Predator-prey interactions, and the fear response, help shape an ecosystem.
Being a predator that has to kill other animals to survive "is such a regal responsibility to have," biologist Austin Gallagher told me recently. Not only do they hunt for a living, they also play a critical ecological role: they help shape the entire food web they're in, partly based on the fear responses of the animals that could become their next meal.
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Gallagher, who splits his time between Miami and Washington DC, has spent most of his career studying ocean predators, like sharks. He recently turned his attention to birds of prey. As a postdoctoral researcher at Carleton University in Ottawa, he visited a research station at Opinicon Lake, in Ontario, and noticed a ton of osprey there. "We thought, why not do a study on osprey, looking at [the effect they have] on the fish in this lake?"
Using real live osprey in this study, though, wasn't really possible. "I'm not a bird wrangler," said Gallagher, who is CEO of the ocean conservation nonprofit Beneath The Waves. So he and his team found a Virginia Beach-based company called Jackite that sells realistic models of all kinds of birds—basically, they're bird-shaped kites attached to long, telescopic poles—used by ranchers and others who need to get rid of pests, he told me.
"You pay $45 and they send you an osprey on a kite," he said. "It flaps its wings and everything. We got it to the lake, and oh boy did it work well."
In a new paper, published in the Canadian Journal of Zoology, Gallagher and his co-authors describe how they used these fake ospreys to terrify a small, freshwater type of sunfish called pumpkinseeds, common in Ontario lakes. Female pumpkinseeds will come in and lay their eggs, and then the males stick around on patrol to defend their nests.
"Their whole life is defending these eggs against other fish," he said. Other fish will try to eat the eggs, or other males will try to sire them. It's kind of weird."
They installed GoPro cameras near the fish nests and watched how, when a fake osprey showed up overhead, male fish behaviour changed dramatically. Most of the fish actually abandoned their nests for some stretch of time, presumably to save themselves from the bird. Researchers observed a major drop in protective parenting behaviours when the fake ospreys showed up, Gallagher explained.
"I think we had 40 fish total, and there were three cases of total abandonment, in which they never came back," he said. (Most of them eventually did return to their posts.) "There were a couple of exceptions of super robust dads that would never leave, but in most cases, they want to live to reproduce another day."
In a follow-up study, which isn't yet published, the researchers took blood samples from fish living near or far from osprey nests, and found that fish living closer to the nests have a higher baseline stress level. But they also appear to recover more quickly from a stressful event—in this case, being exposed for a few minutes to the open air.
This shows the importance of maintaining a "balance" between predator and prey, Gallagher said. "If there were no predators, all these fish would [presumably] be good dads and the egg survival rate would be really high," pumping lots of baby fish out into the lake, and upsetting the system. "All those babies would make a lot more hungry mouths to feed." This is called a trophic cascade. In extreme cases, it can cause an ecosystem to collapse.
Taking predator-prey interactions into account is crucial for conservation, he said.
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