On Whittaker's Components of Diversity

Suppose that the diversity of a collection of organisms is measured either by H', the Shannon—Wiener index, or by s, the species richness. For collections made from discrete habitat types, methods exist for decomposing such quantities into two components describing, respectively, the diversity within habitat types and the amount of turnover in the species composition between habitat types. The definitions of the components of H' and s are modified here to suit collections of organisms inhabiting a continuous gradient of habitat types. Since difficulties may arise in the evaluation of these components, however, an alternate set of components of s is also presented. Values of s and this latter set of components are presented for transect data obtained from salt marshes at latitudes from 44°40'N at Halifax, Nova Scotia, to 58°50'N at Churchill, Manitoba. A latitudinal diversity gradient is shown to be accompanied by a similar trend in the between—habitat component only. The lack of trend in the first component reduces the collection of possible causes of the diversity gradient.