The Allee Effect and the Vortex of Species Extinction

Eggs of the many rare bird species are a vivid target for egg collectors around the world. The classic theory assumes that, as the cost of finding rarer and rarer individuals would outweigh the benefits, such exploitation would stop and the species will stand.

But a new approach adding human behavior to the equation uncovers an unexpected mechanism, with alarming implications for the survival of species. The researchers found that rarity increases a species' value into a classic pattern of resource exploitation used to manage fisheries. Rarity provokes a positive feedback loop with exploitation, driving a species into an extinction vortex. It's exactly like the ecological Allee effect, when lowered plant or animal populations experienced reduced fitness, which rose their extinction risk.

Low populations means reproductive failure when sex mates can not meet, or experience high mortality by losing the benefits of foraging in groups (pack hunting benefits or minimized predation risk). Many thought that the Allee effect is an intrinsic species trait which human activity cannot induce.

But financial value can trigger an "anthropogenic Allee effect" in rare species. The rarer a species, the more expensive, and this creates a positive feed back. This way, the market price surpasses the cost of harvesting the species, harvesting will cause further declines, making the species ever rarer and more expensive, which in turn stimulates even more harvesting until there's nothing left to harvest. As long as someone will pay any price for the rarest species, the last rarest parrots, lizards, or orchids will be harvested. The great auk (Pinguinus impennis) is an example of Allee effect. The flightless, 75 cm high now-extinct bird that laid only one egg a year, was driven to the brink of extinction by overhunting for meat and feathers.

But most likely scientists and museum collectors, anxious to get an increasingly rare specimen, finished the bird off. Even the hunting trend has changed in modern times, from targeting the most dangerous species to the rarest ones, as items made from the rarest species are indicators of social status, because many are coveted as luxury: whether for handbags, exotic cuisine, or dining room furniture or traditional medicines (like the awful use of rhino horns and tiger/panther bones in useless practices of Chinese folk medicine). In Yemen, displaying a rhino horn handled dagger is a sign of high social status ....

The exotic pet trade keeps on threatening orangutans, monkeys, felines, birds, reptiles, fish, arachnids, and insects. Most of the caught individuals die during capture or transport and never even get out alive from the provenience country, which is usually not a consumer of these "nature's products".

Currently, pet trade dealers even consult scientific literature to assess the next "hot" species; for example, soon after a study found the Roti Island snake-necked turtle (Chelodina mccordi) (photo center) and Chinese leopard gecko (Goniurosaurus luii) (photo bellow) as rarities, their prices boomed. Both species are now nearly extinct in the wild.

Even well intentioned activities like ecotourism can harm threatened populations. In the North Pacific an inverse relationship was found between the number of whale-watching boats one year and a reduced killer whale population size the next, thus motorized boats can disturb the wales. The smaller population size one year didn't decrease the number of whale watching tours the next year, but stimulated interest, as proved by the larger number of boats. This is a big puzzle on conservationism: simply declaring a species endangered catalyzes its exploitation.

Since many collectors, pet owners, and ecotourists actually care about biodiversity, maybe education and information could curb these human behaviors. Trophy hunting and luxury consumption could drop if society stigmatized such activities. But for those careless people who prize rarity above anything else, only strengthened legislation will decrease the probability of a species' extinction. Until those legal protections tolls are not firmly in place and enforceable, scientists must think twice before reporting a species as rare.

Image credit: Maria Angulo