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Nature:
"Effects of Household Dynamics on Resource Consumption and Biodiversity"
January 12, 2003

JIANGUO LIU*, GRETCHEN C. DAILY†, PAUL R. EHRLICH† & GARY W. LUCK†

* Department of Fisheries and Wildlife, Michigan State University, E. Lansing, Michigan 48824, USA

† Center for Conservation Biology, Stanford University, Stanford, California 94305, USA

Correspondence and requests for materials should be addressed to J.L. (e-mail: jliu@panda.msu.edu).

Human population size and growth rate are often considered important drivers of biodiversity loss1-6, whereas household dynamics are usually neglected. Aggregate demographic statistics may mask substantial changes in the size and number of households, and their effects on biodiversity. Household dynamics influence per capita consumption7, 8 and thus biodiversity through, for example, consumption of wood for fuel9, habitat alteration for home building and associated activities10-12, and greenhouse gas emissions13. Here we report that growth in household numbers globally, and particularly in countries with biodiversity hotspots (areas rich in endemic species and threatened by human activities14), was more rapid than aggregate population growth between 1985 and 2000. Even when population size declined, the number of households increased substantially. Had the average household size (that is, the number of occupants) remained static, there would have been 155 million fewer households in hotspot countries in 2000. Reduction in average household size alone will add a projected 233 million additional households to hotspot countries during the period 2000–15. Rapid increase in household numbers, often manifested as urban sprawl, and resultant higher per capita resource consumption in smaller households15-19 pose serious challenges to biodiversity conservation.

As a first step towards quantifying the effects of household dynamics on biodiversity, we compared the rates of change in human population size and the number of households in 76 hotspot and 65 non-hotspot countries. We also investigated the sources of growth in household numbers, comparing the relative contributions of changes in aggregate population size and household size. Finally, in six representative hotspot areas, we estimated the relative contributions of changes in population size and household size to the growth in the number of households (see Methods).In hotspot countries, the annual rate of growth in the number of households (3.1%) was substantially higher than the population growth rate (1.8%) between 1985 and 2000 (Fig. 1a). Over 80% of hotspot countries showed this pattern (Table 1). In contrast, population and household growth rates in non-hotspot countries were roughly equivalent (1.7%) (Fig. 1a). The divergence in population and household growth rates is expected to become more pronounced over the next 15 years (Fig. 1b and Table 1), suggesting that it is crucial to consider growth in the number of households when assessing threats to biodiversity.

bar graphs for Household dynamics in 76 hotspot countries (HC) and 65 non-hotspot countries (NHC) Figure 1 Household dynamics in 76 hotspot countries (HC) and 65 non-hotspot countries (NHC).   Full legend
 
High resolution image and legend (50k)

The growth in household number resulted directly from a simultaneous increase in population size and reduction in average household size. In 1985, average household size was larger by 1.0 persons in hotspot countries (4.7) than in non-hotspot countries (3.7). This difference was reduced to 0.3 persons in 2000. By 2015, the average household size in hotspot countries (3.4) is expected to be 0.2 persons smaller than that of non-hotspot countries (3.6). Per cent contribution of reduced household size to growth in the number of households was about 12 times higher in hotspot countries than in non-hotspot countries in 1985–2000 (Fig. 1c), and it is estimated to be about 7 times higher in hotspot countries than in non-hotspot countries in the period 2000–15 (Fig. 1d). Furthermore, this contribution is projected to increase from 43% (hotspot countries) and 3% (non-hotspot countries) in 1985–2000 (Fig. 1c) to 54% and 7%, respectively, in 2000–15 (Fig. 1d). 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.Had the average household size remained at the 1985 level, there would have been 155 million fewer households in hotspot countries in 2000 (Fig. 1e). By 2015, 233 million more households are likely to be added to hotspot countries as a result of continued reduction in average household size alone. If the average household size in 2015 were the same as in 1985, there would be 415 million fewer households in hotspot countries (Fig. 1e); in non-hotspot countries, there would be 4 and 7 million fewer households in 2000 and 2015, respectively (Fig. 1f). In four hotspot countries (Italy, Portugal, Spain and Greece) over the period 2000–15, contributions of reduction in average household size to growth in the number of households are expected to be roughly 120–190% (equivalent to 0.4–2.4 million additional households in each country) even when their corresponding population sizes are projected to decline at an annual rate of 0.1–0.3%.In the six representative hotspot areas listed in Table 2, reduction in average household size contributed approximately 30–73% to the growth in the number of households over the periods of 10 and 40 years. Annual rates of population growth in these six areas ranged from 0.5% to 7.0%, whereas annual rates of growth in household numbers were much higher (1.7–10.0%) owing to a decline in average household size (0.7–1.2% per year). By 1991, Italy had added almost 6 million households as a result of reduced average household size alone since 1951 (Table 2). In Brazil, over 4.6 million households were created between 1991 and 2000 owing to a reduction in average household size. Had the average household size stayed at the 1970 level, Indian River County (United States) would have had 11,103 fewer households in 2000. These extra households are among the factors that have made Indian River County one of the endangered species hotspots in the United States (areas with the largest numbers of federally listed endangered and threatened species)20. Reduction in average household size was an important factor causing the increase of household number and thus rise in the amount of fuel wood consumed in Wolong Nature Reserve (China), which contributed to increased deforestation and loss and fragmentation of habitat for giant pandas21. In Wolong, there would have been approximately 230 fewer households in 1998 if average household size had been kept at the 1975 level (Table 2).Even in regions where population size decreased, the number of households still increased substantially owing to a reduction in average household size. During the period 1976–81, population size in nine government regions of New Zealand declined by about 640–7,200 people, but the number of households rose by approximately 560–7,650 per region (Fig. 2, top). Contributions of reduction in average household size to the increase in the number of households ranged from roughly 112–2,034%, compared with -1,934% to -12% resulting from the change in population size (Fig. 2, bottom). In the other 13 regions of New Zealand where the population increased, reduction in average household size contributed about 29–82% to the growth in the number of households, whereas population growth accounted for approximately 18–71%. Considering all 22 New Zealand government regions, contributions of reduction in average household size and change in population size to growth in household number were about 83% and 17%, respectively.

bar graphs for Changes in household numbers and population size (top), contributions to growth in household number (bottom) Figure 2 Changes in household numbers and population size (top) as well as their contributions to growth in household number (bottom) in nine regions of New Zealand.   Full legend
 
High resolution image and legend (46k)

Reduction in average household size takes a double toll on resource use and biodiversity. First, more households mean more housing units, thus generally increasing the amount of land and materials (for example, wood, concrete and steel) needed for housing construction (see Supplementary Results). Second, smaller households have lower efficiency of resource use per capita (Fig. 3) because goods and services are shared by more people in larger households16-18 (see Supplementary Results). Proximate causes of a reduction in household size include lower fertility rates, higher per capita income, higher divorce rates, ageing populations, and a decline in the frequency of multi-generational families living together (see refs 22, 23–24). Some of these factors can affect both population growth and household growth. Although lower fertility rates may reduce population growth and household numbers, the resulting potential reduction in resource consumption may be off-set by higher per capita consumption in smaller households. Thus, declining fertility rates are necessary but not sufficient to ensure reduced anthropogenic pressure on the environment and natural landscapes. Our study suggests that biodiversity conservation is faced with much larger challenges than previously thought, because reduction in household size leads to higher per capita resource consumption and a rapid increase in the number of households, even when population size declines. This trend is most prevalent in hotspot countries where it may severely limit efforts to conserve biodiversity, thus degrading the ecosystem services25 that biodiversity delivers to humanity.

bar graphs for Rooms per capita versus household size in New Zealand, Italy and Rodrigues of Mauritius Figure 3 Rooms per capita versus household size in New Zealand, Italy and Rodrigues of Mauritius.   Full legend
 
High resolution image and legend (18k)

Methods
Data on population sizes and household numbers for 141 countries over the period 1985–2015 are from the United Nations26. The hotspot countries were identified according to Conservation International (http://www.biodiversityhotspots.org/xp/Hotspots/) and ref. 27, confirmed by N. Myers and P. Langhammer (personal communication), and are listed in Supplementary Table 1. The six areas used in our detailed analysis were chosen from six major regions (Africa, Asia, Europe, North America, Oceania and South America) on the basis of data availability, representation and policy implications (see Supplementary Methods).Rates of growth and per cent contributions in hotspot and non-hotspot countries (Fig. 1) were weighted by population size and number of households in each country because of great variations among countries. To compare the rates of growth in household number (hhn) and population size (pop) (Table 1), we determined the number and percentage of hotspot and non-hotspot countries with hhn(+ ) > pop(+ ) and hhn(+ ) < pop(+ ) (where + indicates positive rates of growth). Using Fisher's exact test, we tested for differences in the frequency of occurrence of hotspot and non-hotspot countries in these categories, for 1985–2000 and 2000–15, respectively.

Changes in the number of households are affected directly by changes in population size and household size. We calculated the per cent contribution of change in average household size (chs) to the change in the number of households (Hchs) as the total contribution (100%) minus the contribution due to population growth alone:

household dynamics equation

where H0 and H1 are the number of households at time 0 and 1, respectively. HP is the growth in the number of households (with the same average household size at time 0 (S0)) due to population growth or the difference in populations (P1 - P0) at times 1 and 0: HP = (P1 - P0)/S0. The average household size was fixed at its value for time 0 (S0) (when HP was computed) to see how many fewer households would exist at time 1 if the average household size remained static. As in standard decomposition techniques28, taking a simple average of household sizes at times 0 and 1 would be another way to calculate HP. However, our choice of S0 yields similar results and is the most appropriate method to address our question.

The contribution of change in population size (cps) to growth in the number of households (Hcps) is the total contribution minus the contribution owing to change in household size:

household dynamics equation

Supplementary information accompanies this paper.

Received 6 September 2002;accepted 12 December 2002

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Acknowledgements. We thank J. Eagle, W. Falcon, M. Feldman, N. Keilman, H. Mooney, R. Naylor, S. Pimm, K. Seto and S. Tuljapurkar for their constructive comments on earlier drafts; P. Langhammer and N. Myers for providing lists of hotspot countries; E. Laurent for technical assistance in producing figures; J. Baca, R. Cincotta, W. Lutz and A. McMillan for providing some references; G. Clarke for providing the housing data of India River County, Florida; and W. W. Taylor and Q. Wang for logistical and moral support. Funding for this project was provided to J.L. by the National Science Foundation (CAREER Award and Biocomplexity in the Environment Program) and the National Institute of Child Health and Human Development.

Competing interests statement. The authors declare that they have no competing financial interests.



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