Featured Image Credit: Defenders of Wildlife
By Eva Gruber
Ecology is the study of organisms and their relationship with the surrounding environment. One of the classic ecological examples that most people (and all biologists) are familiar with is that of the sea otter in the kelp forest.
Sea otters are known as a keystone species – that is, they are critical to the health and stability of the ecosystem that they inhabit – in the kelp forests of the North Pacific Ocean. The sea otters feed voraciously on sea urchins, which in turn feed on giant kelp (Macrocystis pyrifera). By keeping the urchin population in check, they indirectly affect the abundance of kelp.
Without sea otters, urchins explode in number and rove across the seafloor, consuming the holdfasts of kelp – essentially clearing the forest. Kelp forests without sea otters turn into what is known as an urchin barren. The rocky sea bottom where kelp once grew becomes a seascape of purple and overrun with urchins.
In the 18th and 19th centuries, fur hunters from all over the Pacific killed sea otters for their dense, warm pelts, and sea otters very nearly went extinct. The mass hunting wrecked kelp forests from Japan, across the Aleutian Islands, to Alaska, down to Baja California (the historic range of the sea otter) into disarray. The majority of the dense patches of kelp disappeared as well. The far-reaching effects from the loss of sea otters is why ecology holds them as a prime example of a keystone species.
Strict conservation measures in place since the 1970s, listing the sea otter under the Endangered Species Act, have paid off and their populations have seen a steady increase. In fact, in some places sea otters are even quite abundant. Their population has jumped from 2,711 in 2010, to 3,054 in 2016.
Lately, a pronounced uptick in their population has been documented – especially in the central part of their range. Tim Tinker, a marine ecologist with the US Geologic Survey, theorizes that it is a result of sea star wasting syndrome. The disease wiped out the other main predator of sea urchins, and therefore leaving more for the sea otters. This extra food source has led to an increase in the sea otters’ central range, which was already close to carrying capacity. However, one interesting piece of the puzzle presents somewhat of a roadblock to the expansion of the sea otter populations in both the northern and southern parts of their range.
In 1994, California banned gillnetting of sharks, which had been the leading cause of shark mortality and inflicted a serious dent in their populations. This ban, along with an increase in the population of great whites’ main prey – sea lions and elephant seals – has led to an increase in sharks of the nearshore Pacific.
With more sharks, scientists are seeing more attacks on sea otters. Researchers believe that the sharks are conducting “investigation bites.” By biting the sea otter once, the shark can tell if the sea otter is a suitable food item with the thick layer of blubber that they crave. Since sea otters lack blubber, they are not eaten. They usually end up dying and washing up on shore.
Despite the predations, researchers remain optimistic about the continued upward growth and expansion of the sea otter population. The shark gauntlet is a natural obstacle to the expansion of their range, and researchers are studying other possible barriers such as disease, and unnatural, human-caused barriers such as pollution.
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