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Lonesome George, Allee and the invisible line


In 2012, a tortoise named Lonesome George died. He was the last of the Pinta Island giant tortoises, whose numbers were decimated by overhunting and whose habitat was destroyed by feral goats. Following his discovery in 1971, the world held a prolonged memorial service, awaiting the death of Lonesome George and the extinction of his species.

By the time George was the sole survivor, it was obvious his species was doomed. In reality, though, the fate of the Galapagos species was probably sealed many years before, when its population size fell below a critical number, and the few remaining tortoises could not out-reproduce their mortality.

These critical thresholds – minimum population sizes that represent an invisible line between survival and extinction – occur because of Allee effects, the biological term for positive density dependence. That is, the larger a population is, the greater each individual’s reproductive rate, the faster the population grows.

Allee effects stem from a variety of causes. As Lonesome George’s case highlighted – the fewer individuals there are, the harder it is to find a mate. This is also true for plants, whose insect or wind-borne pollen must find fertile ground within a certain radius.

For many species, cooperative behaviors help to ensure survival. For example, emperor penguins form tight huddles to survive harsh Antarctic winters, and honeybees share the duties of maintaining, provisioning and defending their hives. The survival of each individual hinges on the success of the group.

Species are only as healthy as their genetic diversity. That’s because having a variety of genes in the population gives the species resilience should the environment change (for example, due to climate change). Small populations also have a tendency to become inbred, making them increasingly vulnerable to their own genetic disorders (like the European royalty who suffered from hemophilia).

Typically, we only observe Allee effects when populations are small. For big populations, other factors like competition for food or breeding territory, take over and can actually decrease a population’s growth rate. When a plant or animal population is large and healthy, we don’t realize that Allee effects exist. But they’re still there lurking, like invisible tripwires for conservation, making it hard to identify when human intervention is needed.

For example, when trying to identify endangered species, we often rely on estimates of their current population sizes. These estimates can help us predict how fast that population can grow and how well it can withstand pressures like habitat loss and human activity. But Allee effects can throw off these calculations. For a species with Allee effects, a population of size 500 grows less than half as fast as a population of size 1000, so calculations based on data from when the population size was 1000 don’t hold when the it shrinks. And where Allee effects are strong and a critical threshold exists, a population of 500 might be able to recover, but a population of 490 might be doomed to extinction.

Besides Lonesome George, we know that many endangered species probably suffer from Allee effects. Cheetahs are the poster children of inbreeding and may no longer have enough genetic diversity to bounce back from near-extinction. Florida’s grasshopper sparrows, of which fewer than 200 still survive in the wild, don’t have the numbers to find mates or keep a shared watch for predators.

And in a twist of economic irony, endangered species face an additional “anthropogenic Allee effect” driven by humanity’s fluctuating markets. As species become rarer, they become more valuable. This is a result of the so-called “diamond-water paradox:” While water is critical to life, we paradoxically value diamonds more highly because of their rarity. At one point, the Ecuadorian government offered $10,000 to anyone who could find a mate for Lonesome George – a reward that was never claimed. Unfortunately, rare species are also particularly appealing to trophy hunters, collectors and even poachers willing to take tremendous risks to sell parts on the black market. Even increased ecotourism interest can be harmful: whale watching boats can disturb the animals and lower their fitness.

How, then, can we prevent other species from meeting Lonesome George’s fate? It’s clear that Allee effects exist and that they can pose a serious threat to species survival. Unfortunately, it’s almost impossible to identify the population thresholds beyond which extinction is inevitable. For many of the 11,500 species listed as endangered (or critically endangered) by the IUCN, it may already be too late. Lonesome George, perhaps, would recommend a conservative course: slow and steady, taking each step with caution, steering well clear of that invisible line.

Contact Holly Moeller at hollyvm ‘at’ 

Holly is a Ph.D. student in Ecology and Evolution, with interests that range from marine microbes to trees and mushrooms to the future of human life on this swiftly tilting planet. She's been writing "Seeing Green" since 2007, and still hasn't run out of environmental issues to cover, so to stay sane she goes for long runs, communes with redwood trees and does yoga (badly).