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.

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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.

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