CORVALLIS, Ore. – Scientists and resource managers have always been interested in how animals in the ocean find their prey and the relative health of marine ecosystems is often judged by the abundance of food for the myriad species living there.
But new studies focusing on ocean “patchiness” suggest that it isn’t just the total amount of prey that is important to predators – it is the density of the food source, and ease of access to it.
Kelly Benoit-Bird, an Oregon State University oceanographer, outlined the importance of this new way of looking at ocean habitats during a keynote talk Wednesday (Feb. 22) at the 2012 Ocean Sciences meeting in Salt Lake City, Utah.
Sophisticated new technologies are helping scientists document how predators target prey, from zooplankton feasting on phytoplankton, to dolphins teaming up to devour micronekton, according to Benoit-Bird, who received a prestigious MacArthur Fellowship in 2010.
“We used to think that the size and abundance of prey was what mattered most,” said Benoit-Bird, a marine ecologist who studies relationships among marine species. “But patchiness is ubiquitous in marine systems and ultimately dictates the behavior of many animals and their relationships to the environment. We need to change our way of thinking about how we look at predator-prey relationships.”
Benoit-Bird pointed to a section of the Bering Sea, where her research with collaborators had estimated the abundance of krill. Closer examination through the use of acoustics, however, found that the distribution of krill was not at all uniform – and this may explain why two colonies of fur seals and seabirds were faring poorly, but a third was healthy.
“The amount of food near the third colony was not abundant,” she said, “but what was there was sufficiently dense – and at the right depth – that made it accessible to predators.”
The ability to use acoustics to track animal behavior underwater is opening new avenues to researchers. During their study in the Bering Sea, Benoit-Bird and her colleagues discovered that they could also use sonar to plot the dives of thick-billed murres, which would plunge up to 200 meters below the surface in search of the krill.
Although the krill were spread throughout the water column, the murres ended up focusing on areas where the patches of krill were the densest.
“The murres are amazingly good at diving right down to the best patches,” Benoit-Bird pointed out. “We don’t know just how they are able to identify them, but 10 years ago, we wouldn’t have known that they had that ability. Now we can use high-frequency sound waves to look at krill, different frequencies to look at murres, and still others to look at squid, dolphins and other animals.
“And everywhere we’ve looked the same pattern occurs,” she added. “It is the distribution of food, not the biomass, which is important.”
An associate professor in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University, Benoit-Bird has received young investigator or early career awards from the Office of Naval Research, the White House and the American Geophysical Union. She also has received honors from the Acoustical Society of America, which has used her as a model scientist in publications aimed at middle school students.
Her work has taken her around the world, including Hawaii where she has used acoustics to study the sophisticated feeding behavior of spinner dolphins. Those studies, she says, helped lead to new revelations about the importance of patchiness.
Ocean physics in the region results in long, thin layers of phytoplankton that may stretch for miles, but are only a few inches thick and a few meters below the surface. Benoit-Bird and her colleagues discovered a layer of zooplankton – tiny animals that feed on the plankton – treading water a meter below to be near the food source. Next up in the food chain were micronekton, larger pelagic fish and crustaceans that would spend the day 600 to 1,000 meters beneath the surface, then come up to the continental shelf at night to target the zooplankton. And the spinner dolphins would emerge at night, where they could reach the depth of the micronekton.
“The phytoplankton were responding to ocean physics,” Benoit-Bird said, “but all of the others in the food chain were targeting their prey by focusing on the densest patches. We got to the point where we could predict with 70 percent accuracy where the dolphins would show up based just on the phytoplankton density – without even considering the zooplankton and micronekton distribution.”
Ocean “patchiness” is not a new concept, Benoit-Bird says, but may have been under-appreciated in importance.
“If you’re a murre that is diving a hundred meters below the surface to find food, you want to maximize the payoff for all of the energy you’re expending,” Benoit-Bird said. “Now we need more research to determine how different species are able to determine where the best patches are.”