Case Study: Orca vs Otters

Around the Aleutian Islands near Alaska, a major ecological transformation occurred between 1980 and 2000. Where 100,000 sea otters once thrived among the kelp forests is now a barren zone with heavy populations of sea urchins. Tuna and salmon fisheries had been established, eliminating the smaller fish populations. These small fish were prey for seals and sea lions. Orcas preyed on the seals and sea lions for centuries, but those animals disappeared. The orcas found a new food source: sea otters. In order to make up for the calories needed to live, they needed to eat A LOT of sea otters. In just 5 years, 3 orcas had consumed around 40,000 otters! The drop in sea otter numbers resulted in an increase of the otter food: sea urchins. The urchins destroyed the kelp by over-grazing.

What animal was taken out of the ecosystem which then caused the negative trophic cascade effect on this kelp forest ecosystem?

Orcas customarily feed on sea lions and seals, which are packed with high-calorie blubber. But the population of Steller sea lions, the world's biggest sea lions, took a sharp dive in the late 1980s. Harbor seals also declined at a similar rate. By 1992, otters were the only plentiful marine mammals left in Aleutian waters. The orcas, in their hunt for calories, apparently had been forced to switch prey.

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