Research shows a predator affects prey species more through scaring than eating
By Tom Oswald
Published June 15, 2007
In the animal world, sometimes getting eaten by a predator is only the tip of the iceberg when it comes to the predator’s effect on a species.
Research by an MSU professor shows that it is often the nonlethal effects that a predator has on its prey that are more dangerous, at least in the long run, to a species than the lethal effects are.
The work of Scott Peacor, assistant professor of fisheries and wildlife and visiting scientist at the Great Lakes Environmental Research Laboratory, in collaboration with lead author Kevin Pangle, a doctoral candidate in fisheries and wildlife, and a Canadian colleague, is published in the journal Ecology, a publication of the Ecological Society of America.
The research finds that when a predator species invades another species’ territory, the prey perceive the threat of the predator and move to another, often less hospitable environment, something that can have long-term negative effects on that species.
Working in Lake Michigan and Lake Erie, Peacor and his colleagues studied what happened when an invasive predator species – the spiny water flea (bythotrephes) – invaded territory inhabited by a smaller zooplankton prey species – daphnia.
Because the spiny water flea is a visual predator, the daphnia retreated to deeper, darker waters to avoid being eaten, but these waters were also colder and less conducive to reproduction. Laboratory experiments by Peacor and colleagues have shown that “scared” daphnia know when to retreat because they can smell the predators.
“In this case, the daphnia that didn’t get eaten were hiding down in the cold water,” Peacor said. “The fact that the spiny water flea has actually eaten some daphnia might be inconsequential compared to the fact that they’ve affected the behavior of all the others. This induced migration reduces predation risk but also can reduce birth rate due to exposure to cooler temperatures.”
Peacor’s research found that these so-called nonlethal effects the spiny water flea had on the daphnia were comparable or, in some cases, as much as 10 times greater than the effects due to eating the prey.
“So if you were to look at what the spiny water flea was eating to gauge its effect on the ecological community,” Peacor said, “you would arrive at a very incomplete and inaccurate picture.”
These findings could affect the management of the Great Lakes food webs, because they present an important mechanism by which the spiny water flea affects the abundance and location of daphnia. Daphnia are an important food source of larval fish and, therefore, play a key role in the food web and are important to Great Lakes fisheries.
This type of nonlethal predator effect, said Peacor, likely extends to other species in the Great Lakes including predatory fish and other species in other habitats.
“In effect, the cartoons of a big fish eating a smaller fish eating a smaller fish to describe what is happening in nature may often be missing larger factors governing how species affect each other,” Peacor said.
An invasive species, the spiny water flea is native to Europe and Asia. It was introduced to the Great Lakes by ballast water from ocean-going ships in the early 1980s.
Peacor said that this sort of nonlethal effect in ecology is something that humans can relate to, because people change their behavior in many circumstances in relation to threats both on global and individual levels.
Peacor’s work was supported by the Great Lakes Fishery Commission, The Great Lakes Environmental Research Laboratory of the National Oceanic and Atmospheric Administration, the National Science Foundation and the Michigan Agricultural Experiment Station.