It stands to reason that as the cost of finding and an over-harvestedÂ medicinal plant, say,Â increases beyond the market value of the product, people will stop looking for it, hopefully giving the population a chance to recover. Well, yes, but what if the very increasing rarity of the plant – or animal – actually in turn ratchets up its value? The result is a positive feedback loop which can only end in extinction. Or so says a mathematical model described in Nature, backed up by some telling examples.Â So publicizing the fact that a species may be rare and threatened (for example, in a Red Data List) may actually make things worse! Taking species into cultivation may be one way around the problem.