Jianguo Liu of Michigan State University and his colleagues analyzed data compiled by the United Nations for 141 countries covering the period between 1985 and 2015. They calculated the population growth, number of households and average household size for 76 countries considered to include biodiversity hotspots (regions rich in endemic species that are threatened by human activity) and 65 that do not. The decrease in average household size in hotspot countries, they found, resulted in 155 million extra households between 1985 and 2000. If the trend continues there will be another 233 million more households by 2015, the team reports. What is more, even in countries whose overall population is declining, such as Italy, Portugal, Spain and Greece, new homes continue to spring up.
The scientists point out that the trend of fewer people living under each roof affects biodiversity in two different ways. First, more land and materials are dedicated to housing construction. Second, a house with fewer inhabitants is less energy efficient and consumes more resources per person than one that is home to a larger brood. According to the authors, "the divergence in population and household growth rates is expected to become more pronounced over the next 15 years, suggesting that it is crucial to consider growth in the number of households when assessing threats to biodiversity."