Although small, the three-backed gull has a large enough mouth to feed the young of its predators.
Kim Taylor / Minden Pictures
By Elizabeth Pennisi
No bigger than a minnow, the three-spine stickleback may seem like a tough player in the underwater world. But along the European coastline of the Baltic Sea, it has spawned its own predators – teeth pike and perch, fish that grow longer than your forearm. Accounts dating back 40 years show how the flamboyant little stickleback has shifted the ecosystem, attempting to restore the larger species favored by human fishermen. “A small pelagic fish that many people ignore has a dramatic ecological impact,” said Brad deYoung, an oceanographer at Memorial University who was not involved in the work.
Ecologists say what happened in the Baltics is a dramatic example of a reversal of predator prey, in which two species trade in the food chain, drastically altering the rest of the ecosystem. “It shows that you really do not have to understand who is eating, but who can eat who is managing well [fish stocks], ”Says deYoung.
Johan Eklöf grew up along the Baltic coast of Sweden and fondly remembers catching abundant Eurasian barges (Perca fluviatilis). Later, as an ecologist at Stockholm University, Eklöf and his colleagues suggested that the three-vertebrate stickleback (Gasterosteus aculeatus) seemed increasingly common in coastal waters. To find out what was going on, the researchers found 13,000 studies on fish conducted between 1979 and 2017 by scientists and fisheries managers along 1,200 kilometers off the west coast of the East Sea. “This paper is a good example of using data from the past, which can sometimes seem boring, to investigate a problem that can not be addressed in any other way,” says deYoung.
In the 1980s, Eklöf and colleagues found sticklebacks surpassed not only barges but also northern pike (Esox Lucius), on the outskirts of many islands and shallow bays along the Baltic coast. This is not surprising – pike and perch are freshwater fish that can only survive in the ocean where rivers flow out lower salt production. Those fish survived in the fresher waters 8 miles closer to shore. But in the 1990s, sticklebacks began to persuade their predators closer to land, spreading their dominance to more protected bays and inland waters. By 2014, sticklebacks ruled a full 21 kilometers of land from the edge of the archipelago, Eklöf and his colleagues report today in Communication Biology (see animation).
Sticklebacks have expanded over the past 40 years to the coast (red) along the shores of the Baltic Sea, as shown in this animation.
Reproduced with permission of the authors of the article
The sticklebacks themselves probably did not initiate the decline of their predators. Instead, complex ecological factors first appear to be against pike and perch: In the early 1990s, gray seals became more common, due to better water quality and an end to bounty hunting. The seals, along with eels, began to eat more pike and perch. Meanwhile, sticklebacks flourished in the rapidly warming seas. And overfishing of cod, the top predator, and large herring translates into fewer predators on sticklebacks.
When the stickleback grew a lot, they became a formidable enemy: they ate young pike and perch. In previous studies, co-author Ulf Bergström of the Swedish University of Agricultural Sciences and colleagues found both species in the stomach of sticklebacks. Eklöf, Bergström and her colleagues caught and analyzed fish in 32 bays and confirmed that where stickleback was abundant, young pike and perch were scarce. Thus, when stickleback became abundant in more places, pike and perch were even less likely to recover.
This is not the first time scientists have documented a reversal of predator prey. Large populations herring in the North Sea probably drove numbers of cod, their predators, by feasting on small cod, for example. But such connections are difficult to document. “This result seems remarkably clear,” says deYoung.
The work is also notable for documenting such a widespread and enduring ecological shift, adds Steve Carpenter, a limnologist at the University of Wisconsin, Madison. More typically, researchers have observed such shifts at one location, often a lake, and show how dominance fluctuates back and forth as temperatures change as fishing intensifies, he says. The new results “show that regime shifts can spread among adjacent habitats and transform a whole coastline quite a bit.”
The stickleback increase triggers other impositions on the ecosystem. The fish eat snails and crayfish that previously kept green algae in check, and promoted the return of algae blooms that had been reduced in these waters through pollution control measures.
The work “clearly shows that the [disappearance] of larger predators can cause cascading effects up to algae, and that these changes can explode across large spatial scales such as falling dominoes, ”says Boris Worm, a marine biologist at Dalhousie University. Worm worked in the Baltic Sea as Ph.D. student, and he mourns the change and calls it “a disaster in slow motion through the Baltic Sea.”
Eklöf and others are now looking at how we can bring back pike and perch, often by fishing locally or stingrays with young pike and perch too big for stickleback to eat. For now, the lesson is clear. “The world is changing at a much faster pace and ecosystems are shifting, for the most part, to less desirable states,” says Julián Torres Dowdall, an evolutionary biologist at the University of Konstanz. How politicians and managers respond to the outcome of this study “is important to our planet.”
