Field Problems: Biological Diversity and Habitat Disturbance Indicators

 

Species diversity not only describes how many species are in an ecological community (species richness) but also how those species are represented(species eveness). Both characteristics are necessary to distinguish between ecological communities that may have the same number of species but which are dominated by species to different degrees.

 

Using Simpson’s Index of Diversity, based on probability theory, one is able to measure the likelihood of two individuals drawn at random from a community will be the same species.

 

The Equation and a Sample Problem Using Simpson’s Index of Diversity:

 

Forest A has 20 trees of each of 5 species. Forest B has 96 trees of 1 species and 1 tree for each 4 rare species of trees. The diversity of these forest communities is very different even though they each have the same number of trees and tree species.

 

Simpson’s Index of Diversity is calculated as follows:

 

Forest A	Forest B

              

 

Where N = total number of all species, n_i = number of individuals of the i^th species.

 

Your final calculation may be thought of as a measure of dominance (0-1: low to high dominance). However, biologists usually use the reciprocal of Simpson’s Index as a measure of “it feels like” so many species. In this example the calculation for Forest A is .19 and Forest B is .92. The reciprocal of .19 is 5.2 and of .92 is 1.1. Therefore we can say that forest A feels like 5.2 species and Forest B 1.1 species of trees.

 

1.      Butterflies as Disturbance Indicators

 

Your field site may be a mosaic of forest and field patches of varying ages undergoing secondary succession. Disturbance may have been caused by past farming and silvaculture practices which create significant light gaps or “natural” disturbances such as fire, hurricane, or significant storms. Butterflies are used as an indicator of disturbance in that their diversity is inversely proportional to a forest’s integrity or lack of disturbance.

So…determine the species diversity of butterfly species found in your area using the Simpson’s Index of Diversity!