No bigger than a minnow, the three-spine stickleback may seem a puny player in the underwater world. But along the European coastline of the Baltic Sea, it has edged out its own predators—toothy pike and perch, fish that grow longer than your forearm. Records dating back 40 years show how the flamboyant little stickleback has shifted the ecosystem, thwarting efforts to restore the larger species favored by human fishers. “A little pelagic fish that many people ignore is having a dramatic ecological impact,” says Brad deYoung, an oceanographer at Memorial University who was not involved with the work.
Ecologists say what has happened in the Baltic is a dramatic example of a predator-prey reversal, in which two species trade places on the food chain, drastically altering the rest of the ecosystem. “It shows you really need to understand not just who eats who, but who might eat who to properly manage [fish stocks],” deYoung says.
Johan Eklöf grew up along Sweden’s Baltic coast and fondly remembers catching plentiful Eurasian perch (Perca fluviatilis). Later, as an ecologist at Stockholm University, Eklöf and his colleagues noted that the three-spine stickleback (Gasterosteus aculeatus) seemed more and more common in coastal waters. To find out what was going on, the researchers unearthed 13,000 surveys of fish done between 1979 and 2017 by scientists and fisheries managers along 1200 kilometers of the western coast of the Baltic Sea. “This paper is a good example of using past data, which can sometimes seem dull, to explore a problem that cannot be addressed any other way,” deYoung says.
In the 1980s, Eklöf and colleagues found, sticklebacks outnumbered not just perch, but also Northern pike (Esox Lucius), at the outer edges of the many islands and shallow bays along the Baltic coast. That’s not surprising—pike and perch are freshwater fish, able to survive in the ocean only where river outflows lower salinity. Those fish prevailed in the fresher waters 8 kilometers closer to shore. But in the 1990s, sticklebacks began to outnumber their predators closer to land, their dominance spreading toward more protected bays and inland waters. By 2014, sticklebacks reigned a full 21 kilometers landward from the archipelago’s edge, Eklöf and his colleagues report today in Communications Biology (see animation).
The sticklebacks themselves probably didn’t initiate their predators’ decline. Instead, complex ecological factors appear to have first worked against pike and perch: Beginning in the 1990s, gray seals became more common, thanks to better water quality and an end to bounty hunting. The seals, along with cormorants, began to eat more pike and perch. Meanwhile, sticklebacks were thriving in the rapidly warming seas. And overfishing of cod, the top predator, and large herring translated into fewer predators on sticklebacks.
As the stickleback grew numerous, they became a formidable foe: They eat juvenile pike and perch. In earlier studies, co-author Ulf Bergström from the Swedish University of Agricultural Sciences and colleagues found both species in the stomachs of sticklebacks. Eklöf, Bergström, and their colleagues caught and analyzed fish in 32 bays and confirmed that where stickleback were abundant, juvenile pike and perch were scarce. Thus, as stickleback became more plentiful in more places, pike and perch had even less chance to recover.
This is not the first time that scientists have documented a predator-prey reversal. Large populations of herring in the North Sea likely drove down numbers of cod, their predator, by feasting on tiny cod juveniles, for example. But such connections have been difficult to document. “This result seems remarkably clear,” deYoung says.
The work also stands out because it documents such a widespread and lasting ecological shift, adds Steve Carpenter, a limnologist at the University of Wisconsin, Madison. More typically, researchers have observed such shifts in a single location, often a lake, showing how dominance swings back and forth between two species as temperature changes or fishing becomes more intense, he says. The new results “show that regime shifts can spread among connected habitats and transform an entire coastline rather rapidly.”
The stickleback surge is triggering other ecosystem impacts. The fish eat snails and crustaceans that previously kept green algae in check, favoring the return of algal blooms that had been declining in these waters thanks to pollution control measures.
The work “clearly shows that the [disappearance] of larger predators can cause cascading effects all the way down to algae, and that these changes can unfold over vast spatial scales like falling dominoes,” says Boris Worm, a marine biologist at Dalhousie University. Worm worked in the Baltic Sea as a Ph.D. student, and he mourns the change, calling it “a slow-motion disaster through the Baltic Sea.”
Eklöf and others are now considering how to bring back pike and perch, perhaps by locally fishing out stickleback or stocking bays with juvenile pike and perch too big for stickleback to eat. For now, the lesson is clear. “The world is changing at a very fast rate and ecosystems are shifting, most times, to less desirable states,” says Julián Torres Dowdall, an evolutionary biologist at the University of Konstanz. How politicians and managers respond to the result of this study “is important to our planet.”