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).


Figure A Pyramid of Numbers 
Figure B Trophic Pyramid 
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:
 NATURAL:
 time of year
 grazing
 shading
 elevation
 north slope vs. south slope
 road proximity
 flat vs. slope
 INDUCED:
 physical disruption
 addition of fertilizers or nutrients
 mulching
 mowing, harvesting, or clearcutting
 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
 statistics
 Rate of development or rate of change
 population dynamics
 Trophic structure
 pyramid of numbers or biomass
 predatorprey 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 Southfacing Slopes
Example #3  Sampling at Various Distances from a Highway
Example #4  Arthropod Fauna in Different Types of Compost