The long-term monitoring of earthworm populations and species diversity.

There is ample evidence that earthworm populations change over time with changes in soil management practices. Studies in many different parts of the World have shown that earthworm populations increase with reduced soil disturbance (Clapperton et al., 1997; Clapperton, 1999). However, there are far fewer studies that examined changes in earthworm species diversity over time (Muys and Granval, 1997). The reason for this may be because it is likely that it will take longer to show changes in earthworm species diversity than in abundance.

It is critical when you are monitoring over a long period of time that you plan the study in great detail. Sampling sites need to mapped precisely so others can find the sites in years to come. Accurate records of how many earthworms of each species need to be maintained. If you are comparing the persistent effect of soil management treatments, then the treatments and boundaries for the treatments need to be described in detail.

Worm Watch recommends the Modified Flip and Strip and Modified Hand-sorting methods (Sampling Methods 2 and 3) for long-term monitoring. There are other non-destructive methods to extract earthworms such as using a mustard solution to expel earthworms from the soil (Gunn, 1992), electrical currents (Tisdall and McKenzie, 1999), and mechanical vibration (Lee, 1985). However, expelling solutions like mustard do not work effectively on all earthworm species and in all soils, and they leave a residue (Fox et al., in preparation). Electrical currents and vibration can be effective in many soils but you will only recover the more mature earthworms, and no cocoons. If you intend to use any of these other methods, it is important that you verify the efficiency of the sampling by hand-sorting in a number of sites that were exposed to the expelling technique. Then you can calculate the percent recovery of the technique that you choose to use (Baker and Lee, 1993).

If you are returning to the same site each year to monitor earthworms do not sample in exactly the same spot, especially if you are the modified hand-sorting method. A site nearby would be acceptable as long as you map the location accurately.

Clapperton, M.J., Miller, J.J., Larney, F.J. and Lindwall, C.W. 1997. Earthworm populations as affected by long-term tillage practices in southern, Alberta, Canada. Soil Biology and Biochemistry 29: 631-633.

Clapperton, M.J. Tillage practices, and temperature and moisture interactions affect earthworm populations and species composition. Pedobiologia 43: 658-665.

Gunn, A. 1992. The use of mustard to estimate earthworm populations. Pedobiologia 36: 65-67.

Lee, K.E. 1985. Earthworms: Their Ecology and Relationships with Soil and Land Use. Academic Press, USA.

Muys, B., and Granval, P.H. 1997. Earthworms as bio-indicators of forest site quality. Soil Biology and Biochemistry 29: 323-328.

Tisdall, J.M., and McKenzie, B.M. 1999. A method of extracting earthworms from cores of soil with minimum damage to soil. Biology and Fertility of Soils 30: 96-99.