Center for Systems Integration and Sustainability

A new study that examines the world's increasing number of households -- even where populations decline -- draws a strong link between the lifestyles of humans and the fates of animals such as pandas and crocodiles.

Michigan State University scientist Jianguo "Jack" Liu and colleagues at Stanford University, in the Jan. 12 Advanced Online Publication of the British science journal Nature, examine how the growing number of households worldwide - and the declining number of occupants in a household - affects biodiversity and resource consumption.

The paper, entitled "Effects of household dynamics on resource consumption and biodiversity," takes a new look at population dynamics. It explores how increases in the number of households in 141 countries, even where the overall population declines, have a significant impact on wildlife and the environment.

The results, the authors say, point to needed changes in policies intended to protect valuable wildlife habitat and ecosystem services.

"Having fewer people in more households means using more resources and putting more stress on the environment," said Liu, an associate professor of fisheries and wildlife. "Freedom and privacy come at a huge environmental cost."

Liu and his co-authors ecologist Gretchen Daily, population expert Paul Ehrlich and postdoctoral associate Gary Luck examined household dynamics and population changes worldwide, then scrutinized six areas with biodiversity "hotspots," areas with high densities of animal and plant species.

The studies paint pictures of how changes in human lifestyles affect vastly different habitats - from endangered pandas in the mountains of southwestern China to the subdivisions that press against the Florida Everglades.

In a nutshell: Across the world, in both developed and developing countries, households are generally 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. Aging populations mean more "empty nesters."

The result often is urban sprawl. As a household shrinks and as more households form, the economy of scale is lost. Each household requires resources to construct it and takes up space. It requires fuel to heat and cool it. A refrigerator uses roughly the same amount of energy whether it belongs to a family of four or a family of two.

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."

Moreover, households may be shrinking in size, but growing in terms of square footage. Fewer people tend to live in more space, thus further using resources.

For example, in Indian River County of 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 roughly 2,400 square feet built between 1970 and 2000.

The household project grew from Liu's years of research on how humans interact with fragile wildlife habitat in China's Sichuan Province. There, villagers compete for resources with the beloved, and endangered, giant pandas.

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

Among the findings:

• In hotspot countries, the annual rate of growth in the number of households - 3.1 percent - was substantially higher than the population growth rate - 1.8 percent - between 1985 and 2000. More than 80 percent of hotspot countries showed this pattern.
• The difference in household size between hotspot and non-hotspot countries is decreasing - with households in hotspot countries getting smaller faster. 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. In non-hotspot countries, it is expected to be 3.6 persons.
• Even in regions where population size decreased, the number of households still increased substantially due to a reduction in average household size.

The six areas with biodiversity hotspots analyzed were Florida, Brazil, Rodrigues (Africa), New Zealand, Italy and Wolong, China.

The research was funded by the National Science Foundation and the National Institutes of Health.