We study a system of N ≫ 1 degrees of freedom coupled via a smooth homogeneous Gaussian vector field with both gradient and divergence-free components. In the absence of coupling, the system is exponentially relaxing to an equilibrium with rate μ. We show that, while increasing the ratio of the coupling strength to the relaxation rate, the system experiences an abrupt transition from a topologically trivial phase portrait with a single equilibrium into a topologically nontrivial regime characterized by an exponential number of equilibria, the vast majority of which are expected to be unstable. It is suggested that this picture provides a global view on the nature of the May−Wigner instability transition originally discovered by local linear stability analysis.complex systems | equilibrium | model ecosystems | random matrices W ill diversity make a food chain more or less stable? The prevailing view in the midtwentieth century was that diverse ecosystems have greater resilience to recover from events displacing the system from equilibrium and hence are more stable. This "ecological intuition" was challenged by Robert May in 1972 (1). At that time, computer simulations suggested that large complex systems assembled at random might become unstable as the system complexity increases (2). May's 1972 paper complemented that work with an analytic investigation of the neighborhood stability of a model ecosystem whereby N species at equilibrium are subject to random interactions.The time evolution of large complex systems, of which model ecosystems is one example, is often described within the general mathematical framework of coupled first-order nonlinear ordinary differential equations (ODEs). In the context of generic systems, the Hartman−Grobner theorem then asserts that the neighborhood stability of a typical equilibrium can be studied by replacing the nonlinear interaction functions near the equilibrium with their linear approximations. It is along these lines that May suggested looking at the linear modelto study the stability of large complex systems. Here J = ðJ jk Þ is the coupling matrix and μ > 0. In the absence of interactions, i.e., when all J jk = 0, system [1] is self-regulating: If disturbed from the equilibrium y 1 = y 2 = . . . = y N = 0, it returns back with some characteristic relaxation time set by μ. In an ecological context, y j ðtÞ is interpreted as the variation about the equilibrium value, y j = 0, in the population density of species j at time t. The element J jk of the coupling matrix J, which is known as the community matrix in ecology, measures the per capita effect of species k on species j at the presumed equilibrium. Generically, the community matrix is asymmetric, J jk ≠ J kj . For complex multispecies systems, information about the interaction between species is rarely available at the level of detail sufficient for the exact computation of the community matrix and a subsequent stability analysis. Instead, May considered an ensemble of community matrices J assembled at random, whereby ...