Stuck in the middle with oysters and crabs

He and his colleagues—associate pro­fessor Jon Grabowski and assis­tant pro­fessor Ran­dall Hughes, ecology experts with labs at Northeastern’s Marine Sci­ence Center—think a sim­ilar pat­tern shows up in oyster reefs, where the behavior of the “middle child” in the predator-​​prey food chain plays a strong role in deter­mining how the reef as a whole will fare. New research from the team, pub­lished online on Tuesday in the journal Ecology Let­ters, gives that hunch even more support.

The work com­pli­cates the evo­lu­tion of a par­a­digm that has per­vaded ecology since the 1960s, namely that the species at the top of the food web dic­tate the wel­fare of the entire system simply by eating.

For instance, obser­va­tions in the Aleutian Islands in the 1970s showed that when sea otters were doing well, the nearby kelp forests below the ocean’s sur­face also thrived. This was due, theory said, to the fact that the otters’ feeding pat­terns nat­u­rally man­aged the sea urchin pop­u­la­tion, which feeds on kelp.

Fast-​​forward four decades and one sees a large body of evi­dence indi­cating that preda­tors do more than eat; they frighten too. In the early 2000s, Grabowski showed that having a preda­tory fish scare the middle child has the same effect as pre­da­tion itself. Like­wise with the sea otters—just swim­ming around scares the urchins enough to send them into hiding and stop eating kelp.

These obser­va­tions have led researchers to assume that the mere pres­ence of a top predator is always ben­e­fi­cial for habi­tats like oys­ters and kelp, Kimbro said. “But,” he added, “we had a hunch that the lynch-​​pin of behavior is fickle. I mean, I can turn on a dime. Just ask my family when I miss snack time and the blood sugar gets low. So it seems log­ical that it’s not always going to be the same.”

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 each local com­mu­nity individually.

“You could walk away from our paper thinking it’s just so com­pli­cated why even try,” Kimbro said. “But with careful exper­i­ments and so forth you can sep­a­rate a pre­dictable pat­tern from what seem­ingly is just noise.”

The research group’s next step is to see how the behavior of the middle preda­tors affect the ser­vices that oys­ters pro­vide to the urban coastal com­mu­ni­ties nearby. Oys­ters don’t just pro­vide a tasty food source for humans (and job secu­rity for the oyster fishers that col­lect them), but they also filter the water of excess nutri­ents, which can have neg­a­tive effects on the overall body of water.

As to whether the pat­tern Kimbro claims to have observed in The Brady Bunch is really true, well, “just go with it,” he said.