Biodiversity

Biodiversity, or all the various forms of life that currently exist on the earth, is being intensively studied. Biodiversity may include species diversity, genetic diversity, and ecological diversity. Many scientists and lay people believe that the earth's biodiversity is in jeopardy due to the marked increase in the rate of species extinction. The purpose of this project is to familiarize you with one measure of species diversity that uses logarithms in its calculation.

Species diversity is a community characteristic that describes structure within a group of organisms. A community has a low species diversity if only a few species are present, or if only a few species are abundant. In contrast, a community exhibits a high species diversity if many species are present and they occur in equal or nearly equal numbers. A high species diversity often indicates a highly complex community with many interspecific interactions. Some scientists also believe that high species diversity results in high community stability (ability to be unaffected by disturbance); however, this notion is subject to considerable debate.

The simplest measure of species diversity is species richness. Species richness is a count of the total number of species in an area and doesn't consider the relative abundance of each species in the community. More elaborate measures of species diversity consider both the total number of species in an area as well as the number of individuals of each species. These indices are based on information theory and they were first applied to community analysis by MacArthur (1955) and Margalef (1958)¹. The basic premise behind the "uncertainty'' concept of information theory is that if a community has a low species diversity, you can be relatively certain of the identity of the species when one is selected at random from the community. In contrast, it would be much more difficult to predict the identity of a individual randomly selected from a highly diverse community.

The Shannon diversity index is one of the most widely-used calculations in the ecological literature. The equation for the Shannon diversity index is:

where k is the number of different species, p_i = n_i / N where n_i is the number of individuals within species i, and N is the total number of individuals of all species. The symbol means to add up all of the terms following the summation symbol.

For example

means to sum 1+2+3+4+5. Notice that the number under the summation symbol tells you at what number you should start and the number above the summation symbol tells you at what number to end.

If the sum is

then you are to find p₁+p₂+p₃+p₄+p₅+p₆+p₇.

a. Calculate the Shannon diversity index for the Native Prairie and the Shannon diversity index for the Improved Pasture using the following data. Compare these two indices. Which habitat is more diverse?

Number of small mammals captured in native prairie and improved pasture by species

b. Obtain a "bug'' net, kill jar, and zip-lock bag from your instructor. Select two different study sites in which you would expect to find different communities of "bugs'' (e.g., pasture and wooded area; lawn and roadside ditch). To collect "bugs'' in the first area, make a sweeping motion of the net back-and-forth in front of you as you walk 30 paces. As you collect bugs in your net, carefully drop them into the kill jar. Remove the bugs and place them in a zip-lock bag. After you are done collecting your bugs, spread the dead bugs out on a table and separate them into groups of similar-looking bugs. Identify the number of different types of bugs you have (e.g., moths, beetles, ants) and the number of individuals of each type and use this information to make a chart similar to the chart in part a. Repeat this procedure for the second habitat. Use this data to calculate the Shannon diversity index for your two bug communities. What does the Shannon diversity index tell you? How many types did you have in each habitat? Were the communities of bugs in the two areas similar? Explain why the two communities of bugs were similar or different. Do you think your sample was a good indicator of the bugs present in each community?