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This is Scientific American's 60-Second Science. I’m Karen Hopkin. It’ll just take a minute.

When you think about an ecosystem¹, you usually think of the big animals that live there. The Serengeti’s ruled by lions. And estuaries² are populated by fishes, birds, snails³ and maybe the occasional otter⁴. But there’s more to an ecosystem than meets the eye. Because a team of scientists from the US and Mexico has found that parasites⁵ constitute a sizeable chunk⁶ of the biomass of an ecosystem. The scientists catalogued and weighed all of the plants, animals and parasitic⁷ species living in three river estuaries. They found that in terms of sheer bulk, parasites represent about three percent of the biomass of these ecosystems⁸. 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 predators⁹. 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.

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