The scientists catalogued and weighed all of the plants, animals and parasitic species living in three river estuaries. They found that in terms of sheer bulk, parasites represent about three percent of the biomass of these ecosystems. Pound for pound, one parasite—the trematode fluke that infects a certain snail—outweighs all of the estuaries’ birds, which are the ecosystem’s top predators. The results appear in the July 24 issue of Nature. That means that creatures you can’t see might be even more important to the health of an ecosystem, and to its balance of power, than the ones you can see. For example, in these estuaries, snails that are infected with trematodes outnumber those that are fluke-free. Now that’s what you call a controlling interest.

—Karen Hopkin

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