# SO WHAT'S THE BIG DEAL?

## 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).

## 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
elevation
north slope vs. south slope
flat vs. slope
INDUCED:
physical disruption
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
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 #3 -- Sampling at Various Distances from a Highway

Example #4 -- Arthropod Fauna in Different Types of Compost

 Return to ENT 525 HomePage John R. Meyer Last Updated:   30 December 2013 Department of Entomology NC State University