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Demographers refer to such collisions between rising demand and diminishing resources as "train wrecks." As the world adds new billions of people in ever shorter periods, such potential conflicts happen almost everywhere. With most of the world's good land already under plow, a population of 11 billion human beings would probably have to make do with less than half the arable land per capita that exists today. That would set the stage for disaster, as farmers stripped nutrients from the soil, exacerbated erosion and gobbled up water and wild lands.
If population keeps building at the current rate, the most ominous effect is that millions of life-forms will become extinct. Humans, no matter how well behaved, cannot help crowding out natural systems. A survey of 50 countries by environmental researcher Paul Harrison showed that habitat loss, the most important factor leading to extinctions, rises in direct proportion to the density of the individuals that make up various species. Big animals often range over hundreds of square miles and increasingly collide with settlements. Smaller species, which make up most of nature's diversity, are affected by human activities in countless ways. Frogs, for example, are gradually disappearing around the world, perhaps because airborne pollutants are destroying their eggs. The crucial question is whether humankind can afford to exterminate large numbers of other species without ruining the ecosystems that also sustain us.
The world could avoid this question by reducing the burden placed on the biosphere by rising human numbers and the life-styles of rich nations. To do so, however, would require countries to treat these threats far more seriously than they did at the Earth Summit in Brazil last June. The affluent nations must move their economies more rapidly toward patterns of production and consumption that recognize the limits of what the earth can provide and what wastes it can accommodate. The poorer nations must make monumental efforts to remove incentives for people to have large families. This will require massive social change, including better education and improved access to family planning. With each passing year, it becomes more likely that the fastest- growing nations will be forced to adopt coercive measures, as China has, if they are to stabilize their numbers.
If none of this takes place, what might the earth look like? Author Meadows predicts that at its best, the typical landscape might resemble the Netherlands: a crowded, monotonous tableau in which no aspect of nature is free from human manipulation. Other analysts look to the history of island cultures because they tend to reveal how the environment and humans respond when burgeoning populations put stress on an isolated ecosystem.