lotka-Volterra models; the pattern of fluctuation in both populations resembled a sine function, with the predator population lagging behind the prey population. In the model without omnivory (minimal connectance), the range of fluctuation in population density was the greatest for the secondary consumer, followed by the primary, and then tertiary consumers. Low levels of consumption of the primary consumer by the top predator (between 1% and 4%) increased the minimum population density point of all consumer populations. Higher consumption rates produced wider oscillations and populations quickly became extinct. Sensitivity analysis revealed that the model was highly sensitive to initial conditions. A 5% increase in the initial population density of the primary consumer produced an output that diverged exponentially from the original output (λ = 0.012), confirming that the system behaved chaotically. This suggests that pervasive fluctuations in population density that are characteristic of many aquatic insects could be the product of deterministic rather than stochastic processes.