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Conservation biology

Rarity bites

Naturevolume 444pages555556 (2006) | Download Citation


Rare species have to cope not only with habitat loss, genetic bottlenecks and invasive competitors, but also with a self-reinforcing cycle of human greed. This last threat has now been dragged into the spotlight.

It makes sense that, as a valued commodity becomes scarce, its cost rises. Indeed, tests of the economic theories of demand have shown that consumers greatly value the hedonic exclusivity of owning rare objects, such as coins or stamps1. But is this desire restricted to the harmless venture of collecting historical curiosities? Writing in PLoS Biology, Courchamp and colleagues2 suggest not. They describe an under-appreciated thorn in the side of endangered species already teetering on the brink of extinction — the lure of the few.

Standard bioeconomics asserts that wildlife harvests will halt when the expense of locating individuals of a species exceeds the financial gain, thus shielding that species from fatal overexploitation. However, Courchamp et al. develop a mathematical model to show that the exploitation of rare and endangered species can have a paradoxical outcome. If rarity makes them more desirable to some, then ever-increasing pecuniary incentives for poaching can create a positive feedback loop, ending in the species' demise. The authors show how the value placed on rare species for collections, as luxury items, as pets or trophies, in traditional medicine, and even in ecotourism, can keep pace with the cost of acquiring such species.

Courchamp et al. support their case by providing a series of telling examples covering plants, molluscs, insects, fish, amphibians, reptiles, birds and mammals. For instance, the swim bladder of the Chinese bahaba (Bahaba taipingensis) is so valued for traditional medicine that its price per unit weight eventually exceeded that of gold by sevenfold. Even when fewer than half-a-dozen fish were being caught each year in the 1990s, more than 100 boats continued to try to catch them2. There are many other examples3,4,5, illustrating how widespread this problem may be. The Bali starling (Leucopsar rothschildi; Fig. 1) epitomizes how the craving for rare birds in the pet trade can drive a species to the edge of extinction. Despite being listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES)6 since 1970, 19 individuals were reported as being sold illegally in shops in Singapore in 1979, and 16 were observed in cages in Denpasar (Bali) in 1982. This was when fewer than 150 birds were left in the wild. They now number six, restricted to Bali Barat National Park5.

Figure 1: Object of desire — the critically endangered Bali starling.
Figure 1


This species has been heavily collected for the pet trade, and is now on the brink of extinction in the wild.

Courchamp et al.2 call this rarity-value feedback the 'anthropogenic Allee effect'. Allee effects refer to phenomena that strongly influence the dynamics of populations only at low densities, and they include the increasing difficulties of finding mates, of maintaining a cohesive social structure, and of avoiding the genetic hazards associated with inbreeding. By reducing the chances of successful reproduction, Allee effects are synergistic and self-reinforcing, and can plunge rare species into what has been colourfully described as an 'extinction vortex'7. An empirical analysis of the final chapter in the life story of some well-studied species suggests that the extinction vortex is a real problem8.

Courchamp and colleagues' model requires that consumers are always willing to pay what it takes to obtain a rare species, making extinction by collection an inevitable prediction. Yet all that may be required is for over-harvesting to continue long enough to push rare species below a population size that dooms them to an extinction vortex. If so, extinction induced by the anthropogenic Allee effect might be a disturbingly common event, even if collecting ceases before the species' final demise. This may have happened in the case of the Tasmanian tiger (Thylacinus cynocephalus), whose population crashed during a period of bounty hunting, but which then dwindled to extinction after exploitation had ended9.

It is essential to identify threatened species and populations so that action can be taken to protect them. The best-known system of this kind is the IUCN Red List drawn up by the World Conservation Union10. Publicizing the plight of vulnerable plants and animals is crucial because it helps to secure public support and funds. But in so doing, are conservationists unintentionally making things worse? It seems that the very act of identifying new rare species, and describing where they are, can lure collectors and so induce an anthropogenic Allee effect, with sometimes devastating consequences11. Perhaps controversially, Courchamp et al.2 also provide evidence that declaring a species' threatened status on the IUCN Red List and CITES Appendices amplifies the black-market demand for that species.

Breaking this vicious circle will be difficult and potentially costly, but necessary. The possibilities include keeping secret the location of newly designated rare species; commercial rearing and subsequent legal sale of otherwise rare species or their products, such as traditional medicines; and enforcing measures that make poaching too risky, as is the case in some African game reserves where armed guards shoot to kill. Of course, the most effective (but idealistic) strategy would be to curb demand. As Mahatma Gandhi famously said: “Earth provides enough to satisfy every man's need, but not every man's greed.”


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  1. School for Environmental Research, Charles Darwin University, Darwin, 0909, Northern Territory, Australia

    • Barry W. Brook B W
  2. Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, 117543, Singapore

    • Navjot S. Sodhi N S


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