Ancient humans as major predators in marine food webs

By synthesizing 5,000 years of biological, archeological, ethnographic, and other data from marine systems in the North Pacific, Dunne and colleagues have characterized how humans fit into complex marine food webs, how they compare to other predators, and how their behaviors might have affected long-term ecosystem sustainability.

The results are surprising: Despite being "super-generalist" predators that fed on more species than other predators, the Aleut of Sanak Island, Alaska frequently switched among their many food sources, a flexibility that likely helped stabilize the entire ecosystem. This research provides a network-based perspective on how modern-day economic pressures might drive ecologically dysfunctional overharvesting of rare species and potential destabilization of whole food webs.

Dunne presented the work in a talk entitled "The Roles of Human Hunter-Gatherers in North Pacific Food Webs," at the American Association for the Advancement of Science meeting in Vancouver, B.C. Her talk was part of a symposium on "Historical Biocomplexity in the North Pacific Ocean: Lessons from the Past."

The Sanak Island data are the first highly resolved food webs that include humans, Dunne says. The data include more than 500 species and over 6,000 feeding links. The detail of the Sanak data raises the prospect of addressing many interesting questions in new ways, such as how humans compare to other species, and how they might have affected the sustainability of the system.

"Our analyses show that humans played special roles in the marine food webs," Dunne says. "As super-generalists, they fed on more kinds of species than most other predators." In the intertidal web, for example, the Aleut fed on 50 of 171 taxa. "They also were highly omnivorous," she says, "eating everything from kelp to sea otters."

While that may sound like a bad thing, it can be just the opposite, she explains. "We know that the Aleut lived on this island for thousands of years without other species going extinct. A super-generalist can co-exist with other species if it focuses on just a few of its possible prey at a time and switches prey regularly."

This is because generalists tend to focus on a preferred food source until it gets too difficult to find, and then they switch to something less preferred but easier to find. In the case of the Aleut, seasonal salmon may have been on the menu for awhile, followed by marine mammals, and when the weather got too stormy for hunting they could gather mussels and snails from the intertidal.

"This is a natural behavior for predators. It's stabilizing for the system because it allows populations to recover," Dunne says.

But food switching tends to be the opposite of what often happens in modern economic systems. Take blue-fin tuna, for instance. As the premium sushi tuna gets scarcer, its value goes up, and fishing becomes more profitable, leading to more, rather than less, pressure on tuna populations. This "increased rarity-higher value-more harvesting" cycle tends to drive species toward extinction and introduces dynamics that might destabilize the whole food web.

Dunne believes it is important to make sense of human impacts within an ecological framework, rather than treating humans as external to natural systems. "This type of network-based research provides new ways to understand human roles and impacts within complex ecosystems."