Some years ago Norman Myers coined the term “biodiversity hotspots” to indicate areas of the globe which contain a high degree of biodiversity, and which should get high priority for conservation (Myers, 1988b). A similar concept is Russell Mittermeier’s “major tropical wilderness areas, of which he identified three: the Upper Amazon, the Congo Basin and New Guinea/Melanesia (Mittermeier, 1998). He also uses the term “megadiversity countries” for nations with extremely high biodiversity (Mittermeier, 1988; Mittermeier, et al., 1998; Mittermeier, Myers & Mittermeier, 2000). Recently, Olson, et al., (2001), suggest a finer-tuned method of evaluating terrestrial biodiversity by utilizing a concept based on biogeography, taxonomy, and ecology. These new units they call “ecoregions.” By this means they have subdivided the globe into 14 biomes, eight biogeographic zones, and 867 ecoregions. Under this system, the tropics contain 463 ecoregions. These scientists hope that the finer resolution attained by this method will assist in identification, comparison, and conservation of the most biodiverse regions and those with high endemism. (See also Gentry, 1992.)
Myers’ hotspots now consist of only 10% of their former area. They also contain only about 12% of their former primary vegetation, in comparison with 50% for tropical forests overall (Pimm and Raven, 2000). Nevertheless, they still contain 44% of vascular plant species and 35% of terrestrial vertebrate species (Hardner and Rice, 2002). Unfortunately for their continued existence as areas of great biodiversity, some of them also have high (human) population densities. In 1995, the population density in Myers’ designated 25 hotspots averaged 73 people per km2 (Cincotta, Wisnewski & Engelman, 2000). These areas also contain 44% of known vascular plant species (133,149) and 35% (9,645)of known vertebrate species (Myers, et al., 2000), as well as highly-depleted habitats. Between 1995 and 2000, nineteen of these areas had population growth rates higher than the global average. In 1995, 75 million people lived in the three major tropical wilderness areas indicated by Mittermeier, about eight people per km2. But the population growth rate in these areas is 3.1% per year, more than twice the global average (Cincotta, Wisnewski & Engelmann, 2000). It seems likely that the hotspots will lose 40% of their species if current rates of habitat loss continue, even if for only another decade.
Human population growth has emerged as the driving force behind tropical forest loss and species extinction. Although generalized population growth rates mask many features of population increase, such as non-uniform distribution, size of initial population (larger populations will add more individuals than smaller ones even if the growth rate is the same), and affluence (and therefore ability to consume goods), high growth rates in tropical areas are good indicators of trouble for forests. Many of the areas where human population density is the highest are also those which are biodiversity “hotspots,” areas of great biodiversity. Another factor is urbanization. Urban areas can disturb forests far distant from regions of population concentration – over a radius of 100 km or more. Just as pertinent, affluent people in temperate regions create a demand for tropical products, a demand which provides great impetus for destructive activities in tropical forests. War and civil disturbances (as in the Republic of Congo) are also highly destructive, as people flee from conflicts into the forests, and those seeking food sources strip the forests of edible plants and animals. The rapid growth of human numbers in tropical areas represents trouble for rainforests.
Land-use change is the major factor in biodiversity loss; somewhat less significant are climate change, alterations in nitrogen cycles, the introduction of exotic species into ecosystems, and disruptions of the carbon cycle by alterations in atmospheric carbon dioxide levels.