The Hierarchical Nature of Community Persistence: A Problem of Scale

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Abstract
Community persistence exists within a hierarchial framework wherein the numerical scale of analysis can influence judgments about the stability of assemblages. Numerical resolution refers to whether data are analyzed in terms of the absolute abundances of species, abundance rankings, or species'' presence and absence. A community would be judged most stable when the absolute abundance of each species remains constant over time. At a lower level of stability, the abundance of individual species fluctuates, but abundance rankings remain constant over time. An even lower level of stability would involve assemblages in which both the absolute abundances and the abundance rankings of species fluctuate, but the same species are always present. The least stable condition would occur when even the presence and absence of species are unpredictable over time. Analysis of simulated communities showed that assemblages may be judged stable at some levels within this hierarchy but not at others; thus, statements about community persistence depend on the scale being used. Judgments about persistence were also shown to depend on the level of numerical resolution for a variety of real assemblages. Most assemblages were stable in terms of species'' abundance rankings or presence and absence but unstable when absolute abundances were analyzed. Most studies fail to examine persistence across this hierarchy and thus make claims about assemblage stability that may be appropriate only at certain analytical scales. The hierarchial nature of community persistence suggests that judgments about persistence may be appropriate only for a particular analytical scale, and therefore, community patterns should be analyzed and interpreted at more than one scale. Such multiscale comparisons help to resolve paradoxes that arise when different investigators, using different scales to examine the same communities, arrive at different conclusions about the factors structuring those communities.