Great Lakes Worm Watch

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Analyzing Earthworm Data

Introduction

Density, is the average number of earthworms per unit area, this is commonly given as the number of earthworms per square meter. It is very simple to calculate, all you need is:

Earthworm data is generally reported on a per square meter basis (#/m²). The Great Lakes Worm Watch’s Single and Multiple Plot Studies use a plot size of 33cm x33cm, using this area makes it very easy to convert counts of earthworms into per m2 densities. Density is commonly used in geography as a measure of population. It is a simple and effective method of judging the extent of earthworm invasion in your plot, site or even habitat. It is also useful for cross comparing sites.  What condones high or low earthworm density is actually quite vague, density is quite relative to the individual areas were you have collected your earthworms (i.e. site or plot has high density compared to surrounding environment). BUT as a general rule areas with high densities of earthworms are often more heavily invaded than areas of low-density sites.

Density is also a useful method for evaluating the average earthworm conditions at your site or for the habitat as a whole. More often than not, people are interested in the average density of earthworms for a given habitat or site from the individual plot data.

Methods

Converting your plot based data to “per m²"

If you have used the 33cm x 33cm plot (use by the GLWW) then simply multiply the number of earthworms for that plot by 9 to get the same data “per m²”. Each 33 x 33cm plot is 1/9th of a m² (see illustration below)

Number of earthworms per 33x33 plot * 9 = Earthworm Density per m²

Example

If you collected 13 earthworms from your 33cm x 33cm sample plot, then the density of earthworms per m² is 9 x 13 = 117/m²

Number of earthworms (per 33 by 33 plot) * 9 = Earthworm Density per m²
13 * 9 = 117 Earthworms per

Average Density

Before we use or share your data we might calculate the average. Using an Average is useful if you want to summaries your site or habitat conditions. The number of sample plots determines the reliability of your average. When you are calculating you average it is important to remember that earthworms can be apparent in one plot and absent from a plot one meter away! If you have collected earthworms from 3 or less plots your average may not be representative of the whole habitat.  But an averages density is easy to calculate:

Sum of earthworm density from all plots / Number of plots sampled = Average Earthworm Density per m²

Extra - Density of an individual Species

You can also calculate the density of juveniles/adults, species or genus. Instead of using the total number of earthworms per plot (or for your entire site) you use the number of earthworms in that particular species or other category.

E.g. if you have 100 worms in total 7 of which are earthworm species B, therefore the density of earthworm species B per square meter is 7 x 9 or 63 species B per square meter.

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