Regular patterns link individual behavior to population persistence.

Resisting and recovering from disturbances is a necessity for most species. The strategy is sometimes collective, depending on the aggregation of interacting individuals into regular patterns. However, relating patterns of abundance across scales to both individual behavior and population persistence remains a major challenge for ecology. Such patterns are found in many ecosystems (1), ranging from microbes to forests, with their regularity taking the form of evenly sized and spaced bands and patches of aggregated individuals. Regular patterns are said to be selforganized when they emerge from local interactions among individuals that are a combination of positive and negative feedbacks (2). Positive feedbacks mean that growth and survival increase with the density of individuals. Such “safety in numbers” is found in many natural systems (3), including saltmarshes (4), arid vegetation (5, 6), and mussel beds (7), where individuals can gain protection from physical disturbances, such as waves or erosion. However, aggregation also means competing for limited resources, which leads to negative feedbacks between density and growth. The combination of positive and negative feedbacks illustrates the “balance of nature” (8), and could lead to a homogeneous distribution, but their properties can produce much more complex dynamics. First, their nonlinearity means that growth and survival can show abrupt changes with small changes in density, which can prevent populations from reaching an equilibrium state. Second, most ecological interactions among individuals occur over limited spatial scales (i.e., between neighbors). When the spatial extent of positive effects is shorter than the extent of negative competitive effects, regular patterns of aggregation can emerge (Fig. 1B). When it is the temporal scales of feedbacks that differ instead, self-organized patterns can emerge as a scale-free distribution of aggregated individuals (Fig. 1C). The regular “pattern” can then be defined as the ratio of small to large patches, which remains constant independent of the scale of observation (9). In this case, the regularity is one across scales rather than over space, and it has been documented in highly disturbed rain forests (10), arid ecosystems (11), and mussel beds (12), where the local propagation of a disturbance, such as fire or waves, is much faster than the local aggregation of individuals. In PNAS, de Paoli et al. (13) document a new class of pattern formation mechanisms based on scaleand density-dependent movement (Fig. 1A), which is shown to operate on fast temporal scales and to act synergistically with scale-dependent feedbacks to form large-scale regular patterns in natural mussel bed ecosystems. Self-organized patterns can occur over large spatial scales and result from collective interactions among individuals, often from modifications to their physical habitat. These patterns are also relevant to conservation and for the understanding of whole ecosystems when pattern-forming species are ecological engineers: By forming dense aggregates and modifying their physical environments, species such as Spartina (14) and mussels (15) create novel habitats for the assembly of communities and drive the productivity and stability of whole ecosystems. Theory and empirical evidence abound to document benefits of self-organized patterns for the persistence of ecosystem engineers and their association with maximum biomass and productivity in whole communities and ecosystems (5). However, establishing causal relationships linking individual behavior, pattern formation, population persistence, and ecosystem functions is a daunting task because of the range of scales involved and the emerging complexity that must be harnessed through experimental control. de Paoli et al. (13) provide experimental support for the behavioral basis of aggregation in marine mussel beds leading to self-organized patterns over multiple scales, as well as for their ecological implications for the persistence of mussel populations disturbed by waves and predation. Mussels feed on phytoplankton by filtering the water, and competition can thus arise between nearby individuals that deplete food transported with currents. However, competition builds up over a distance because it takes many individuals to deplete phytoplankton transported by current down to concentrations that will induce competitive interactions (16). On