A large body of scientific literature deals with the vexing and ever-increasing problem of invasive species. But until now studies of native predators and invasive prey have been relatively one-sided, focusing mainly on how predators can keep non-native species in check or, by not eating them, enable them to establish and become invasive.

Ecologists Lauren Pintor of the Ohio State University and James Byers of the University of Georgia wondered instead about the effect of non-native prey species on native predators. That hasn’t been studied much, perhaps because it seems obvious that more prey should be good for predators. But reality is a little more complicated: non-native prey only benefit predators when lots of native prey species are also present, the researchers reported last week in Ecology Letters.

The researchers sifted through the scientific literature and found 109 studies that reported the effect of non-native animal prey on native predator abundance, growth rate, survival, or reproductive success. Altogether, the studies covered 93 predators and 47 non-native prey species.

About three-quarters of the studies involved observing what happens when non-native prey species are introduced into complex ecosystems that also contain a variety of native prey. The remainder were laboratory experiments in which researchers gave predators access to a single native or non-native prey species and compared the effects of each.

In real-world ecosystems, predator populations increase as much as 57% after introduction of a non-native prey species. The availability of non-native prey also increases predator growth rates.

But laboratory experiments tell a different story. When native predators only have non-native prey to eat, their reproductive success declines compared to predators that are eating native prey. (The laboratory studies showed no effect of non-native prey on predator survival or growth rates.)

The contrast suggests that non-native prey are the ecological equivalent of junk food: okay to eat in moderation, but not something you’d want to survive on exclusively.

“It may be that the new prey isn’t as nutritious, or that the predator hasn’t evolved the ability to eat or digest it well,” says Pintor. So even though non-native prey may have fewer defenses against predators in their new environment and be easier to catch, the predators continue to seek out their favored native prey species.

But the availability of non-native prey can still be good for predators overall when consumed as an occasional snack. “To me, the most interesting finding is that non-native species seem to really aid native predators when they serve as a supplemental food source,” Byers says.

In fact, non-native prey has contributed to the recovery of the Lake Erie watersnake, which began to increase in population when it learned to prey on the round goby, an invasive fish introduced from Europe. In this case, the non-native prey seems to have functioned more like a protein shake, helping to bulk up an infirm species.

In the studies Pintor and Byers identified, about 70% of the non-native prey species were either insects or crustaceans, and a little over 60% of the native predators were either birds or insects. So it’s not clear whether the same patterns will apply more broadly to other taxonomic groups.

And the studies don’t capture the indirect effects of non-native prey on ecosystems, the researchers caution. For example, if the availability of non-native prey increases the number of native predators, this could in turn increase pressure on native prey species and drive down their populations. Of course, this should come as no surprise to humans, who know all to well that junk food, once introduced, can be hard to control. – Sarah DeWeerdt | October 20, 2015

Source: Pintor L.M. and J.E. Byers “Do native predators benefit from non-native prey?” Ecology Letters DOI: 10.1011/ele.12496

Header image: An invasive round goby from Lake Erie. Credit: Ohio Sea Grant via Flickr.