On the Evidence Needed to Judge Ecological Stability or Persistence

To see whether real ecosystems (as opposed to model ones) are stable, i.e., whether they exhibit resistance to short-term perturbations or adjustment following them, stable equilibria must be identified. To do this and still avoid trivial results, certain criteria of scale must be satisfied. To judge resistance, the strength of the perturbation capable of overcoming it must be estimated, and this usually requires experimentation. On a temporal scale, the fate of all adults of the population or community must either be followed for a minimal period of at least one complete turnover, or their replacement probabilities estimated. In regard to space, if one finds instability, this may apply only to the area studied, not to larger areas. However it is useful to define the spatial scale for which instability versus stability applies. An analysis of census data from many long-term studies revealed a continuum of temporal variability in the dynamics of natural populations and communities. There is no clear demarcation between assemblages that may exist in an equilibrium state and those that do not. Only a few examples of what might be stable limit cycles were found. There was no evidence of multiple stable states in unexploited natural populations or communities. Previously published claims for their existence either have used inappropriate scales in time or space, or have compared populations or communities living in very different physical environments, or have simply misconstrued the evidence. Rather than the physicist's classical ideas of stability, the concept of persistence within stochastically defined bounds is, in our opinion, more applicable to real ecological systems.