Final answer:
Krill are considered a keystone species in the Southern Ocean because they are a crucial food source for many marine animals, helping to maintain biodiversity and ecological balance in the region.
Step-by-step explanation:
Krill can be considered a keystone species for the Southern Ocean due to their significant role in the marine food web. These small crustaceans are a primary food source for a variety of marine organisms such as fish, penguins, seals, albatrosses, and whales. The krill thrive by feeding on phytoplankton, which is abundant in the nutrient-rich waters due to the upwelling caused by ocean currents at the Subtropical Convergence. This area is where the cold, dense waters of the Southern Ocean meet the warmer waters of the adjacent Atlantic, Pacific, and Indian Oceans.
The concept of a keystone species was exemplified by studies on the intertidal sea star, Pisaster ochraceus, in the northwestern United States. In this case, the removal of the sea star led to an overpopulation of mussels, dramatically altering species composition and reducing biodiversity. Similarly, in the Southern Ocean, if krill populations were to decline significantly, it would likely have far-reaching effects on the structure of the ecological community and its overall biodiversity.