Earthworm diversity linked to latitude and isolation

The collage shows worms of the aquatic genus Sparganophilus (left) and the terrestrial genus Diplocardia (right) as well as their habitats. USDA Forest Service photo by R. Carrera-Martínez.

Earthworms don’t get enough attention, according to USDA Forest Service research ecologist Mac Callaham.

“Earthworms have profound influences on soil habitat and other soil animals,” says Callaham. “They’re called ecosystem engineers. Their behavior and influence can help us understand how the systems are functioning and how we can best manage natural resources.”

But until we know more about which worms live where, it’s hard to know how well they’re doing.

“There’s only been one earthworm considered by the U.S. Fish and Wildlife Service for the endangered species list. The species ultimately wasn’t listed because there wasn’t enough information about its distribution and abundance to justify that status,” notes Callaham.

“We probably have had some extinction of species that we don’t know about. What we do know is that there are species represented by specimens in natural history museums that we can’t find in nature anymore. Whether that means those species died out or we’re just not digging in the exact right place, we don’t know. We could potentially be losing undocumented species.”

Callaham and others examined the diversity of two groups of species: aquatic (Sparganophilus spp.) and terrestrial (Diplocardia spp.) earthworms. The researchers chose these groups because both are native to North America, and both were expected to harbor species that had never been collected before.

The Japan Society for the Promotion of Science funded the research, published in Journal of Biogeography.

Hiroshi Ikeda led the study while with Callaham’s team as a visiting scientist. “Ikeda wanted to test a theory about speciation patterns and latitude,” says Callaham. “Specifically, he wanted to see if earthworms that live in saturated soils next to rivers have similar speciation patterns as earthworms that live in drier soils.”

The team collected earthworms over a wide swath of North America. Their hunt was organized by latitude. Latitude lines are also called parallel lines and wrap the earth in an imaginary net running east and west.

graphic shows where worms were captured
Geographic distribution pattern of each potential species of Sparganophilus and Diplocardia in North America. USDA Forest Service graphic.

For a large portion of the study area, researchers found a different species of Diplocardia, the soil-dwelling worms, practically everywhere they dug. One of the main findings from the study is that more Diplocardia species are likely to be discovered, particularly at lower latitudes (below 32°N).

The Sparganophilus species dominated high latitudes (above 40°N). They weren’t there during the last glacial period, meaning that one or two species with better dispersal ability must have migrated there from lower latitudes within the past 12,000 years.

On the other hand, Sparganophilus species that were locally distributed were often collected at mid-latitudes (32° to 40°N) in the Appalachian Mountains, where headwater regions saw more frequent genetic speciation. For these worms, the geographic isolation of each watershed may affect diversity more than latitude.

“On a more practical level, for these groups in North America, we don’t have a good handle on what our native diversity is,” Callaham says. “We’re trying to increase our knowledge about soil biota.”

In addition to building the body of knowledge on population range and genetic diversity of earthworms, the results help to untangle the mystery of soil biodiversity in North America and show how geologic events like glaciation influence the biogeography of this important group of animals.

Read the full article.

For more information, contact Mac Callaham at mac.a.callaham@usda.gov.

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