Could predator adaptation be a bad thing?

For a thousand years, humans systematically hunted the world’s great whales – humpbacks, right whales, sperm whales, and so on – for their meat, oil, and bones. Some whale populations were reduced by as much as 90 percent, and the planet’s total whale biomass might have been reduced by some 85 percent. Southern hemisphere blue whales were hunted to within just 1 percent of their existence. That was a problem for killer whales, which are really the world’s largest dolphins. Prior to our overexploitation of the great whales, their calves were the primary food source for many of the world’s killer whale populations. Facing a dwindling food supply, killer whales had a chance: go hungry or find something new to eat.

Killer whales are smart, social, adaptable creatures. Rather than go extinct, the blackfish simply learned to hunt something else: pinnipeds. In large part, killer whales are probably to blame for the sudden decline in the western populations of Steller sea lions in the northern Pacific Ocean and Bering Sea. But seals and sea lions didn’t have the luxury of thousands of years of evolution to evolve anti-predation defenses. Killer whales were a new threat, so the pinnipeds declined. Ever undeterred, the killer whales changed their tastes again and began hunting sea otters. As sea otters declined, due both to the whales and to their value in the fur industry, the entire kelp forest ecosystem off the western US coastline suffered. Sea urchin populations exploded. They ate up all the kelp, and scores of species that had come to rely on the kelp were suddenly in trouble as well.

Ecologists call the killer whales’ ability to adapt their foraging techniques and prey species “adaptive rewiring.” It was long thought that adaptive rewiring following the loss of a preferred prey item would buffer against the possibility of a larger trophic cascade. But now researchers David Gilljam, Alva Curtsdotter, and Bo Ebenman, of Sweden’s Linköping University have discovered that the opposite is actually true. While predatory flexibility may allow a hungry species to thrive in the short term, it ultimately leads to a loss of ecosystem stability.

“Generally, we find almost no positive effects of rewiring on food web persistence,” they wrote in the journal Nature Communications. The researchers came to that conclusion after studying four real-world food webs as well as three types of computerized food web models. “On the contrary, the effect of rewiring on the persistence of both natural and computer-generated food webs is predominantly negative; the proportion of species going extinct increases with an increasing fraction of adaptive consumers in the webs.” That is, as food webs are stocked with more and more killer whale-like species, able to adjust their predation to their circumstances, the more likely the ecosystem is to suffer from a trophic cascade. The effects are especially strong if the predator in the food web is a specialist. If it was a generalist, then the extinction of one of its prey sources would cause is to simply redistribute its predation effort across its remaining prey species. Instead, specialist predators shift their attention almost entirely onto the new species. It’s almost as if the predatory species is a sort of “native invader”: a native species that has some of the same consequences as an invasive one.

It’s particularly worrisome, especially because humans are among the most adaptable predators on our planet. When one food resource dries up, it’s fairly easy for us to pick up and hunt something new. That’s been happening in Africa. As coastal fisheries have become depleted (through our own overfishing), West Africans have largely turned to hunting so-called “bushmeat” inside of nature reserves. While that’s been sufficient to keep them well fed, it’s led to severe declines in more than 40 mammal species, including baboons, duiker, hartebeest, kob, and porcupine.

So don’t get too excited that polar bears seem able to find alternative sources of nutrition as the ice-free summer grows longer. Indeed, the strain from climate change endangers not only polar bears, but also their food sources, especially if they can’t sustain the increased pressure of predation following the bears’ adaptive rewiring. It might save them for a while, but the long-term health of their ecosystem is still very much an open question. – Jason G. Goldman | 09 October 2015

Source: Gilljam, David, Alva Curtsdotter, & Bo Ebenman. (2015). Adaptive rewiring aggravates the effects of species loss in ecosystems. Nature Communications 6:8412. DOI: 10.1038/ncomms9412.

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