Killer Whales are Causing a Trophic Cascade

Killer whales are causing a tropic cascade in the Northern Pacific ocean. A tropic cascade is when a top carnivore decreases the abundance of a lower carnivore, which leads to an increase in herbivores and then a decrease in primary producers.  

Over the past two decades, Alaskan Sea Otter populations have been steadily declining.  This decline is caused by killer whales.  Normally killer whales feed on fish, seals, sea lions, and walruses.  In the past two decades, sea otters have been added to the list of killer whale prey.

Sea otters live among the kelp forests in the North Pacific. They are considered a keystone species in kelp forests. They prey on sea urchins.  Sea urchins are herbivores and eat kelp, but the presence of sea otters control the sea urchin populations, which allows the kelp forests to grow. Now since sea otters are declining, it is allowing an overgrowth of sea urchins, which is causing the destruction of kelp forests.  The destruction of the kelp forests is also affecting bald eagles, as they feed on kelp forest fish.

Scientists are unsure what is causing killer whales to prey on sea otters. There are many different theories about the cause. Some scientists have speculated that killer whales used to feed on whales, but stopped because of overexploitation cause be commercial whalers.  With whale populations low, killer whales turned to smaller mammals such as seals and sea lions. However, seal and sea lion populations have decreased. They believe that this decrease has led to sea otters becoming prey to killer whales. Other scientists believe that killer whales would eat the harpooned whales left by commercial whalers, but did not actively seek out live whales. When commercial whaling stopped, they believe that killer whales turned to smaller mammals as food, which in turn started the tropic cascade.

Sarah Tebo (3)

http://www.nature.com/nature/journal/v445/n7129/full/445703a.html