Trace Laskey discusses how species can seemingly recover from the brink of extinction, only to feel the effects of a genetic bottleneck years later.
Growing up along the coast in California, I was taught at an early age the conservation success story of sea otters. Extreme fur hunting throughout the Pacific pushed them to the brink of extinction. In the 1920s, the species were believed to be extinct until a group of around 50 otters were found off the coast of Big Sur, California. From this raft (the collective noun for a group of sea otters) the population has now grown to over 3,000 and have been removed from the endangered species list, now classed as “threatened”.
However, the recent appearance of local legend Otter 841, sparked my interest for how the species is really recovering after such a devastating blow to their population. Otter 841 was observed exhibiting unexplainable abnormal behaviour towards swimmers and surfers, approaching humans without hesitation and even going as far as “stealing” surfboards. While the species as a whole is now highly protected from human interference such as hunting and pollution their history of a small population may still put them at high risk.

Big Sur, California. Image Credits: Kudinov via 500px.
Genetic bottlenecks, or population bottlenecks refer to when a species population greatly decreases at a rapid rate. This could be caused through environmental factors such as fires, droughts, or disease, or through anthropogenic factors like hunting in the case of sea otters. Small population sizes result in a reduced gene pool and genetic diversity.
This decreases the rate at which random mutations occur, reducing a species ability to adapt and survive environmental changes such as a shift in food availability or climate change. A minimum viable population refers to the lowest a population can be and still survive. Many conservation biologists follow the “50/500” rule stating at least 50 adults are required to avoid damaging effects of inbreeding, and at least 500 to avoid extinction from inability to evolve and cope with change.

A population bottleneck. Image Credits: Mysid on Wikimedia Commons.
While the aggression displayed by Otter 841 was determined to likely be due to hormonal surges (she’s a mother now!) initial theories believed it could be down to genetics. Beyond this example the larger population doesn’t seem to be improving at an anticipated rate. Trends show the species beginning to flatline rather than return to their historic population of 150,000-300,000.

A close-up of a sea otter, taken in Morro Bay, California in 2016. Credits: Marshal Hedin via Wikimedia Commons.
Unfortunately sea otters are not the only species to have been put through a bottleneck due to human activity. Culling and breeding with cattle reduced the American bison population down to a herd of only 100. Conservation efforts, with the help of private ranchers and a small herd of surviving bison in Yellowstone National Park have helped the population rebound to 500,000.
While this should be celebrated as a conservation victory, research has found only an estimated 1,000 of these are known to have non-cattle ancestry due to hybridization in the 1800s. Some research goes as far as saying all “pure” American bison have gone extinct due to the bottleneck.
Throughout the 19th century, the northern elephant seal population was hunted for oil, forcing them through a bottleneck. A population estimated to be only around 10-20 seals was found living near Baja California, Mexico. Despite a current population of around 124,000, testing shows a dangerously low genetic diversity. As ocean temperatures warm, many believe that the species does not have the gene pool to allow them to evolve and adapt to climate change.

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