biotic factors

Biotic Factors Lab

Distribution Patterns

Distribution patterns result from the manner in which the individuals of a given population orient themselves spatially with respect to each other under a given set of environmental conditions. Measures of such patterns can provide information about the future of the population under study. Three basic types of distribution patterns will be studied:

Binomial (normal distribution)
Random (poisson distribution)
Clumped (negative binomial distribution)

These patterns can be described by their unique relationships between the calculated mean (central tendency) and calculated variance (degree of scatter around the mean). the index which formally describes this relationship is referred to as the Coefficient of Dispersion (CD).

CD = variance/mean

Coefficient of Dispersion	Observed Pattern	Expected Distribution
< 1	Regular	Normal (binomial)
= 1	Random	Poisson
> 1	Clumped	Negative Binomial

To begin calculations, separate the observed data into frequency classes and prepare a frequency table.

Number of Plants per Grid Square (x)	Frequency of the Grid Squares with this Number (f)	Total Frequency of the Class (fx)

A histogram of the frequencies should be constructed. This will allow a visual impression of what kind of distribution is being observed. The horizontal axis should be the Number of Plants per Grid Square and the vertical axis should be the Frequency of the Grid Squares with this Number.

Calculate the CD using the mean and variance functions of your calculator. Determine the Distribution Pattern based on your Coefficient of Dispersion.

The Plant

The investigator should know as much about the species being studied as possible, even before taking the actual field measures.

Yucca glauca, common name soapweed, is native to the western Great Plains. It is found from South Dakota to Oklahoma. It produces stiff, broad, short, sword-like leaves from green rosettes. Yucca prefers full sun with dry soil. It is a perennial and grows from 90 cm to 1.8 m high.
The plant produces 20 to 60 white flowers per stalk.
Yucca plant and the Yucca moth have a mutualistic relationship. The plant uses a unique method of pollination. The moth is genetically programmed to stuff a little ball of pollen into a cup-shaped stigma of each flower. The moth uses the flowers to hide inside during the day and the Yucca moth caterpillars must eat yucca seeds or starve.
Yucca can also be used as an indicator for the amount of overgrazing in a pasture.

The Lab

This lab will determine the Distribution pattern of the Yucca Plants found at the site of the Biome Project.

Write a HYPOTHESIS: Using your knowledge of the biology of the Yucca glauca what distribution pattern will it show in the Nebraska grasslands?
Your group will be assigned a section of grids where you will be collecting data. Locate the grid on your data sheet. In the grid area, record the location of all Yucca plants as close as you can to their relative position in the grid square.
Combine data from all groups.
Prepare a frequency distribution and histogram for the collected data.
Calculate the CD and determine the pattern of distribution.

Name _____________________________________________

Honors Biology

Biotic Factors - DATA SHEET

HYPOTHESIS:___________________________________________________________________________________________________________________

Frequency Table

Number of Plants per Grid Square (x)	Frequency of the Grid Squares with this Number (f)	Total Frequency of the Class (fx)
0
1
2
3
4
5
6
7
8
9
10
11
12
13

Bar Graph

Coefficient of Dispersion (CD) = variance/mean = ___________________

Distribution:________________________________________________________________________________

Accept or Reject Hypothesis:_____________________________________________