Succession of Species within a Community: Chronological Clustering, with Applications to Marine and Freshwater Zooplankton

Abstract

A clustering method is presented to describe the discontinuities in a multivariate (multispecies) series of biological samples, obtained from a single station at successive times. The method takes into account the sequence of sampling (time contiguity constraint) and makes it possible to eliminate singletons. Such singletons can be found in most ecological series, due to random components or to external forcings such as a temporary shift of water masses or immigration and emigration at a fixed station. The clustering proceeds from a sample x sample association matrix, built with an appropriately chosen similarity or distance coefficient. Agglomerative clustering is applied with the time constraint, and a randomization test is performed to verify whether the fusion is valid. This test compares the number of "high" distances in the between-group matrix to that in the fusion matrix of the two groups tested. When a singleton is discovered, with this same test, it is temporarily removed from the study and the clustering is started again from the beginning. This procedure is used because a singleton can well disturb the whole clustering geometry, as a result of the contiguity constraint. The end result is a nonhierarchical partition of the series into nonoverlapping homogeneous groups, which are the steps of the ecological succession. Interval graph tests and a posteriori tests help to understand the relationships between the groups, as well as the origin of the singletons. Examples are presented of a 78-wk Mediterranean zooplankton (chaetognaths) series and also of a 28-mo series of the zooplankton community in a freshwater reservoir. The results are compared to those obtained with several other methods of analyzing succession data. In the Appendixes we discuss various mathematical aspects of the work.