Effects of Nutrient Recycling and Food-Chain Length on Resilience

Abstract

Food-web resilience, or the rate at which a food web returns to steady state following a perturbation, was investigated using both a simple abstract food-chain model and a complex food-web model for an ecosystem in which nutrient limitation and recycling occur. The food-chain model demonstrated that both nutrient input, or loading, and food-chain length have important effects on resilience. For very small nutrient inputs, resilience (or the inverse of the return time to steady state TR) was in all cases an increasing function of nutrient-input rate. However, the resilience of food chains in which the autotroph was the highest level was lower and constant over a wider range of input levels than the case in which herbivores were also present. The presence of a carnivore level complicated the dependence of resilience on nutrient input, leading to cases in which increased input could actually decrease resilience. All of these behaviors are consistent with the generalization that the return time, TR, tends to be similar to the turnover time of nutrient in the system. The top-down effects in the food chain influence how the nutrient turnover time, and thus the return time, TR, respond to changes in nutrient input. To test the robustness of the results of the abstract model, similar investigations were made of a detailed aquatic food-web model, the Comprehensive Aquatic-Simulation Model (CASM). The results of the study of resilience of CASM were in relative agreement with the abstract model for systems in which autotrophs and herbivores, respectively, constituted the highest trophic levels. The situation was more complex when carnivores were added. The results of both models strongly suggest that resilience may not always increase as nutrient input is increased and that resilience may not always decrease as the number of levels in a food chain increases.