A Human Taste for Rarity Spells Disaster for Endangered Species, Liza Gross
The shady pursuit of endangered bird eggs made international headlines in May 2006
when Colin Watson, widely considered Britain’s most notorious illegal egg collector,
died after falling from a 12-meter tree, allegedly while hunting a rare egg. (Watson’s
son Kevin has publicly claimed that his father hadn’t collected an egg since the practice
was banned in 1985.) The Royal Society for the Protection of Birds estimates that up to
30 of Britain’s most vulnerable species are targeted by collectors.
Classical economics theory predicts that such exploitation is unlikely to extinguish
a species because the cost of finding the last individuals would outweigh the benefits.
But a new theoretical study shows that adding human behavior to the equation
specifically, the human penchant for rarity—reveals an unexpected mechanism of
exploitation, with alarming implications for species survival. Franck Courchamp,
Elena Angulo, and their colleagues incorporated the assumption that rarity increases
a species’ value into a classic model of resource exploitation used to manage fisheries.
Prizing rarity, they found, triggers a positive feedback loop between exploitation and
rarity that drives a species into an extinction vortex.
This phenomenon, the authors explain, resembles an ecological process called the
Allee effect, in which individuals of many plant and animal species suffer reduced
fitness at low population densities, which increases their extinction risk. Reduced
survival or reproduction can occur if individuals fail to find mates, for example, or
suffer increased mortality by losing the benefits of pack hunting (more access to
prey) or foraging in groups (minimized predation risk). Most studies assume the
Allee effect is an intrinsic species trait that human activity cannot artificially induce.
But the authors’ model shows that humans can trigger an “anthropogenic Allee
effect” in rare species through a paradox of value. When rarity acquires value,
prices for scarce species can skyrocket, even though continued exploitation will
The model predicts that as long as there is a positive correlation between a species’
rarity and its value, and the market price exceeds 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. And as long as someone will pay any price for the rarest of the
rare, market price will cover (and exceed) the cost of harvesting the last giant
parrot, tegu lizard, or lady’s slipper orchid on Earth.
The authors describe multiple human activities that could precipitate the
anthropogenic Allee effect. Hobby collections of the sort Watson allegedly gave
his life for top their list. Overhunting for food and feathers pushed the great auk,
a flightless, now-extinct bird that laid only one egg a year—to the brink of extinction.
But it was likely scientists and museum collectors anxious to nab an increasingly rare
specimen, the authors suggest, that finished the bird off. And trophy hunting collectors
have placed increasing pressure on rare species as their focus has shifted from killing
the most dangerous animals to killing the rarest.
The pursuit of social status and health can also trigger the anthropogenic Allee effect,
as many rare species are coveted as luxury items—whether for handbags, exotic cuisine,
or dining room furniture—or traditional medicines. The exotic pet trade continues to
threaten orangutans, monkeys, reptiles, birds, and wild cats, as well as a wide variety of
arachnids, insects, and fish. The great majority of targeted animals die during capture
or transport and never even reach the consumer. And it appears that pet trade dealers
read the scientific literature for clues to the next hot species: immediately after an article
recognized the small Indonesian turtle and Chinese gecko as rarities, their prices soared.
The turtle is now nearly extinct and the gecko can no longer be found in its southeastern
Even well-intentioned activities like ecotourism can destabilize threatened populations.
A recent study of killer whales in the North Pacific found an inverse relationship between
the number of whale-watching boats one year and a reduced whale population size
the next, in keeping with evidence that motorized boats can lower whale fitness. The
study also found that the smaller population size one year didn’t discourage whale
watching tours the next year, but stimulated interest, based on the larger number of
How to conserve biodiversity when simply declaring a species endangered catalyzes
its exploitation? Since many collectors, pet owners, and ecotourists actually care about
biodiversity, the authors hope that education may go a long way toward curbing these
human activities. Education could even mitigate the damage of trophy hunting and
luxury consumption if society stigmatized activities responsible for driving a species
to extinction and people could no longer take pride in displaying such “treasures.” But
for those who prize rarity above all else, only strengthened regulations and interventions
will decrease the probability of a coveted species’ extinction. And until those protections
are firmly in place and enforceable, biologists may do well to think twice before reporting
a species’ decline.
Gross, Liza. "A Human Taste for Rarity Spells Disaster for Endangered Species." PLOS Biology (November 28, 2006): n. pag. Journals.plos.org. Web. 20 Mar. 2016.
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