Stable coexistence in indefinitely large systems of competing species.

The Lotka-Volterra system is a set of ordinary differential equations describing growth of interacting ecological species. One of the debated questions is understanding how the number of species in the system, influences the stability of the model. Robert May studied how large systems may become unstable when species-species interactions do not vanish. This outcome has frequently been interpreted as a universal phenomenon and summarised as "large systems are unstable". By exploring general interaction networks, we show that the competitive Lotka-Volterra system may maintain stability even for large networks, despite non-vanishing interaction strength. We establish sufficient conditions for stability as a threshold on the interspecific interaction strength, formulated in terms of the maximum and minimum degrees (or weights) rather than the network's size. For values below this threshold, coexistence of all species is attained, regardless of the system size.Our finding generalises May's result by showing that it is the outlier nodes with large degree that cause instability, rather than the large number of species in the system.