24 Feb 2012

The Tangliest of Food Webs

The idea of food chains and food webs is one of the first ecological concepts that children often learn. (I'm sure most of us have played a version of a food web game, where, in the end, everyone is connected to everyone else, if a few trophic levels away.) But at last weeks' AAAS conference I saw food webs in a new light. And learned just how very, very complex they can be.

Research by the Santa Fe Institute's Jennifer Dunne added human hunter-gathers to marine food webs. To do this, she synthesized 5000 years of biological, archaeology, ethnographic and other data from marine systems in the northeast Pacific. Her results showed that the Aleuts of Sanak Island, Alaska were "super-generalist" predators and they ate foods from a wide spectrum of sources. Dunne said that this flexibility "likely helped stabilize the entire ecosystem." In essence, when one food source became low, they moved to another giving species time to recover.

This flexible grazing (or, "prey switching") method is in stark contrast to the modern-day economics-driven pressures that can destabilize food webs. As Dunne explained in a briefing note, "[Switching prey when a population is low] is natural behaviour for predators. It's stabilizing for the system because it allows populations to recover." This is in stark contrast to modern economic systems. Again Dunne explains using blue-fin tuna as an example, "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."

Remarkably, this is one of the first studies of food webs that includes humans. In the end, the very tangled food web had more than 6000 feeding links. Dunne's research found that the humans in her study fed on 50 of the 171 taxa available to them. And they lived on Sanak Island for thousands of years without other species going extinct.

No comments: