Biomass data can be valuable when asking certain questions but is a bit trickier to calculate for earthworms than you might think. So if you are interested or motivated to do some simple measurements and use a spreadsheet to calculate biomass, we would appreciate the additional data. Learn more about how to calculate earthworm biomass or download a spreadsheet that will help you.
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:
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).
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.
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 here. 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
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