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Smaller households fuel global housing boom and threaten biodiversity, study finds

Stanford University

A new study in the journal Nature concludes that the average household is shrinking - a worldwide trend that is fueling an international housing boom, which threatens the survival of plants and animals in dozens of countries including the United States.

According to the study, housing units throughout the world are being built at a rate that outpaces population growth, resulting in a loss of habitat, natural resources and biodiversity.

''We had hoped to find that, where human population growth was slowing, biodiversity might be given some breathing room,'' said Stanford University ecologist Gretchen C. Daily, co-author of the study. ''But instead, we've found that urban and suburban sprawl are accelerating faster than population growth is decelerating.''

A research scientist with Stanford's Center for Conservation Biology, Daily also is an associate professor (research) in biological sciences and a senior fellow with the university's Institute for International Studies.

''To our knowledge, this is the first study to look at the environmental impact of households on a global scale,'' Daily added.

Writing in the Jan. 12 edition of Nature online, she and her colleagues noted that the housing boom is largely being driven by a global trend toward smaller households. Throughout the world, the average number of people living together in a household is shrinking - primarily because of lower fertility rates, higher divorce rates, higher per capita income, aging populations and a decline in multi-generational family units, the authors noted.

''Reduction in average household size takes a double toll on resource use and biodiversity,'' they wrote. ''First, more households means more housing units, thus generally increasing the amount of land and materials (for example, wood, concrete and steel) needed for housing construction.''

Second, fewer people per household leads to higher per-capita consumption of water, fuel, land and other natural resources - even when population size declines, they noted.

''This easily overlooked trend presents a particularly serious threat to biodiversity - the plants, animals and microbes underpinning our life-support systems,'' Daily added. ''The threat is particularly acute in so-called biodiversity 'hotspot' countries, where extraordinarily rich stocks of native species are threatened by human activities.''

United Nations data

In their study, the researchers analyzed demographic data compiled by the United Nations between 1985 and 2000. The data were obtained from 141 countries, 76 of which have been identified as biodiversity hotspots - including Australia, India, Kenya, Brazil, China, Italy and the United States.

The data revealed that the growth in the number of new housing units worldwide increased at a rate more rapid than the population growth - particularly in hotspot countries. Had the average number of occupants per household remained constant during that 15-year period, there would have been 155 million fewer households in hotspot countries in 2000, the authors concluded.

''Ignoring population growth, reduction in household size alone is projected to add 233 million households to hotspot countries between 2000 and 2015,'' Daily said - a trend that could have a particularly damaging effect in biologically sensitive ares such as the Wolong Nature Reserve in China, where the growth in new homes had led to deforestation and the loss and fragmentation of habitat for giant pandas.

The authors found that, even in hotspot nations such as Greece, Italy and Spain where the population growth rate is declining, the number of households is mushrooming because fewer people are living together.

Urban sprawl

The threat to global biodiversity is likely to escalate, the authors concluded, because current household trends, such as higher divorce rates and increased affluence, are expected to continue.

''Most countries containing hotspots have relatively low population growth rates, and the primary demographic pressure on their biodiversity will come from urban sprawl and other impacts associated with increased household numbers,'' they wrote.

For example, in Florida's Indian River County north of Miami, the number of residents per household has declined by 22 percent in the last 30 years, while the floor area in a single story one-family house has increased by one third.

''Had the average household size stayed at the 1970 level, Indian River County would have had 11,000 fewer households in 2000,'' the researchers observed. These extra households are one of the factors that have made the county a biodiversity hotspot - a refuge for manatees and a variety of other endangered and threatened animals and plants.

''We all depend on open space and wild places, not just for peace of mind but for vital services such as crop pollination, water purification and climate stabilization that are key to health and economic prosperity,'' Daily observed. ''The alarming thing about this study is the finding that, if family groups continue to become smaller and smaller, we might continue losing biodiversity - even if we get the aggregate human population size stabilized.''


The lead author of the Nature study is Jianguo Liu of Michigan State University. Two other co-authors are affiliated with Stanford's Center for Conservation Biology: Paul R. Ehrlich, the Bing Professor of Population Studies, and postdoctoral fellow Gary W. Luck.

Funding for the study was provided by the National Science Foundation and the National Institute of Child Health and Human Development.

By Mark Shwartz 650-723-9296 or 831-239-3312,

COMMENT: Gretchen C. Daily, Biological Sciences: 650-723-9452,

Gary Luck, Biological Sciences: 650-725-1783,

Jianguo (Jack) Liu, Michigan State University: 517-355-1810,

EDITORS:The study, ''Effects of Household Dynamics on Resource Consumption and Biodiversity,'' will be available to Nature subscribers at on Jan. 12 at 2 p.m. EST. To see the report, enter the identification code10.1038/nature01359. A photo of Professor Gretchen C. Daily is available at (slug: ''Households'').

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