Lab 3

LABORATORY THREE

COMMUNITY ECOLOGY: ANALYSIS

OVERVIEW:

Last week you collected soil and litter samples from the local forest community. This week you will identify and quantify the animals present in those samples and use those data to calculate the species diversity in the community. To see how this works, imagine two communities with the same number of species, for simplicity, three. Community 1 has 95 individuals of species A, two of species B, and two of species C. Community 2 has 95 individuals of each species. We want to be able to distinguish between communities in which one species may contain the majority of all the individuals in the community, with the remaining minority divided among numerous additional species, and communities in which the populations of the various species are nearly equal. In this example, Community 2 should be treated as more diverse than Community 1. Because the number of individuals and the number of species are important, a mere inventory of the species would not express this difference. Today in lab we will determine the animal species diversity within the litter layer of the local forest community.

OBJECTIVES OF LABORATORY:

•To describe and quantify local soil and litter animal community structure.

•To learn how to use a taxonomic key.

•To learn methods for calculating species diversity in a community.

DEFINE:

Taxonomic key, Berlese funnel, dimensionless number, and diversity index

SPECIES IDENTIFICATION

Usually, years of training and experience are required to learn a particular group, such as insects, birds, or mammals. There may be as many as 20,000,000 species of insects, only about 10% of which have been named and described. Obviously, one afternoon cannot make you an expert on the many animal groups found in our samples.

We have designed and provided a taxonomic key for you to use. It will allow you to identify the major groups of animals present in the sample. Keep in mind that each group may consist of numbers of species which can look quite different from one another. Your instructor will demonstrate how to use a taxonomic key to make identifications and will help you if you run into difficulty. Useful illustrations accompany the key. Accurate identification of species is central to developing an accurate measure of species diversity.

SPECIES DIVERSITY

One way of expressing species diversity is as an index, or dimensionless number. A dimensionless number is simply a number that does not measure anything (like inches, seconds, or meters) but expresses a relationship. The index used here is Simpson's Diversity Index. Simpson's Index can assume values from 1.0 upwards. If a community contains only one species, the value will be one; as diversity increases, the value of the index increases. If a community contained thousands of species, each with only one or two individuals, Simpson's Index would become very large.

As you can see from the formula below, Simpson's Index takes into account not only how many species are present, but what proportion of the total number of individuals occurs in each species.

_N (N – 1)

D = Σn₁ (n₁ – 1)

In the formula, N is the total number of all individuals of all species found, and n₁ is the number of individuals in a single species. The denominator of the formula can be read as "the sum of n₁(n₁ - 1) for all species found."

Here's an example. Suppose we found four species in a sample (A, B, C, and D) represented by 20, 5, 10, and 45 individuals respectively. The total number of individuals in the sample is obviously 80, so the formula's numerator becomes 80 (80 - 1). For the denominator, we have to calculate n₁(n₁ - 1) for species A, B, C, and D separately, then sum them up:

Species A: 20(19) = 380 Species C: 10(9) = 90

Species B: 5(4) = 20 Species D: 45(44) = 1980

The total for all species is 2470. Our formula becomes:

D = 80 (79) = 6320 = 2.56

2470 2470

This value, 2.56, is quite useless by itself. A dimensionless number can only be used for comparative purposes. To give meaning to the number, one of two things has to be done. There must be comparisons between samples, or comparisons between communities. Let’s look at another example for comparison. Suppose a sample from a neighboring (but similar) community also had the same four species (A, B, C, and D) represented this time by 6, 5, 4, and 65 individuals, respectively, the total number of individuals remains 80. If you set up the calculation as above, you will see that D = 1.50. This indicates that the first community has greater species diversity than the second, at least by this measure. Of course, if the samples were collected by inexperienced observers, it is possible that the difference merely represents errors in identifying species or collecting samples.

LAB EXERCISE:

Your job is to accurately identify as many major groups as you can from sample you collected and prepared last week. Part of your grade for the lab this week will be based on the number of groups you correctly identify. However, there is a penalty for guessing. An incorrect identification will cancel out a correct one.

1. Working in groups of two or three, obtain your sample of soil animals from last week. Sort the animals out into groups that seem to you to be species. You might just label them A, B, C, etc. for this step.

2. Follow the dichotomous key from last week’s lab in order to accurately identify species; do not merely try to match animals with the pictures. The drawings are provided to aid in identifying key features. Your instructor will provide a card on which to write your identifications. Ask the instructor to check and initial each identification you make; at the end of the afternoon the cards will be collected for grading.

3. Record the numbers of individuals of each of the ten most common species.

4. Calculate the diversity index for your sample. Use the data sheet at the end of the exercise to record and calculate your data. Hand the sheet in for grading.

5. Record the total number of groups that you identified in your sample and the calculated index number on the board so that it can be compared with other teams. Record the values from all of the teams.

Assignment

Hand in the answers to questions 1 through 3, next week. Your instructor may ask you for a more formal report. Follow instructions carefully.

1. Compare your results with a second sample from a nearby community that was analyzed by another team. Explain any differences you observe.

2. Several determinations of Simpson’s Index were developed from a variety of communities. The values were: 5.4; 64.5; 17.3; and 2.1. Rank these communities from richest in species diversity to least rich. From these numbers, propose and describe examples of ecosystems that might reasonably produce such results (e.g., rain forest, desert, deciduous forest, coral reef, tundra, littoral). Explain your rationale.

3. Why might indices vary from several plots sampled from the same community? What are some sources of error in determining the index for each sample?

4. Read Laboratory Four, Interactions Among Organisms, before lab next week.

Notes:

Name ______________________________

Laboratory #3

n n-1 n(n-1)

Species A

N = Σn (n-1) =

D = N (N – 1) = ________________

Σn₁(n₁– 1)