Researchers Worry About Worms Worldwide, By: Joe Eaton

Tuesday March 14, 2006

Add this to your list of things to worry about: Native California earthworms. Like many native California creatures, they’re not doing well. 

Scientists aren’t even sure how many species of earthworms we have here, or what roles they play in the complex ecology of the soil. Oligochaetology—the study of the class within the phylum Annelida that includes night crawlers, red wigglers, and the little black tubifex worms you feed to tropical fish—would appear to be a wide-open field. There are just a couple of practicing earthworm taxonomists in the United States, and the technical literature is sparse. A Southern California survey in 1990-91, the first ever in that region, turned up several undescribed species and a new genus. I don’t know whether anything comparable has been done for the Bay Area. 

The only comprehensive collection of native California earthworms was destroyed in the 1906 earthquake and never rebuilt. I’m assuming this was in the California Academy of Science’s old downtown quarters. The intrepid botanist Alice Eastwood ventured into the ruins of the building to retrieve rare plant specimens before it burned, but it appears that no one went for the worms; they may have been in glass jars that were irretrievably smashed.  

The worms in your garden, or your worm bed if you’re that serious about them, are aliens. Lumbricus terrestris, the classic earthworm of the biology textbooks, is native to Europe. Like other weedy species, exotic earthworms thrive in disturbed environments. When they encounter native worms, the invaders generally outcompete them. 

It’s not clear what impact the exotics have had on our native vegetation, but studies in the Great Lakes region indicate they may be contributing to the decline of the rare goblin fern.  

Functionally, there are three kinds of earthworms: epigeic worms that live in the uppermost layers of soil and feed on undecomposed plant litter; endogeic worms that make horizontal tunnels further down; and anecic worms that inhabit deep vertical burrows from which they come to the surface in search of leaf litter, manure, and other organic matter. Lumbricus terrestris is one of the anecic types, the worms that do the bulk of the work of soil formation. But its relative L. rubellus and the red wiggler Eisenia foetida are considered to be better composting worms.  

From what little is known of their habits, most of the native Californian species are endogeic, with an activity pattern that peaks in the rainy season. Tolerant of drier, leaner soil than the exotics, they’re most abundant in oak-savanna habitats, but also occur in chaparral and wet coastal forest. The natives, sensitive to disturbance, disappear where soil has been tilled or tree cover cut down.  

Although California native earthworms are on the small side, there are or were giants in the earth to the north of our borders. Driloleirus macelfreshi, found in riparian woodlands in Oregon’s Willamete Valley, attained a length of three feet and smelled of lilies. Its relative in the Palouse Valley of Washington, D. americanus, was of similar dimensions. I use the past tense here since the Oregon giant earthworm was last seen in 1981 and is most likely no longer with us, although people are still looking. (Just for the record, the world-champion earthworm is the 10–foot-long Megascolides australis of Gippsland, Australia, rare but still extant). 

Charles Darwin was the first to appreciate the ecological importance of earthworms. He had addressed the subject in a short paper in 1837, just after his return from the Beagle voyage, concluding: “It will be difficult to deny the probability, that every particle of earth forming the bed from which the turf in old pasture land springs, has passed through the intestines of worms…” He returned to earthworm studies late in life, and the last book he wrote, The Formation of Vegetable Mould Through the Actions of Worms, was a surprise bestseller.  

As with all his other projects—the pigeons, the insectivorous plants, the orchids—the whole Darwin family was drawn into the worm research. He was curious about their sensory apparatus and tested their reaction to tobacco fumes and the proximity of a red-hot poker. To determine whether they had a sense of hearing, he had his son Francis play the bassoon to a pot of earthworms. He also set worm pots on top of the piano and asked his wife Emma to play fortissimo. Emma remarked to one of her correspondents that Charles “has taken to training earthworms, but does not make much progress, as they can neither see nor hear.” 

The worm work became for Darwin yet another example of what gradual processes can accomplish given enough time. He calculated that earthworms had completely buried the ruins of a Roman villa in Surrey, their castings—the pellets of soil that passed through their guts—accumulating at the rate of one inch every 12 years. This fit nicely with his view of natural selection working slowly through deep time to create the multifarious living world. 

Ironically, the worm species Darwin studied may now be in trouble at home, even if it’s thriving in North America. Two flatworms introduced to western Europe from Australia and New Zealand have been found to prey on Lumbricus terrestris, reducing some populations to the point of local endangerment—the kind of thing that can all too easily happen as biological globalization scatters exotic species around the world.