Ask a question : follow the now approach to write your summary in no more than four sentences: Safety in numbers may not be enough
In the early 18th century, flocks of migrating passenger pigeons had darkened the skies over eastern North America, taking three days to pass by. Hunters simply pointed a gun upwards, fired, and then got out of the way as the pigeons tumbled to earth. When the birds stopped to roost, trees broke under their combined weight. With an estimated population of somewhere between 3 and 5 billion, the passenger pigeon was the most abundant bird that ever lived. Yet, by the late 1890s, the species was almost extinct. A few birds found their way to zoos, but they languished in captivity and refused to breed. It was a result that perplexed the eminent conservationist Wallace Craig and his contemporaries, and today's conservationists often face a similar problem. It isn't that living in a zoo can ruin an animal's sex drive. When wild species experience a population crash they can go into freefall, even though you would think that by removing the pressure of overcrowding the survivors would flourish. Now conservationists are beginning to realize that undercrowding itself can help drive species to extinction. It's a counter-intuitive idea, but it's not a new one: the consequences of low population density were first studied more than half a century ago by American biologist Warder Allee. For decades his ideas were largely forgotten, but now an awareness of these 'Allee effects looks set to transform conservation practices. 'They alter our perception about the risks facing populations that have declined markedly, even if they are not numerically tiny,' says Georgina Mace from the Institute of Zoology in London.
Source: Barnett, A. (2001). Safety in numbers may not be enough. New Scientist, 169, 38-41.