Discontinuities in ecological data.

H istorically, ecology has focused on continuous distributions and smooth transitions. Only recently have discontinuities and thresholds become an explicit focus in some areas of ecology, especially in the realm of complex systems. The study of animal body mass distributions has been recognized for its potential to provide insight into the underlying processes shaping animal communities. Hutchinson (1) formalized the understanding of species niches and the potential for competition to shape body mass distributions. However, despite a long history of theoretical and empirical pursuit, the mechanisms driving patterns in body mass distributions remain poorly understood. The work of Scheffer and van Nes (2) in this issue of PNAS demonstrates that community interactions alone can create discontinuous, lumpy distributions of simulated species along a niche axis. Their contribution comes at a time of heightened interest in understanding the mechanisms that may lead to discontinuities in body mass or biomass distributions. Much of the renewed interest in body mass distributions has followed the publication of a provocative ecological monograph that suggested animal body mass distributions are entrained by landscape structure (3). Holling’s paper (3) initially spawned skepticism that body mass distributions are characterized by what Holling termed ‘‘lumps’’ and ‘‘gaps.’’ Currently, many ecologists accept that body size distributions are discontinuous, but there remains disagreement regarding the mechanisms responsible. One mechanism proposed focuses on interactions among species living in the same habitat. The strongest and clearest species interaction, other than predation, is competition. However, facilitative interactions are also increasingly recognized for their potential to shape community structure. Scheffer and van Nes (2) demonstrate that species interactions may result in both repulsion and attraction along a niche axis. Attraction occurs when species are similar enough to avoid competition and results in aggregations (lumps) of species, and competition also repulses and disallows species of moderate similarity, resulting in species distributions that are both discontinuous and aggregated. Roughgarden (4) also recognized that species interactions have a strong effect on the distribution of species and that competitive interactions can lead to both aggregation and discontinuity along a niche axis. Interestingly, similar patterns have been demonstrated for social–economic systems. Discontinuities have been found in international economic data (5), where the variable of interest was gross domestic product per capita. A discontinuous distribution was found to persist over time, and the overall structure seemed to bound the growth trajectories of individual countries. Explaining the mechanisms behind discontinuities in economic processes is difficult. Barro (6), for example, has hypothesized the existence of a limited number of ‘‘convergence clubs’’ in gross domestic product