Ecological non-monotonicity and its effects on complexity and stability of populations, communities and ecosystems

In traditional ecological models, the effects of abiotic and biotic factors are often assumed to be monotonic, i.e. either positive, negative or neutral. However, there has been growing evidence that non-monotonic effects of environmental factors and both intra- and inter-specific interactions can significantly influence the dynamics and stability of populations, communities and ecosystems. In this paper, we present a review and synthesis on both theoretical and empirical studies on ecological non-monotonicity. There are various non-monotonic relations observed in populations, communities and ecosystems. The non-monotonic function of per capita population increase rate against intrinsic or extrinsic factors is a significant driving force in determining the complexity and stability of biological systems. There are several mechanisms such as the law of tolerance, adaptive behaviors, or opposing dual or pathway effects which may result in non-monotonic functions. Ecological non-monotonic functions are often closely related to spatial and temporal scale processes which may explain why ecosystems are often highly variable and unpredictable in both space and time. Recognizing ecological non-monotonicity would greatly change our conventional monotonic views on the effects of environmental factors and species interactions on ecosystems. We appeal for more effort to study ecological non-monotonicity and re-think our strategies to manage ecosystems under accelerated global change.