Worm Farm Census

I keep a worm farm for the purpose of vermicomposting kitchen waste. Recently, I started new bins to increase the project's capacity. I had enough extra worms that I also helped some friends to begin their worm farm. The project suffered a set-back as locally we had a very early heat wave that killed many of the worms. My expansion bin has recovered a bit and I decided to make an estimate of the worm population. It is 2020 and everyone should be counted.

Method

The expansion project is shown below.

There are two 20-liter bins. Each is approximately half full. I used a small measuring cup (.75 ml) and took six samples from each bin. Two samples each at the bottom, middle and top of the bins. This sample cup size was chosen because this amount can be spread over a paper plate to facilitate the counting of the worms. 

Also noted, but not counted, was the presence of pods which is an indication that the worm population is reproducing and that new worms will soon emerge.

Results

The results are given in the table below:

Bin	Position	Worm Count	Pods Present Y/N
Top	Top	2	N
Top	Top	1	N
Top	Mid	6	N
Top	Mid	4	Y
Top	Bottom	3	Y
Top	Bottom	3	Y
Lower	Top	7	Y
Lower	Top	4	Y
Lower	Mid	7	Y
Lower	Mid	4	Y
Lower	Bottom	2	Y
Lower	Bottom	1	Y
Average		44/12 = 3.67

Given an average sample count of 3.67 per 75 ml, yields an average worm density of 3.67/.075 = 49 worms/liter. The total project volume is approximately 20 liters so my current worm census is 980 worms.

Conclusion

We now have a baseline against which future counts can be compared. The presence of pods throughout the bins indicates the worms are healthy enough to reproduce. As a goal for future measurements, my first worm farm has a worm density of about 225 worms/liter so the expansion farm still needs some careful monitoring until the population increases.

Update: 6-11-2020

A repeat census was conducted. Since the prior month, a new bin (or layer) was added to the original two bins. Samples were taken from 3 locations in each of the 3 bins. The average count was 6 per 75 ml for a average density of 80 worms/liter. Additionally, pods were present in every sample of the two lower bins but not in any of the samples of the top (most recently added bin). 

Update: 1-23-2022
In the book, The Math of Life & Death, Kit Yates describes the capture-recapture method of estimating wildlife populations (method reviewed in this post).