To find out, the trio set up large experimental reefs along the eastern coast from Florida to North Carolina. At each site, they created three reefs: one containing just oysters; one containing oysters and their immediate predators, small mud crabs; and one containing the oysters and the mud crabs, as well as a scary fish and large crabs that feed on the mud crabs.
What they found surprised them. In North Carolina, the picture looked exactly the same as it did in the early 2000s, when Grabowski first did his behavior experiments. But from South Carolina to Florida, the fear of predation on the mud crabs actually had a negative effect on the overall reef.
That's because as you move southward, the rivers that flow into the estuaries become muddier. And more mud means the healthy, thriving oysters start to get buried under layers of sediment. When top predators are scarce, the mud crabs feel safe enough to wander around the reef to eat baby oysters. As they do so, they kick off the river sediment and free the reef from being buried alive. It's not so good for the reef when the crabs' fear of being eaten prevents these reef strolls.
In North Carolina, there isn't as much sediment on the reef so the oysters don't benefit from the mud crabs' walking around.
"Georgia and South Carolina are super-sized value meals," Kimbro said. "It's like the whole system is on steroids. There's so much oyster food there, the crabs are so well fed they don't wander far from their home in the reef anyway." So, fear isn't as important there.
Each of these hyper-local situations causes a unique effect, complicating the age-old story of the certain benefits of top predators. "This shows that you can't just apply something across the board," Kimbro said. Instead, researchers have to consider ea
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