Smaller Households Lead to Vanishing Biodiversity

By Cat Lazaroff

WASHINGTON, DC, January 13, 2003 (ENS) - A worldwide trend toward smaller households is fueling a global housing boom and threatening biodiversity around the world, a new study finds. The report is among the first to link trends such as a rising divorce rate and a movement away from multigenerational households to changes in resource consumption and sprawling development.

Even where the human population is declining, the number of households continues to grow, concludes the study by scientists from Michigan State (MSU) and Stanford Universities. The results, the authors say, point to needed changes in policies intended to protect valuable wildlife habitat and ecosystem services.


New research shows that in many cases, the same number of people now require more houses than they did in past decades. (Photo courtesy USDA Natural Resources Conservation Service)
"Having fewer people in more households means using more resources and putting more stress on the environment," said Jianguo "Jack" Liu, an associate professor of fisheries and wildlife at MSU. "Freedom and privacy come at a huge environmental cost."

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 Daily, a co-author of the study. "But instead, we've found that urban and suburban sprawl are accelerating faster than population growth is decelerating."

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

Liu, Daily, and Stanford population expert Paul Ehrlich and postdoctoral associate Gary Luck, examined household dynamics and population changes in 141 countries worldwide, then scrutinized six areas with biodiversity hotspots: areas with high densities of animal and plant species.


Dr. Jianguo "Jack" Liu's research on giant pandas and their habitat at the Wolong Nature Reserve led to an interest in how the region's household dynamics affect the pressures on the pandas. (Photo by Sue Nichols, Michigan State University)
The studies paint pictures of how changes in human lifestyles affect different habitats - from endangered pandas in the mountains of southwestern China to the subdivisions that press against the Florida Everglades.

The researchers found that across the world, in both developed and developing countries, households are getting smaller, and there are many more of them. Multigenerational living arrangements are giving way to couples or individuals moving out on their own.

Rising divorce rates mean families that used to live in one dwelling now occupy two, and aging populations mean more parents living in households separate from their grown children.

The result is often urban sprawl and less efficient housing for the same number of people. For example, a refrigerator uses about the same amount of energy whether it belongs to a family of four or a family of two.


Dr. Liu talks with a merchant at a roadside stand in the Wolong Nature Reserve. These stands spring up as the area's tourism increases, one sign of increased human impact on this biodiversity hotspot. (Photo by Sue Nichols, Michigan State University)
Each household takes up space, requires resources to construct, and fuel to heat and cool it. Increased energy consumption also increases the emission of greenhouse gases, which is believed to contribute to global warming.

"In larger households, the efficiency of resource consumption will be a lot higher because more people share things," Liu said. "Usually, many people will share living space and other resources. This is true in all countries."

While households may be shrinking in number of residents, most are growing in terms of square footage. Fewer people tend to live in more space.

For example, in Indian River County, Florida, the average area of a one story, single family house increased 33 percent in the last three decades, from an average of about 1,800 square feet in houses built before 1970 to an average of about 2,400 square feet built between 1970 and 2000.

"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.

The household project grew from Liu's years of research on how humans interact with fragile wildlife habitat in China's Sichuan Province, where villagers compete for resources with the endangered giant panda. In Wolong, Liu learned, a reduced average household size was tied to an increase in household numbers and a rise in the amount of fuel wood consumed by the local populace for cooking and heating, which has contributed to deforestation and loss and fragmentation of habitat for giant pandas.


An adult panda overlooks the forest at Wolong, China, where household size is steadily decreasing. (Photo by Sue Nichols, Michigan State University)
"The numbers of households increased much faster than the size of the population at Wolong," Liu said. "What was discovered from the panda reserve helped me to conclude that considering population size and growth alone is not enough, and made me want to find out whether other areas in the world have similar phenomena."

"The issue of the number of households and their impact on the environment basically has been ignored. It was even difficult to unearth the data," Liu added. "Everyone looks at population size and growth rate, but the number of households and household size are crucial factors affecting the environment."

The team of researchers found that in 76 countries containing biodiversity hotspots, between 1985 and 2000 the number of households grew by 3.1 percent a year, whereas the population increased only 1.8 percent. Meanwhile, the number of people living in a single dwelling dropped from 4.7 to 4.0.

The scientists estimated that had average household size remained the same in hotspot countries during the 15 year period, there would have been 155 million fewer households overall, meaning less pressure on biodiversity. Hotspot countries studied included Australia, India, Kenya, Brazil, China, Italy and the United States.


Housing sprawls over California's congested Santa Clarita Valley, consuming land, water and other resources. (Photo courtesy Sierra Club)
"Ignoring population growth, reduction in household size alone is projected to add 233 million households to hotspot countries between 2000 and 2015," said Daily.

In the 65 non-hotspot countries, similar trends were found, although the magnitudes of change were less. In 1985, the average household size was 4.7 in hotspot countries and 3.7 in non-hotspot countries. By 2015, the average household size in hotspot countries is expected to be 3.4 persons, and in non-hotspot countries, it is expected to be 3.6 persons.

Liu contends that the increase in household numbers even in non-hotspot countries directly influences important biodiversity on a national and local scale. Indirectly, he says, global environment is affected in such patterns as more energy consumption and release of more greenhouse gases.

Even in regions where population size decreased, such as in New Zealand, the number of households increased because of a reduction in household size.

The "double toll" a reduction in average household size takes on the environment, the scientists said, includes more land use and more materials consumed for construction, and a lower efficiency of resource use per person. In hotspot countries, where this trend is most prevalent, the authors believe there may be severe limits on efforts to conserve species, thus "degrading the ecosystem services that biodiversity delivers to humanity."


Each new house built gobbles up more natural resources. (Photo courtesy Catawba Riverkeeper)
In the past, the business community took most of the heat for many environmental problems, Liu explained.

"While there is still a need to reduce pollution and ecological destruction caused by factories and companies, this study provides a wake up call, and suggests that efforts at the individual and household levels are also needed to reduce impacts on the environment," Liu said.

Changes in government policies such as tax incentives for sharing housing and resources could be helpful to influence personal and household decisions and actions, he added.

"In China and many other countries around the world, incentives created to help the environment are based on households," Liu said. "These incentives have good intentions, but they also encourage households to break into smaller households."

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.


Professor Gretchen Daily of Stanford University says the research suggests that human population decline alone may not be enough to reverse trends of vanishing biodiversity. (Photo courtesy Stanford University)
"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 research, funded by the National Science Foundation and the National Institutes of Health, appears in the January 12 online edition of the British science journal "Nature."

For more information on Liu's panda habitat research in China, visit: