Population Density

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!

Ecological Pyramids

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

Species Diversity

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.

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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.

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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:

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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 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

statistics

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