Stability criteria for complex ecosystems

Analysis of stability criteria for different types of complex ecological network shows key differences between predator–prey interactions, which are stabilizing, and competitive and mutualistic interactions, which are destabilizing. Writing in Nature 40 years ago, Robert May questioned a central belief in ecology. The accepted wisdom was that complex ecosystems were more stable than simpler ones, but May proved that sufficiently large or complex ecological networks tend towards instability. Since then, much work in theoretical ecology has centred on learning why specific non-random networks are stable in practice. Stefano Allesina and Si Tang have analysed stability criteria for different types of realistic networks, and found key differences between predator–prey interactions, which are stabilizing, and competitive and mutualistic interactions, which are destabilizing. Forty years ago, May proved1,2 that sufficiently large or complex ecological networks have a probability of persisting that is close to zero, contrary to previous expectations3,4,5. May analysed large networks in which species interact at random1,2,6. However, in natural systems pairs of species have well-defined interactions (for example predator–prey, mutualistic or competitive). Here we extend May’s results to these relationships and find remarkable differences between predator–prey interactions, which are stabilizing, and mutualistic and competitive interactions, which are destabilizing. We provide analytic stability criteria for all cases. We use the criteria to prove that, counterintuitively, the probability of stability for predator–prey networks decreases when a realistic food web structure is imposed7,8 or if there is a large preponderance of weak interactions9,10. Similarly, stability is negatively affected by nestedness11,12,13,14 in bipartite mutualistic networks. These results are found by separating the contribution of network structure and interaction strengths to stability. Stable predator–prey networks can be arbitrarily large and complex, provided that predator–prey pairs are tightly coupled. The stability criteria are widely applicable, because they hold for any system of differential equations.