Abstract
Biologists have identified 25 areas, called biodiversity hotspots, that are especially rich in endemic species and particularly threatened by human activities. The human population dynamics of these areas, however, are not well quantified. Here we report estimates of key demographic variables for each hotspot, and for three extensive tropical forest areas1 that are less immediately threatened. We estimate that in 1995 more than 1.1 billion people, nearly 20% of world population, were living within the hotspots, an area covering about 12% of Earth's terrestrial surface. We estimate that the population growth rate in the hotspots (1995–2000) is 1.8% yr -1, substantially higher than the population growth rate of the world as a whole (1.3% yr-1) and above that of the developing countries (1.6% yr-1). These results suggest that substantial human-induced environmental changes are likely to continue in the hotspots and that demographic change remains an important factor in global biodiversity conservation. The results also underline the potential conservation significance of the continuing worldwide declines in human fertility and of policies and programs that influence human migration.
In 1988, ecologist Norman Myers introduced the term 'biodiversity hotspots' to distinguish a global set of high-priority terrestrial ecoregions for conservation2. Myers and others argue that, because their 25 hotspots are high in species endemism and low in pristine vegetation (< 30% remaining), wise conservation investments in these ecoregions could help minimize future extinctions2, 3, 4. Primatologist Russell Mittermeier subsequently developed a complementary concept, the 'major tropical wilderness areas'5. These three areas of tropical forest (Upper Amazonia/Guyana Shield, the Congo Basin, and the New Guinea/Melanesian Islands) are the most pristine of all terrestrial ecoregions exhibiting a high degree of species endemism. Together they cover 6.3% of Earth's terrestrial surface, an area larger than the United States or China. Myers, Mittermeier and others propose a strategy of conservation investments in these areas as a back-up strategy for the hotspot approach1. By using mapped world distributions of humans (Fig. 1), various census sources and ecoregional boundary data, we calculated population density and growth rates for each of the biodiversity hotspots and major tropical wilderness areas (see Methods).
Figure 1: World population density (1995) and the 25 biodiversity hotspots (outlined in red, numbered), and three major tropical wilderness areas (outlined in green, lettered).
Hotspots: (1) Tropical Andes; (2) Mesoamerica; (3) Caribbean; (4) Atlantic Forest Region; (5) Chocó-Darién-Western Ecuador; (6) Brazilian Cerrado; (7) Central Chile; (8) California Floristic Province; (9) Madagascar; (10) Eastern Arc Mountains and Coastal Forests of Tanzania and Kenya; (11) West African Forests; (12) Cape Floristic Region; (13) Succulent Karoo; (14) Mediterranean Basin; (15) Caucasus; (16) Sundaland; (17) Wallacea; (18) Philippines; (19) Indo-Burma; (20) Mountains of South-Central China; (21) Western Ghats and Sri Lanka; (22) Southwest Australia; (23) New Caledonia; (24) New Zealand; and (25) Polynesia and Micronesia. Major tropical wilderness areas: (A) Upper Amazonia and Guyana Shield; (B) Congo River Basin; and (C) New Guinea and Melanesian Islands.
High resolution image and legend (48K)We estimate that in 1995 population density in the hotspots was 73 people km-2, a figure 71% greater than that of the world as a whole (excluding ice- or rock-covered land). We found that 16 of the 25 hotspots (Fig. 2a) have population densities at or above the world average (42 people km-2). According to our estimates, from 1995 to 2000, human population was still growing in all but one of the hotspots (the Caucasus), with 19 of the hotspot populations growing more rapidly than that of the world as a whole (Fig. 2b). Although population growth rates were, in general, highest in the 19 hotspots wholly within developing countries, growth rates in the hotspots within developed countries were in most cases substantially higher than the worldwide average for developed regions (0.3% yr-1).
Figure 2: Human population densities (a) and annual growth rates ( b) in the 25 global biodiversity hotspots (1–25; see map for names and locations, Fig. 1) and the three major tropical wilderness areas (A, B, C).
![Figure 2 : Human population densities (a) and annual growth rates (
b) in the 25 global biodiversity hotspots (1|[ndash]|25; see map for names
and locations, Fig. 1) and the three major tropical
wilderness areas (A, B, C). Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com](/nature/journal/v404/n6781/images/404990ab.0.jpg)
High resolution image and legend (60K)
In 1995, nearly 75 million people (1.3% of world population) were living within the three major tropical wilderness areas, representing an average density of about 8 people km-2. (Area boundaries enclose several major cities.) These areas are experiencing population growth at a rate of 3.1% yr-1, which is more than twice the global rate.
If population numbers are examined in isolation of other factors, the three hotspots with the most elevated risks, as assessed by high human population density, are the Western Ghats/Sri Lanka, Philippines and Caribbean hotspots. Chocó-Darién-Western Ecuador, Tropical Andes and Madagascar head a list of hotspots facing elevated risks on the basis of rapid population growth alone. Notably, the latest hotspot analysis by Mittermeier et al . concludes that the Philippines, Caribbean and Madagascar hotspots appear to be the highest-priority of these ecoregions on the basis of their extreme endemism and the intense packing of species into a much reduced area of original vegetation6.
Human population variables are imperfect indicators of risk to biodiversity. Population density figures, for example, obscure patterns of population distribution within areas. Roughly 90% of the population of the southwest Australia hotspot lives in and around Perth, a single metropolitan area covering less than 2% of the ecoregion; however, such uneven distribution does not negate risk to biodiversity. There is considerable evidence of the capacity of urban populations to alter ecosystems, which are sometimes more than 100 km away, through demand for wood fuel (principally in developing countries), water, food (including wild foods), waste disposal and recreation (mostly in developed countries)7, 8.
Another problem is that disturbance caused by humans can occur in the absence of widespread human settlement. This is the frequent result of over-logging, burning, grazing, mining and commercial hunting that have extracted or degraded natural resources, abetted biological invasion or polluted soil and water resources9. Population density remains low, for example, in the most arid hotspot, the Succulent Karoo, which experiences heavy grazing and the over-harvesting of its flora for the international trade in ornamental plants.
Population growth rates can also be misleading indicators of risk to biodiversity. Because growth rates are calculated as the annual percentage change in a population, low rates of growth in dense populations add more individuals than much higher rates of growth in sparse populations. Population growth rates mask spatial distributions of growth and the trend of that rate. And both density and growth rates hide the culture, affluence and technology of the numbers of people they represent.
Despite these caveats, however, population trends in the biodiversity hotspots and major tropical wilderness areas indicate a high risk that habitats will continue to degrade as ecosystems dominated by humans expand and species become extinct in the world's most biologically diverse terrestrial regions. Results of the analysis also suggest that, whatever species conservation strategies ultimately emerge, conservation scientists and policymakers should take human population dynamics into account. Especially relevant are trends and potential changes in population growth, density and migration, and the social and economic factors known to influence population variables. One hopeful sign for the conservation of biodiversity is that declines in human fertility are gradually slowing population growth worldwide.
Methods
Population density
We estimated population densities for biodiversity hotspots and major tropical wilderness areas (ecoregions) using the Gridded Population of the World, 1995, a geographic information systems (GIS) layer developed by geographers at the National Center for Geographic Information and Analysis, University of California, Santa Barbara10. The authors of this layer call attention to numerous sources of potential error in these data, including extrapolation from census-year estimates to 1995 projections, the mapping of census geographical boundaries and census estimates themselves. For census data in most industrialized countries, demographers regularly assume an error (most often an undercount) of less than 3% of the actual population11. Errors exceeding 10% occasionally occur in censuses in the poorest countries, particularly those experiencing political instability. Moreover the populations enumerated vary from country to country, with some countries including, for example, military personnel living outside the country.
Population growth
We partitioned hotspots into countries and sub-national political divisions (provinces or states), used available growth rates or census and projection data to determine the growth of each unit, and then calculated average growth rates for the composite ecoregion. Data on a provincial level were used in ecoregions covering parts of Argentina, Australia, Brazil, Bolivia, Colombia, China, Ecuador, France, India, Indonesia, Mexico, Panama, Peru, South Africa, Spain, Turkey, United States and Venezuela. Where provincial data were unavailable or unnecessary (where the entire country fell within the hotspot), country population growth rates were obtained from estimates generated by the United Nations Population Division12. These United Nations data are also the source for 1995 world population density and 1995–2000 world population growth rates.