Temporally Concordant Structure of a Fish Assemblage: Bound or Determined?

Statistically significant values of Kendall's W as a measure of temporal concordance in the rank order of fish abundances have recently been used to infer that many local assemblages tend toward equilibrium and "deterministic" regulation via species interactions. This criterion has also been criticized as being overly simplistic and misleading. For example, concordances significantly different from zero may be expected despite marked random fluctuations simply because large numerical gaps occur between common and rare species in most natural communities. We used simulation models to estimate the likelihood of obtaining significant values despite random yearly changes in the densities of five commonly co-occurring marine reef fishes (surfperches). Significance was obtained even when "realistic" bounds to the amount of yearly change in populations, suggested by 12 yr of censusing on one reef, were severely relaxed. Observations of surfperch populations on reefs at other localities indicate that, despite uniformly high concordances of local guild structures, annual variation in species densities was either large or small depending on environmental constraints to offshore migrations. Thus, concordance is not a reliable measure of the intrinsic regulation of local species populations. It may simply reflect broad-scale regional patterns. It cannot be reliably used to distinguish deterministic (intrinsic) from stochastic (extrinsic) factors affecting assemblage structure through time.