Bottlenecking: A Blast From the Past

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.

Northern elephant seals (Mirounga angustirostris) at Ano Nuevo State Park. Credits: Rhododendrites via Wikimedia Commons.

However, a bottleneck isn’t always the end for a species. Research has shown evidence that cheetahs have lived through at least two bottleneck events. The first of which, roughly 100,000 years ago, occurred during the rapid range expansion into Asia, Europe and Africa. It is believed that this expansion was so quickly widespread that it restricted the species ability to effectively exchange genes. 

The second event likely occurred 10,000-12,000 years ago during the last ice age resulting in the extinction of European populations. Despite the resulting inbreeding and low genetic diversity, the species managed to survive and has grown to as many as 100,000. It is still unclear how the cheetahs have managed to survive both bottleneck events, but their story provides hope for other species.

Human intervention can also help species overcome bottleneck events. Southern California freeways have isolated a small population of mountain lions in the Santa Monica Mountains. The isolation led experts to believe the population would die off within 50 years if no action was made to connect them with a larger gene pool. Just last year, conservationists were able to spark initiative to fix the problem. Construction began for the world’s largest wildlife crossing to be built. The 200 by 165 foot bridge will cross 8 lanes of traffic, greatly expanding the habitat for mountain lions as well as many other species.

While environmentally driven bottleneck events may be unavoidable, it is imperative that we learn from past mistakes to reduce the frequency at which they occur. These events can be detrimental to a species, thus human conservation efforts are crucial in mitigating their impact.

About the Author: Trace Laskey is a 3rd year Environmental Science student at the University of York with an interest in conservation and marine science. You can find him @the_real_traceadilla on Instagram or via his LinkedIn.