Students can estimate population density for each taxon by multiplying the number of individuals per sample by a unit of area (e.g. 1 meter) and then dividing by the fractional part of that unit that was actually sampled.
Example: Suppose there are 36 mites collected in a sample measuring 15 x 15 cm, (225 sq cm = 0.0225 sq m), then there should be (36)(1)/(0.0225) = 1600 mites per sq. meter.
Pool class data and average all values from similar habitats to give a more accurate estimate of population density. Extrapolate this estimate to number/acre or number/hectare by using an appropriate conversion factor:
1 hectare = 10,000 square meters = 2.471 acres
1 acre = 43,560 square feet = 0.4047 hectare
Seven (7) individuals per square foot equals more than 300,000 individuals per acre!
Either raw counts or density estimates can be used to construct ecological pyramids. For a pyramid of numbers, sort the collection into size categories, add the number of individuals in each category, and then draw a horizontal bar graph (like Fig. A) to illustrate the inverse relationship between body size and population density. A trophic pyramid is similar to a pyramid of numbers, but the animals are grouped by their position in the food chain rather than by body size (see Fig. B).
Pyramid of Numbers
Ecologists have devised several numerical methods for comparing the species diversity between two different samples or communities. Jaccard's Index, the simplest of these comparisons, is calculated by dividing the number of species found in both of two samples (j) by the number found in only one sample or the other (r) and then multiplying by 100. This gives a percentage of faunal similarity:
For example, suppose sample #1 has 12 species and sample #2 has 14 species. If only 3 species are common to both samples, then 9+11=20 species are represented in only one of the samples. In this case, Jaccard's Index would equal 3/20 X 100 = 15% similarity.
Sorensen's Quotient of Similarity (Q/S) is another diversity index that computes the percentage similarity between two samples:
where a is the total number of species in sample #1, b is the number of species in Sample #2, and j is the number of species common to both samples.
Jaccard's Index and Sorensen's Quotient of Similarity are both rather sensitive to differences in sample size. This problem is less severe in Mountford's Index of Similarity (I) which uses the same three variables (a, b, and j) defined above:
More information about these diversity indexes can be found in Chapter 2 of The Distribution and Diversity of Soil Fauna by John A. Wallwork (see Bibliography). Most ecology textbooks also have at least one chapter dealing with the numerical methods used in the study of species diversity.
Independent Student Projects
Students can select their own study site and formulate a hypothesis about the effects of some variable on one or more measurable elements of the community structure.
- Study sites might include:
- agricultural fields
- organic farms
- compost piles
- grasslands or pasture communities
- orchard ground cover
- hardwood forest leaf litter
- coniferous forest pine straw
- bogs or swamps (wetlands)
- lawns and golf courses
- Variables might include:
- time of year
- north slope vs. south slope
- road proximity
- flat vs. slope
- physical disruption
- addition of fertilizers or nutrients
- mowing, harvesting, or clear-cutting
- introduce or exclude natural predators
- pesticides (students should not apply, protective clothing must be worn)
- Measurable elements might include:
- Population age structure
- head capsule measurements
- counting body segments in anamorphic organisms
- Species diversity
- use of diversity indexes
- Population density of selected species
- extrapolation of sampling counts
- Rate of development or rate of change
- population dynamics
- Trophic structure
- pyramid of numbers or biomass
- predator-prey relationships
Examples of Student Research Projects:
Example #1 -- Effect of Grazing Cattle on the Population Density of Oribatid Mites in Pastureland
Example #2 -- Density of Millipedes in North and South-facing Slopes
Example #3 -- Sampling at Various Distances from a Highway
Example #4 -- Arthropod Fauna in Different Types of Compost