The lowly earthworm poses a dire threat to this California island

San Clemente Island has been invaded by foreign intruders, threatening inhabitants of the volcanic isle owned by the Navy.

The island is home to the only ship-to-shore bombardment training range in the United States, but this invasion isn’t part of a military exercise. The intruders are thousands of lowly earthworms.

While they may seem harmless, scientists say, the worms pose a mortal threat to the native species clinging to life on the island about 75 miles northwest of San Diego.

Scientists believe the sticky, writhing menace made its way there in 2008, probably as stowaways in truckloads of topsoil imported from mainland Southern California as part of a road construction project.

The invasion of earthworms in this earthworm-free region is a major concern. The worms alter the soil and microbial communities where they dwell, laying the groundwork for invasions of nonnative plants that would alter the island’s unique ecosystem and threaten biodiversity there, said Travis Longcore, a spatial scientist at USC.


“Even though we think of earthworms as being good for gardens, they are bad for ecosystems where they are not native because they end up changing vegetation and plant distributions,” said Longcore, coauthor of a recent study that described the situation in the journal Diversity and Distributions.

The study, led by researchers at Oklahoma State University in Stillwater and funded by the Navy, represents the first attempt to systematically quantify the distribution of invasive earthworms on an isolated island where local species have no room to hide from the interlopers, he said.

None of the earthworm species found on the island were native to North America, instead hailing from Europe and South America. The researchers found evidence suggesting the invasion is still in its early stages.

For example, the earthworms were first spotted on San Clemente in 2013, having never been seen during archaeological excavations throughout the island over the last 35 years. In addition, all of the earthworms detected so far were near the road that was being constructed along the spine of the 21-mile-long island in 2008.

Complete eradication of the invasive earthworms is unlikely, Longcore said. However, he did not rule out the use of experimental treatments, such as application of tea seed oil, that have produced positive results at golf courses and airports.

In the meantime, native plants and animals on San Clemente are showing signs of recovery, thanks to a series of steps taken by the Navy.

The first one came in 1992, when it removed hundreds of feral goats and pigs — descendants of animals brought to the island by seamen as far back as 200 years ago.

The Navy also launched aggressive efforts to replant native plants nurtured in greenhouses.

A captive breeding program has boosted the number of San Clemente loggerhead shrikes, a bird species, to more than 70 pairs. Across the island, signposts urge people to watch for more than 1,100 San Clemente Island foxes, up from a few hundred a decade ago.

An estimated 21 million San Clemente night lizards occupy the 75-square-mile-island, one of the highest densities of any lizard on Earth.

Scientists hope the earthworms won’t undo this progress.

“Time will tell the full effect of the invasion of earthworms on San Clemente Island,” Longcore said in a recent blog post.

“I hope this research provides a warning to those proposing and doing construction on oceanic islands with high biological diversity and natural values,” he said, “to sterilize any building materials being imported for use lest they introduce unexpected species to environments where they can do damage.”

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