Species Abundance
The Lognormal Distribution
If a community is thoroughly sampled and the number of species in the community is plotted (Y axis) against the individuals per species (X axis), a bell shaped curve is often produced.
Figure 1. A hypothetical lognormal distribution.
A bell shaped curve (called a normal distribution) indicates that most species have an intermediate number of individuals. A few species have very many individuals and a few species have very few individuals. In order to produce a normal distribution, the categories on the X axis are plotted so that each one is twice as great as the preceding one (1, 2, 3, 8, 16, 32, etc.), creating a log2 scale. The distribution thus produced is called a lognormal distribution.
Example
Suppose that a community was thoroughly sampled and the following data were collected.
Number of Species | Number of Individuals |
2 | 1 - 2 |
6 | 3 - 4 |
17 | 5 - 8 |
20 | 9 - 16 |
8 | 17 - 32 |
3 | 33 - 64 |
1 | 65 - 128 |
The table above tells us that 57 species were found in this community. Two of the species were rare with only one or two individuals. One of the species was abundant with more than 65 individuals. Twenty species had between 9 and 16 individuals. Notice that each category of Number of Individuals is twice as large as the preceding one. These data are plotted below.
Figure 2. Lognormal distribution of species in a hypothetical community.
Plotting proportional abundance against rank in the community produces a rank-abundance curve. This curve can provide information on species diversity and richness. An example will illustrate how this is done. Suppose that two communities (community A and community B) are sampled and the number of individuals of each species is tabulated. The two tables below show the number of individuals of each species in each of the two hypothetical communities.
| Community A |
| Species | Number of Individuals |
| Species #1 | 22 |
| Species #2 | 12 |
| Species #3 | 36 |
| Species #4 | 1 |
| Species #5 | 2 |
| Species #6 | 7 |
Community B |
Species | Number of Individuals |
Species #1 | 12 |
Species #2 | 4 |
Species #3 | 51 |
Species #4 | 3 |
Species #5 | 1 |
To create a rank-abundance curve, determine the proportional abundance of each species. This is done by dividing the number of individuals of a species by the total number of individuals in the community. This is done below. The tables below also show the species ranked by abundance. The most abundant species is ranked #1.
Community A |
Species | Number of Individuals | Proportional Abundance | Rank |
Species #1 | 22 | = 22/80 = 0.275 | 2 |
Species #2 | 12 | = 12/80 = 0.15 | 3 |
Species #3 | 36 | 0.45 | 1 |
Species #4 | 1 | 0.013 | 6 |
Species #5 | 2 | 0.025 | 5 |
Species #6 | 7 | 0.088 | 4 |
Total | 80 | | |
Community B |
Species | Number of Individuals | Proportional Abundance | Rank |
Species #1 | 12 | 0.169 | 2 |
Species #2 | 4 | 0.056 | 3 |
Species #3 | 51 | 0.718 | 1 |
Species #4 | 3 | 0.042 | 4 |
Species #5 | 1 | 0.014 | 5 |
Total | 71 | | |
Above: The proportional abundance and rank of the species in two hypothetical communities.
These data should be plotted on semi-log paper with Proportional abundance on the Y axis and Rank on the X axis.
Figure 3. Rank-abundance plot for two hypothetical communities.
The slope of a line on the graph are associated with evenness. If most of the species have a similar number of individuals (high evenness), the line on the graph will have a smaller slope. A horizontal line indicates that all of the species have the same number of individuals. A community that is dominated by one or a few species will have a steep slope.
Species diversity is a function of species richness (number of species) and evenness. Community A in the graph above is more diverse than Community B because the line has more evenness (less slope) and because it has a higher species richness (6 species).
Materials Needed
A 0.10 m2 sampling ring may be needed for part of this lab This ring was previously used in the plot sampling lab. Refer to this lab if you need to construct a ring.
Data Collection
The sampling described in this section will be done outdoors. The remainder of the laboratory exercise involves calculations and graphing and can be done indoors. The data will be used to create lognormal and rank-abundance plots as described above.
Choose one of the options below and sample the organisms as described. Try to choose an area that has had minimal disturbance by humans for two or more years. Some examples of places to avoid are lawns, gardens, and recent agricultural fields.
Count the number of individuals of each species that are present in all of the samples. If you do not know the names of the species, assign them a number such as species #1, species #2, etc.
Choose only one of the options listed below. If you prefer to use another ecosystem instead of one listed below (ex: a marine ecosystem), please ask your instructor before beginning.
Option 1: Herbaceous Plants in a Forest Floor or Abandoned Field
Count the number of nonwoody plants growing on a forest floor or in an abandoned field by taking 25 random samples using a 0.1 m2 sampling ring. Do not count mosses, lichens, or fungi. If you do not know the name of a species, give it a number (example: species 1, species 2, etc.). To aid you with consistency in identification, it might be helpful to write a brief description of each species as you proceed with the samples. For this exercise it is not necessary measure the diameter of plants as was done in the plot sampling lab. Enter your data in table 1.
You may encounter a cluster of stems of the same species and cannot tell if they are all the same individual or are several individual plants. It may be necessary to move the soil near the base of the stems and explore with your finger. If the stems are all attached near the base, they should be counted as a single plant. However, if the stems appear to have separate root systems, count them as individual plants.
If you sample the same community that you sampled in the plot sampling lab, you can use those samples here to reduce the number of samples necessary.
Refer to the plot sampling lab for more information on the sampling technique.
Option 2: Forest Trees
Use the Point-Quarter method to estimate the number of individuals of each tree species in a woodlot. Sample locations can be determined by walking through a forest and stopping after a random number of paces. Use the table of random numbers to determine the number of paces. Two-digit random numbers between 00 and 25 may be sufficient but if trees are far apart or if the forest is large, you may wish to use random numbers between 00 and 50.
At each sample location, identify the nearest individual in each of four imaginary quadrats. If you do not know the name of the species, give it a number (example: species 1, species 2, etc.). To aid you with consistency in identification, it might be helpful to write a brief description of each species as you proceed with the samples. For this exercise, it is not necessary to measure the size (circumference) of the trees as was done in the Point Quarter Method lab. Use table 1 to help you record the number of each species as you sample. Repeat the pacing procedure and sampling until 30 samples (120 trees) are taken.
If you sample the same community that you sampled in the point-quarter method lab, you could use those samples here to reduce the number of samples necessary.
More details on this sampling technique are given in the laboratory exercise on the point quarter method.
Option 3: Insects in an Abandoned Field
Use insect nets to sample the insects on vegetation in an abandoned field. This can be done by swatting the nets against the vegetation and then observing the individual insects that are collected in the net. Continue sampling insects until you have counted at least 100 individuals.
Calculations
After table 1 is completed, determine which species had the most number of individuals. This species will be ranked #1. Enter the data for this species in table 2. Next, determine which species had the second most number of individuals and enter this information in table 2. Repeat this for each of the species listed in table 1. Calculate proportional abundance for each species. This can be done by dividing the number of individuals of a species by the total number of individuals. See the example above.
Lognormal Plot
Print the graph paper provided with this lab and create a lognormal plot for your data. The Y axis (vertical axis) of your plot should be Number of Species and the X axis (horizontal axis) should be Number of Individuals. The categories on the X axis should be 1-2, 3-4, 5-8, 9-16, 17-32, 33-64, 65-128, 129-256, 257-512, etc.
Rank-Abundance Curve
Print the semi-log graph paper included with this lab and create a rank-abundance curve for your data. Plot proportional abundance (table 2) on the Y-axis and rank on the X-axis. The example above shows this for two hypothetical communities A and B. If you have many proportional abundance values in the range from .001 to .01, use the graph on the bottom of the page. If most of your values are greater than 0.01, use the graph on the top of the page.
Lab Report
Turn in the following:
1. Data sheets - Table 1 and table 2
2. Lognormal plot of species abundance - This may be mailed, scanned, or created using Excel. If scanned, submit a jpeg file (*.jpg). If created using excel, submit an Excel file (*.xls).
3. Rank-abundance plot on semi-log paper - This may be mailed or submitted online as described in step 2 above. Plot the rank-abundance data for community A above on the same graph as your data. The graph will have two lines- one for your data and one for community A.
4. Answers to the questions below - These must be submitted online.
Questions
1. Which of the sampling options (herbaceous plants, forest trees, etc.) did you use? Describe your sampling area and indicate your total number of samples taken.
2. Is your lognormal plot a bell-shaped curve?
3. Are most species in your community very common? Are they very rare? Are they intermediate? Explain.
4. Explain why the curve below is not bell-shaped. The answer to this question is in the reading assignment in the textbook.
5. Does your rank-abundance plot appear to be a straight line? If not, does the community seem to be dominated by one or two species?
6. Online Students: Plot the data for Community A (above) on the same rank-abundance graph that you created for your data. How do the two curves compare? Which community has more evenness? See the reading in the textbook for help with interpreting your graph.
Campus Students: Plot the data for both communities that the class sampled on the same rank-abundance graph that you created for your data. How do the two curves compare? Which community has more evenness? See the reading in the textbook for help with interpreting your graph.
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