June 15, 2007 — 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.
