Question

compare and contrast Shannon-Weiner Index and Simpson Index in 200 words.

Answer 1

Measuring biodiversity of ecological communities Biodiversity is one of the primary interests of ecologists, but quantifying the species diversity of ecological communities is complicated. In addition to issues of statistical sampling, the rather arbitrary nature of delineating an ecological community, and the difficulty of positively identifying all of the species presents, species diversity itself has two separate components: 1.) the number of species present (species richness), and 2.) their relative abundances (termed dominance or evenness). As a result, many different measures (or indices) of biodiversity have been developed, and whole books have been written explaining and comparing them (Magurran 2004). Here, we will explore two measures of biodiversity: The Shannon index (Hʼ, also termed the Shannon-Wiener index) and the Simpson index (D). We will then consider one measure used to compare diversity among communities, Jaccardʼs similarity index. Shannon index The idea behind this index is that the diversity of a community is similar to the amount of information in a code or message. It is calculated in the following way: € H'= − p ∑ i ln pi where pi is the proportion of individuals found in species i. For a well-sampled community, we can estimate this proportion as Pi = ni/N, where ni is the number of individuals in species i and N is the total number of individuals in the community. Since by definition the pis will all be between zero and one, the natural log makes all of the terms of the summation negative, which is why we take the inverse of the sum. Interpretation: Typical values are generally between 1.5 and 3.5 in most ecological studies, and the index is rarely greater than 4. The Shannon index increases as both the richness and the evenness of the community increase. The fact that the index incorporates both components of biodiversity can be seen as both a strength and a weakness. It is a strength because it provides a simple, synthetic summary, but it is a weakness because it makes it difficult to compare communities that differ greatly in richness. Due to the confounding of richness and evenness in the Shannon index, many biodiversity researchers prefer to stick to two numbers for comparative studies, combining a direct estimate of species richness (the total number of species in the community, S) with some measure of dominance or evenness. The most common dominance measure is Simpsonʼs index.

Simpsonʼs index -Since evenness and dominance are simply two sides of the same coin, their measures are complementary. Simpsonʼs index is based on the probability of any two individuals drawn at random from an infinitely large community belonging to the same species: € D = pi 2 ∑ where again pi is the proportion of individuals found in species i. For a finite community, this is € D = ni (ni −1) N(N −1) ∑ Interpretation: Now D is a measure of dominance, so as D increases, diversity (in the sense of evenness) decreases. Thus, Simpsonʼs index is usually reported as its complement 1-D (or sometimes 1/D or –lnD). Since D takes on values between zero and one and approaches one in the limit of a monoculture, (1-D) provides an intuitive proportional measure of diversity that is much less sensitive to species richness.