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Invasive species can decrease biodiversity and drive resident species to the brink¹ of extinction². But how do these interlopers fare so well in unfamiliar³ territory? One idea is that they’ve escaped their enemies, for example, the parasites⁵ that keep them in check on their home turf. But a study in the journal Biology Letters suggests that notion doesn’t always stand up. Because at least one kind of invasive shrimp⁶ is an even bigger pest when it has a parasite⁴ on board.

The scientists were studying a freshwater shrimp commonly found in England. Fishermen brought the shrimp to Northern Ireland in the 1950s as a tasty treat for local trout⁷. Well, the shrimp liked the new digs. So much that they’ve all but eliminated the native shrimp species. The strange thing is, they may have parasites to thank.

In some streams, 70 percent of the invaders⁸ are infected with an intestinal⁹ worm that doesn’t go for the native shrimp. But this gut¹⁰ worm doesn’t slow them down. In fact, infected shrimp eat 30 percent more prey¹¹ than those that are parasite-free.

Which perhaps is not surprising. If you had a worm that constituted a quarter of your body weight, you’d eat more too.

Thanks for the minute for Scientific American's 60-Second Science. I'm Karen Hopkin.