As noted in Chapter 1, it has been estimated that humans make use of nearly 25% of the entire photosynthetic output on land through the crops and livestock we harvest, the biomass we burn, and the primary production we eliminate when we clear-cut forests. Ecologists also estimate that crops cover 10% to 15% of Earth’s land surface, and pastureland another 6% to 8% (Fig. 49.19).

The conversion of natural ecosystems to cropland decreases the area of natural habitats, as does the expansion of cities and towns. As discussed in Chapter 46, the theory of island biogeography predicts that as habitable area decreases, species diversity within the habitat declines. Habitat loss in tropical rain forests poses a particularly important threat to biodiversity (Fig. 49.20). These forests represent the greatest concentrations of species diversity on land, but clear-cutting to make room for grazing and croplands has destroyed more than half of their pre-industrial area. It is estimated that deforestation over the next century could eliminate 40% to 50% of all tree species in the Amazon forest.

As the forest disappears, so, too, will untold numbers of insect, mite, and other animal species. The renowned biologist E. O. Wilson has estimated that Earth is losing 0.25% of its species annually—that is, 5000 to 25,000 species per year, depending on how many species actually exist. And even where ecosystems remain physically intact, pollution can erode populations. For example, recent studies in Europe show that levels of neonicotinoid pesticides in soil and water accurately predict patterns of population decline in bees and birds.

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In the words of another distinguished biologist, Paul Ehrlich, “The fate of biological diversity for the next 10 million years will almost certainly be determined during the next 50–100 years by the activities of a single species.”