We consider an evolving network of interacting species which exhibits self-organization. The system is characterized by repeated crashes in which a large number of species are extinct and subsequent recoveries. We investigate the macroscopic properties of this system before such crashes, concentrating on the variance of the relative population sizes of species and its evolution over time. A simple score function is constructed to determine the probability of a crash within a certain time interval to be used as a predictor for crashes. © 2004 Wiley Periodicals, Inc. Complexity 9: 24 -30, 2004 Key Words: crash; prediction; self-organization; evolving networks M any time series are characterized by rare but repeated large and sudden changes (crashes) with subsequent slow reversals (recoveries). Examples of such behavior include earthquakes, sand piles, the extinction of species in biological evolution, and chemical processes as well as social systems, where stock markets have been of particular interest. Empirical as well as theoretical contributions in most cases focus on the distribution of the size of such crashes and the waiting time between them. However, for practical applications it would be of great importance to find properties of these systems that allow to predict the occurrence of crashes. Recently it has been proposed that earthquakes are preceded by log-periodic oscillations [1,2]. A similar result has been reported for crashes in stock markets [3], and the literature mentioned therein, but the empirical evidence for these precursors in stock market crashes is thus far not conclusive. Furthermore, the origin of these oscillations remains undetected such that these precursors up to now lack a sound theoretical foundation for the mechanism causing the crash.In this article we consider a model of evolving networks of interacting species as has been found to be useful for a wide range of applications, e.g., in social systems [4 -6], evolutionary models [7,8], or chemical processes [9]. Investigating the network structure of such a model [10], finds that large crashes are the consequence of a structural change in the network itself, called a core-shift. A further result is that the network structure also affects the robustness of the network to minor exogenous changes causing crashes. In deriving these results it was necessary to know the complete structure of the network at any point of time; this we will call the microscopic properties of a network.