Earthworms by numbers

Last week I finished sorting and identifying ‘my’ half of the Soil Biodiversity Group’s samples from the BESS Earthworm Project. The tubes from the last couple of fields were rather full, and because earthworms contain a high percentage of water this had diluted the alcohol, reducing its preserving qualities. The liquid in the samples was dark and pungent and the earthworms soft, this made identification more difficult as the features can be obscured and the earthworms fragile to manipulate. The final earthworm I identified from the samples was a somewhat underwhelming, squishy, headless and tailless Aporrectodea longa!

My final BESS earthworm sample

The final tube was a particularly ‘squishy’ sample

Some previous samples were more exciting however, in particular during the project I have identified by first ever Ap. limicola, Ap. caliginosa subsp. noctura and Octolasion tyrtaeum, although sadly I did not meet the Ap. icterica I was also hoping to find. Here are some of the facts and figures from my time volunteering on the BESS earthworm project:

Sorting and identifying earthworms from 14 fields from seven farms took around 90 hours, and I counted a total of 5940 earthworm specimens, 2124 which could be identified to species. I identified 16 earthworm taxa in total, with the highest number (12) occurring at the Dorset site. This was also the site with the highest number of earthworms per single pit – a whopping 110 individuals, but the highest number of species per pit was from the Dumfries and Galloway site, with seven.

The most common species in my samples were, as expected, Aporrectodea caliginosa (742) and Allolobophora chlorotica (506), but the sites differed in abundance between the two and it will be interesting to see if any environmental preferences emerge when the whole dataset in analysed. The rarest earthworms in my half of the data are Ap. caliginosa subsp. nocturna and Octolasion tyrtaeum with just a single specimen each, both from the fields in Dorset. Other uncommon species were Satchellius mammalis ( 4) and Dendrodrilus rubidus ( 3) both more commonly found in leaf litter, which is sparse in pasture.

Subjectively each field and farm had a different ‘character’ of earthworm diversity, and I am particularly looking forward to seeing what the statistics have to say about the variation between and within the different sites. Some preferences are quite obvious, Ap. limicola was found in large quantities exclusively (so far at least) from a field on a floodplain, and is known to have a preference for wet conditions (as suggested by its name, limnic = referring to freshwater). An old favourite of mine from my MSc project, Murchieona muldali, previously associated with hedgerows and field edges did seem to occur more frequently in low intensity fields with some scrub. There are so many other questions about earthworm diversity that the project will go some way to answering, like does earthworm diversity decrease with latitude? What are the environmental tolerances of each species and is there evidence for competitive exclusion? Do more fertile pastures have more individuals but fewer species? I am so excited to see the results!

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