It hardly seems fair, but water creatures nimble enough to avoid being gobbled up by predators might harm their species more than help, new research suggests.

Fish, amphibians and even tiny zooplankton do many things to escape hungry enemies, from finding new homes to changing their physical characteristics. Such tactics may save individual lives - but in the long run might leave the population worse off, Michigan State University scientists say.

"When you introduce a predator into a system ... the potential prey don't sit around and say, 'Eat me,'" fisheries biologist Scott Peacor said. "They have adapted to get out of the way. But that comes at a cost."

Though the study focused on two particular species in the Great Lakes, it has implications for other predator-prey relationships, Peacor said Thursday.

It also illustrates the complexity of the danger from invasive species, which scientists describe as one of the biggest threats to the lakes' delicate ecological balance. At least 183 such species have been detected in the lakes. Most apparently were carried across the ocean in ballast tanks of freighter ships.

Peacor collaborated with Michigan State doctoral candidate Kevin Pangle and Ora Johannsson of the Great Lakes Laboratory for Fisheries and Aquatic Sciences in Ontario, Canada.

They studied what happened when the spiny water flea, an invasive predator native to Europe and Asia, encroached on locations in lakes Erie and Michigan inhabited by a zooplankton species called daphnia.

As the water flea's numbers increased, many daphnia withdrew to deeper, darker waters - apparently having learned to smell their pursuers. The tactic enabled many to survive. But because the water was colder, their community's birth rate plummeted.

Some also grew longer spines, making them harder to eat. But it also slowed them down, hampering their own ability to catch food.

The study found that such "nonlethal" effects from evading predators could do 10 times more damage to the daphnia population than for some to get eaten.

It also showed that invasive species can cause harmful behavioral changes in their prey even when they've been exposed to each other for relatively short periods, Peacor said.

The same principles likely hold true for other predators, he said.

"All animals evolve ways to sense predation risk and avoid it, from the (single-cell) paramecium to elephants," he said.

Earlier studies documented sharp declines in two other daphnia species after the spiny water flea arrived in the Great Lakes in the 1980s, said David Bunnell, a fisheries biologist with the U.S. Geological Survey's Great Lakes Science Center.

But those studies presumed the only reason was that the water flea was eating daphnia, Bunnell said. The Michigan State team's findings offer an additional explanation: a slower growth rate.

"Certainly the hypothesis makes sense," he said.

Still, Bunnell said there had been no apparent reduction in the type of daphnia that Peacor's group observed. If that happened, it could affect the lakes' food web. The daphnia provides crucial nourishment for juvenile fish such as alewives and smelt that, in turn, are eaten by larger sport fish.

Todd Atwood, a research biologist with the Arizona Game and Fish Department, said the Peacor team's findings about damaging effects from avoiding predators offered insights for his studies of interactions between wolves, cougars and their prey in the West.

When wolves return to an area where they've long been absent, elk spend less time in open spaces - where they're easier targets - and more in places with heavier tree growth.

But that leaves the elk less time to forage. And it boosts their exposure to another predator: cougars.