Making a earthworm observatory is a great way to see for yourself what earthworms do and how they might change ecosystems when they invade. You can set up a demonstration observatory in your classroom or nature center, or students can use small observatories to conduct their own experiments!
The objective of this demonstration is to illustrate what earthworms do and how they affect soil conditions. We do this by setting up an experiment where we simulate a natural soil system and then add worms to one half but not the other. We then observe the demonstration for changes (See Figure 1). This demonstration can be used as an organizing point for many lessons in life science (animals, soil, ecology, ecosystems, etc.), science as inquiry, and has many potential links with math, chemistry and even art and writing (a few suggested lessons are noted with *).
The demonstration described here is intended to approximately replicate the soil layering (called soil horizons) seen in earthworm-free hardwood forests of the Great Lakes region, and the affects that earthworms can have on this soil structure.
Note: All of the soil and litter components must be completely earthworm free. Sift soil to get rid of any worms that may be there. Use only very dry leaf litter since wet litter often contains worms. If you can spread the soil and litter out on a tarp in a thin layer, in the blazing sun for a few days or a week to really cook it, this will kill or drive off any tiny worms present.
For one side of the container (remember you are leaving one side earthworm free!) use enough worms to equal 200-400 individuals per square meter…
Using the same layers as in the demonstration observatory, you can use clear 2 liter plastic pop bottles to make small observatories, which are ideal for small experiments because you can make replicates for different treatments! (See Figure 2)
Using either the demonstration observatory or the small observatories is a great opportunity to introduce “the scientific method” and the concepts of an experimental “control,” experimental “treatments,” and “replicates.”
The basic idea is that all the observatories are set up exactly the same. Then, some get no treatment (the controls) and some get a “treatment.” For example earthworm-free (control) vs. worms added (treatment). The control is required to understand what would have happened if the worms weren't present. For example, the soil layers in the experiment may settle. How much is natural and how much is due to the worms? Without the control we might assume that all settling in the container is due to earthworm activity when that may only be part of the answer. Conversely, if the container without worms does not settle and the one with the worms does, we can probably conclude it was due to the worms.
Of course, with only one control and one experimental set-up, we can't always be so confident that the difference we see is the result of our treatment. Since some unknown factors may have affected our results. But, if we “replicate” the experiment and get the same results in all (or most) of them, then we can say with confidence that the differences we observed were real.
It is very important when using an experimental design with controls and/or replicates that all of the set-ups get exactly the same treatment (ie. water, light, temperature, etc.) so that we are confident that the only difference between our controls and our experimental set-ups is the variable we are testing. However, weird things always happen and when they do it's an opportunity to perhaps learn something unexpected.
I think it was Isaac Asimov that said “the greatest thing to hear in the laboratory is not eureka!, but, humm, that's funny.”
Calculate earthworm growth rates. Weigh the worms and the leaf litter before you put them in the demonstration. Note whether or not the worms have a clitellum (light colored collar or band around their body, near the head). This indicates that the earthworm is sexually mature and may produce cocoons and/or young during the run of the experiment.
Then, after a month or more, you can take the demonstration apart and sift out the worms. Be sure to watch for tiny white juvenile worms! Have students count and weigh the worms and the remaining litter. Compare this to what you put in. You can then calculate earthworm growth rates, reproduction rates (if there are juveniles), etc. and correlate those changes to the changes in mass of the litter.
Make measurements of pH and various nutrients in the different layers at the beginning of the experiment and then in each layer on each side at the end of the experiment (inexpensive test kits are available through many greenhouse or garden supply stores). Compare and contrast the changes in the earthworm-free versus earthworm wormed treatment.
To look at possible nutrient losses from leaching as a result of earthworm activity, put small drain holes in the bottom of your containers. Cover these holes with a very fine screen or landscape cloth so water can get through but earthworms can't. Then assemble the layers as described above. Have a separate catch basin under each container. Collect the water (leachate) that drains through at different time intervals during the experiment. Water both sides of the demonstration equally and at a rate that produces small amounts of leachate (maybe a cup a week?). Be sure to sprinkle water in slowly so it does not run straight through and you are actually collecting water (leachate) that slowly moves through the soil after having a chance to interact with the soil particles (hours to days). If you don't get any leachate from either side after the first few weeks, gradually increase the amount of water added to the demonstration until you get some leaching.
Measure the relative amount of leachate, the pH and the nutrient content of the leachate (inexpensive test kits are available through many greenhouse or garden supply stores). Compare the results of the control vs. the earthworm treatment. Don't be surprised if the two sides produce different amounts of leachate when you put the same amount of water in on each side. Earthworm burrows conduct water more rapidly through the soil horizon than soils without burrows. The nitrogen in earthworm casts is in a form that is more easily leached from the soil than the nitrogen in an intact litter layers.
Consider what the implication might be for the long term nutrient budget of a forest with and without worms based on the results of this experiment. For example, the average nitrogen content of some hardwood forests is 65 kg / hectare (58 lbs / acre). Lets assume that in earthworm free conditions nitrogen loss rates due to leaching are equal to nitrogen added to the system by fixation (estimated to be about 10% of the total pool annually) so there is no net change over time in the amount of nitrogen in the system. If earthworms increase the nitrogen content of the leachate (outputs) by, say 10% or 20% a year. Then how much nitrogen would be lost from the system in a decade and is that a significant amount?
Well, the current loss rate is 6.5 kg / hectare (5.8 lbs. / acres) per year. Ten percent of that amount is .65 kg / hectare (.58 lbs. / acres) and 20% is 1.3 kg / hectare (1.16 lbs. / acres) per year. Over a decade, that represents 6.5 kg / hectare (5.8 lbs. / acres) at the 10% level, or 13 kg / hectare (11.6 lbs. / acres) at the 20% level, of nitrogen lost from the system leaving a total of 58.5 kg / hectare (52.2 lbs. / acres) or 52 kg / hectare (46.4 lbs. / acre), respectively. At this rate of loss, in one or two decades enough nitrogen will have been lost from the system to limit the viability of many hardwood tree species, favoring conifer tree species which grow more slowly and have lower nitrogen content in their needles than do hardwood leaves. This is only one of many possible examples.