Samuel Johnson and Miguel A. Muñoz There has recently been much interest in a property of ecological networks known as nestedness, and its relation to the conservation of biodiversity [Bastolla et al., 2009]. However, there seems to be a lack of consensus on the precise meaning of the term. We propose an interpretation of nestedness as a local property, related to the global definition proposed by Bastolla et al., which allows us to theoretically estimate the expected value for the typical heterogeneous-network null model known as the configuration ensemble. We go on to relate the concept to other topological features, asses the significance of nestedness in some real networks, and discuss mechanisms by which such a property might come about in nature. U. Bastolla et al., The architecture of mutualistic networks minimizes competition and increases biodiversity, Nature 458, 1018 (2009).