Until recently, when scientists studied changes in open-ocean ecosystems, they looked at changes in the bottom of the food chain that gradually make their way to the top, like food shortages and shifts in the ocean environment. A recent study, however, suggests that more attention needs to be given to top-down influences as well.

For example, over the past few decades, there's been a major collapse of the populations of harbor seals, fur seals, sea lions, and sea otters living on the coasts of western Alaska. Now a new study suggests that all this destruction can be traced back to humans overfishing sperm and baleen whales between 1946 and 1979.

Killer whales, who live at the top of the ocean food chain, used to prey on sperm and baleen whales. When these were no longer available, the killer whales were forced to fish farther down the food chain. First they fed on seals, which were the easiest to catch and the most nutritionally valuable.

Then, when seals became rare, the killer whales turned to sea lions. When the sea lion population crashed, they started fishing sea otters. The domino effect reaches even farther. Without sea otters keeping sea urchins in check, their population has exploded. They've destroyed the kelp forests, and with it, the kelp forest ecosystem in southwestern Alaska.

Animals switching food sources can have a very serious effect on the environment. We humans, as the ultimate predators, need to give a lot of thought to the choices we make. They may affect the food web in unintended ways.