Great Lakes Worm Watch

Research Methods

Calculating Earthworm Biomass

At first thought, calculating earthworm biomass seems very easy, simply weigh the earthworm right?

Well, yes and no. Like humans, earthworms are mostly made up of water. But unlike humans, the fresh weight of an earthworm can change up to 20% depending on how much moisture there is in the soil. In addition, whether an earthworm has a full or empty gut can also affect its fresh weight by 10 to 20 percent. So, if you want a measure of earthworm biomass that is comparable to other sites, other times of the year or across different years, you need to use a measure of biomass that eliminates the problem of moisture and gut contents.

To eliminate the problem of variability due to moisture content, you can dry your earthworms at 60°C for 24-48 hours to get “dry earthworm biomass”. That’s not so hard.

To eliminate the problem of variability due to gut contents you can do one of three things:

  1. Keep the live earthworms in containers until they empty their guts (24-48 hours, if they don’t die in the process)
  2. Dissect the preserved earthworm and flush their opened gut until all contents are removed (a LOT of work).
  3. “Ash” the dry earthworm in a muffle oven at 500°C, which removes everything except the gut contents, which is mostly composed of mineral soil. You would then subtract the weight of the ash from the dry weight of the earthworm to get “ash-free-dry earthworm biomass”.

Learn more about determining “ash-free-dry earthworm biomass”.

Now if that seems like a lot of work, you are right. The other problem with using ash-free-dry biomass is that the process destroys your earthworm specimens. This is a problem if you want to keep them for future reference, or send them to us when you are documenting an earthworm occurrence (see Join the Research Team for more information).

Allometric Equations for Estimating Biomass

Scientists from many areas of study have dealt with this same problem by creating equations that estimate the biomass value of a given organism using certain body measurement that are strongly correlated with biomass. Because earthworms are basically a cylinder, it is possible to determine an allometric equation that reliably estimates ash-free-dry biomass using preserved length (mm). Measuring the length of your earthworm is much easier than determining ash-free-dry biomass directly! We have provided you with an equation that allows you to use a spreadsheet to calculate “ash-free dry grams” (AFDg) of biomass of earthworms using the length (in millimeters) of the preserved earthworms.

lenght vs biomass graph

In this figure, each symbol represent one individual earthworm’s length (on the bottom, or X-axis) and ash-free-dry biomass (on the left, or Y-axis). You can see that as length increases, ash-free-dry biomass increases along a predictable pattern. A statistical method called “regression” was used to find an equation that describes the relationship between length and ash-free-dry biomass so that by plugging the length measurements in to the allometric equation, you can estimate ash-free-dry biomass for an earthworm. To view or download the full article describing this equation and how it was determined click here. (see Hale et al 2004).

So, how can I use this allometric equation to estimate the biomass of my earthworms?

We have provided a spread sheet that you can use to do the calculations. Download the spreadsheet . There are a couple steps you need to take to calculate ash-free-dry earthworm biomass using our equation. First, you must use preserved earthworms, because it is impossible to get an accurate length measurement of a squiggling, stretching and contracting earthworm…at least not to the nearest millimeter! Once you have preserved earthworms

  1. Measure the length of your preserved earthworms to the nearest millimeter (mm). The equation is only accurate if the length measurement is in millimeters. If the earthworm is curved, use some forceps to straighten it out along the ruler.
    earthworm measurement
    Once this earthworm was straightened out, it’s full length measurement was 50mm.
  2. Enter your length measurements into a spreadsheet. Download the spreadsheet.
  3. Next, copy and paste the formulas that we provide in our example spreadsheet into the rows with your data to calculate the “ash-free dry grams” (AFDg) of biomass of each earthworm. Then sum the AFDg of biomass of all the worms in a given plot to get total earthworm biomass for that plot.

    Read more about what you are seeing in the spreadsheet or to see how to do the calculation manually.
  4. Finally, you can convert the total earthworm biomass of each plot to an estimated total earthworm biomass per m² by multiplying the plot total by 9 (assuming it is a 33cm x 33cm plot).

    For example, in our example spreadsheet, there were three earthworms collected with lengths of 18, 20, and 24 mm. Their AFDg of biomass of each were 0.005, 0.0064, and 0.0096, respectively and the total biomass for the plot was 0.021 AFDg. To convert the plot value to a per square meter basis, we simply multiply the plot total by 9.

    0.021 x 9 = 0.189 AFDg/m²

The University of Minnesota is an equal opportunity educator and employer.
© 1999-2011 University of Minnesota Duluth Privacy Statement
Natural Resources Research Institute
5013 Miller Trunk Highway
Duluth, MN 55811       218-788-2710