This is Scientific Americans 60 Second Science. I'm Jason Goldman. Got a minute?

Predators like wolves affect their ecosystems by eating their prey.

But a more subtle impact involves fear. Predators also

terrify prey species. And when, for example, elk are hiding, they don't spend as much time eating leaves. The impact of a predator down

through the food web all the way to plants is called a trophic cascade. Meanwhile, fish at a coral reef near the Fiji archipelago in the South Pacific generally

graze on the seaweeds that grow on the reef. But when reef sharks emerge from deeper waters, it's best to quit foraging and hide instead.

And thinking about these other ecosystems like wolves, their effects in their ecosystem don't play out in all places at all times.

They happen to be most pronounced in risky habitats,

like river valleys or gorges.

Marine scientist Douglas Rasher, from the nonprofit Bigelow Laboratory for Ocean Sciences in Maine.

So it got me thinking that maybe these shallow habitats might be the place where sharks have their most pronounced effects on the ecosystem.

At one time, researchers did not think trophic cascades even existed in the real world,

and many still debate whether sharks can drive trophic cascades on coral reefs.

By observing reef communities in Fiji's Votua Marine Reserve, Rasher and his team discovered

that sharks do in fact influence plant growth on the reefs by scaring the herbivorous fish away from eating them.

Here's how it works.

When the tide rises, sharks make hunting raids into the shallow lagoons.

The fish stop eating and hide instead.

But during low tide, the predators are isolated in deeper waters,

unable to access the reef and close the goons.

That's when the fish can safely graze.

All of the fish in the system have a very keen sense of when the tide is coming up and when the tide is going out.

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