(This exercise is based on Gray, J. S. 2000. The measurement of marine species diversity, with an application to the benthic fauna of the Norwegian continental shelf. Journal of Experimental Marine Biology and Ecology 250: 23–49.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
As discussed in Chapter 15 of the textbook, species diversity is a measure of both the total number of species in the locality (species richness) as well as their frequency. Two sites that each had ten species would have the same species richness, but their species diversities could be different (and sometimes dramatically so). For example, a site where one species constituted 91% of the total individuals and all the other species constituted 1%, each would be considered much less diverse than a site in which each of the ten species constituted 10% of the total number of individuals.
Ecologists have developed several indices to measure species diversity. One of these is the Shannon index, which is discussed in the textbook. Also sometimes known as the Shannon–Weaver or Shannon–Wiener index (H′), this measure takes the sums of the products of the frequency of each species times the logarithm of the frequency.
In the textbook, the natural logarithm (log base e) is used, but many ecologists use log base 2 (log2) instead, which is what we will consider here.
John Gray at the Biologisk Institutt of the Universitetet i Oslo (Norway) argues in his paper that other indices of species diversity might be more informative. One of these is the Simpson’s index (HD2), which is the reciprocal of the sum of the squares of the frequencies.
HD2 = 1/(p12 + p22 + …pn2) where p1, p2, …pn represent each species.
Table 1 (Click image to enlarge.)
Question 1
Gray applied these and other indices to a particular set of data from marine species taken along transect (a straight line) across the Norwegian continental shelf. Table 1 lists the five general areas (or regions) surveyed. In which area were the most individuals sampled?
Question 2
In which area were the most species sampled?
Question 3
Which area had the lowest number of species per unit area?
Question 4
Which area had the lowest number of total individuals per unit area?
Question 5
Which area had the lowest number of total individuals per species sampled?
Figure 1 Plot of species richness at various points across the transect. The points are arranged north to south. Key: Open triangles, Heidrun; diamonds, Snorre; solid triangles, Gullfaks; squares, Ekofisk; circles, Tommeliten. (Click image to enlarge.)
Question 6
In relative terms, what is the species richness of sites within Heidrun? Does it show any trend with respect to direction?
Question 7
As one goes from south to north in Tommeliten, what happens to the species richness?
Figure 2 Plot of the Shannon index of species diversity across the transect. The points are arranged north to south. Key: open triangles, Heidrun; diamonds, Snorre; solid triangles, Gullfaks; squares, Ekofisk; circles, Tommeliten. (Click image to enlarge.)
Question 8
Describe the patterns seen in the Shannon index of species diversity across the transect.
Figure 3 Plot of the Simpson’s index of species diversity across the transect. The points are arranged north to south. Key: open triangles, Heidrun; diamonds, Snorre; solid triangles, Gullfaks; squares, Ekofisk; circles, Tommeliten. (Click image to enlarge.)
Question 9
Describe the patterns seen in the Simpson’s index of species diversity across the transect.
Question 10
Gray notes that the Simpson’s index is most affected by the frequencies of the most common two or three species. In contrast, the Shannon–Weaver (or Shannon–Wiener) index is most determined by the frequencies of species of medium commonness. Based on this, what conclusions about this study can you draw?
