MANY of the world鈥檚 biodiversity hot spots are under threat because people no longer live in such large family groups. The trend turns out to be even more damaging to the environment than continuing population growth.
According to UN figures for 141 countries, more than half the increase in the number of households between 1985 and 2000 was due to people shifting from larger households to smaller ones, rather than population growth. Even in countries where populations are falling, the average number of people living under one roof is shrinking faster, pushing up demand for houses and apartments.
Jianguo Liu, an ecologist at Michigan State University in East Lansing, and his colleagues at Stanford University in California found that if the trend continues it will mean an extra 233 million households by 2015 even if there is no overall population growth (Nature, DOI:10.1038/nature01359).
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The trend places an increasing burden on natural resources. A three-bedroom house, for example, uses the same amount of land and materials to build and the same amount of fuel to heat whether it is home to two people or four.
The trend is already causing significant damage. In China, a fall in average household size has led to more wood being used for fuel, and a drastic loss of habitat for native giant pandas.
Ominously, Liu鈥檚 team found the effect is most dramatic in regions with rich biodiversity, including areas of Brazil, New Zealand and China. In these hot spots, the number of households is rising at nearly twice the rate of the population, whereas in other regions, the two are roughly equal.
The problem may not be easy to solve. 鈥淚 don鈥檛 think anybody says we should try to herd people into larger households. So I don鈥檛 see any quick policy options,鈥� says John Bongaarts at the Population Council, an international group concerned with population issues. Liu suggests using tax incentives to encourage people to buck the trend and live in larger groups.