  The traditional assumption of a general positive correlation between diversity (as richness) and stability has been seriously challenged on both theoretical and empirical grounds since the  1970s.[52]  However, more recently, Tilman has suggested an empirical connection between richness and stability, interpreted as constancy, in grassland  habitats.[53]  The scope of this generalization, even whether it can be replicated for other grasslands than those that Tilman studied, remains to be investigated. Meanwhile, Pfisterer and Schmid have produced equally compelling empirical evidence that richness is inversely correlated with stability, interpreted as resilience and  resistance.[54]  Much remains to be found out. All that is certain is that McCann's confident 2000 verdict in favor of a positive diversity-stability relationship was  premature.[55]  Note, finally, that virtually no theoretical or practical exploration of this question has used concepts of diversity other than richness. At least at the theoretical level, this remains an open field for philosophers. (Clear formal results would not go unnoticed by ecologists.)