An international trend towards living alone is accelerating the extinction of endangered species and eating up natural resources at an alarming rate. Rare animals such as the giant panda, the bald eagle and the golden lion tamarin are facing fresh risks as a result.

Scientists say the problem of shrinking home units has become so acute that it now poses a more serious challenge to conservation than human population growth.

Households of just one or two occupants are very inefficient users of natural resources, when compared with the traditional nuclear family unit of five or more.

Declining household size means that more homes have to be built to accommodate the same number of people, bringing urban sprawl and the destruction of vulnerable habitats, a study at Michigan State University and Stanford University has shown.

Small households also use much more energy per head than larger ones and create more waste, making their ecological footprint still worse. A one-person flat and a four-person house, for example, will typically each have one refrigerator, at considerable cost to energy efficiency and greenhouse gas emissions.

“This emerging fashion for living in smaller units has become one of the greatest challenges for conservation that we face today,” Jack Liu, who led the research, said.

“It starts a chain reaction of environmental damage. As well as land use, there are negative effects on greenhouse gases, waste and forest clearance. More roads are needed, which divide threatened populations. Everything is connected.”

In Britain between 1985 and 2000 the number of households increased at 4.5 times the rate of population growth, and demand for housing is rising even in countries such as Romania, where the population is falling.

Late marriage, high divorce rates and ageing populations are largely responsible for the phenomenon in developed countries, while migration to cities in search of work is the prime driver of change in the developing world.

The impact is most serious in countries that contain “biodiversity hotspots” — regions with many native species that are threatened by human activity — because these can least afford the damage to their fragile ecosystems.

Had the number of households in such countries remained static between 1985 and 2000, rather than growing by 3.1 per cent every year, some 155 million homes would not have had to be built, Dr Liu said. By 2015, another 233 million households will be set up in hotspot countries if current trends continue, according to the study published in the online version of the journal Nature.

The phenomenon is adding to the risks faced by many of the world’s most critically endangered species. In the Wolong Nature Reserve in China, giant pandas and red pandas live alongside human settlements that are growing more quickly than expected from population trends.

“Over the past two decades the population has grown by 70 per cent, but the number of households is up by 110 per cent,” Dr Liu said. “As this number grows, the villages use up more land, cut more firewood for fuel and put increasing pressure on the environment.”

In the Cerrado and Atlantic forest regions of Brazil, home to rare species such as the golden lion tamarin and the buffy-headed marmoset, a similar process is taking place.

While the human population has grown at 1.7 per cent per year, the number of households has increased at a rate of 3.2 per cent, driven by an annual reduction in household size of 1.2 per cent. Similar trends are under way in other biodiversity hotspots examined: New Zealand, the Mediterranean Basin parts of Italy, the island of Rodrigues in Mauritius, and Indian River County in Florida. Dr Liu called on governments to think of ways to persuade people to live in larger households. “One thing we can do is to give tax incentives to people to share their houses,” he said. “When a family’s children move out, they could offer that space to a lodger. If governments provide economic incentives, people will think about it.”

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