Opposite trends in response for the Shannon and Simpson indices of landscape diversity

Abstract

Among the most popular of metrics used to quantify landscape composition are Shannon’s index, believed to emphasize the richness component of diversity, and Simpson’s index, emphasizing the evenness component. These indices can show considerable variation in response to changes in landscape richness and evenness. However, the possibility of encountering opposite trends in the responses of these indices to assemblages that differ in only a single component of diversity has not been sufficiently acknowledged. An opposite response of these indices was observed for two Indian landscapes with the same richness, differing only in evenness. Using a numerical simulation, the likelihood of encountering landscapes with an opposite response was demonstrated to increase with increasing richness, from about 4% for landscapes consisting of three cover types, to about 6% for ten types. This emphasizes the need for caution when choosing an index of landscape diversity. Rare cover types provide habitats for sensitive species and facilitate critical ecological processes. The Shannon index, sensitive to their presence, is therefore recommended for landscape management within an ecological framework. Simpson’s index, more responsive to the dominant cover type, can be used for specific situations where the dominant cover type is of interest, such as single-species reserve design.