We’ve all seen earthworms and think we know all about them. But do we? We think they’re good for our gardens, and that’s true. They aerate the soil and provide nutrients. But what’s good for the garden isn’t good for most forests in North America. That’s because earthworms are an invasive species. When the ice sheets moved across the continent more than 10,000 years ago, they wiped out native earthworms. Today, most earthworms north of Pennsylvania are non-native. They arrived with the European explorers.

The northern forests evolved with other ways to get nutrients from the ground, like from fungi and other soil organisms. When earthworms turn soil and provide nutrients, it’s easier for invasive weeds and shrubs to establish, changing forest dynamics.

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