By now it should be clear that resource use and conservation biology are closely intertwined: The fate of many species will depend on the decisions we make about food and energy for a burgeoning human population. With this in mind, sustainable development has emerged as an important principle for trying to balance priorities for humans and the natural world. In principle, the concept is simple: The rate at which we use resources should be no higher than the rate at which they can be replenished.

Sustainable development is most easily understood in terms of species that grow and are harvested by humans. Thus, logging in the forests of western North America or the Amazon basin is sustainable only if tree harvest is matched by growth, keeping the forests dynamically intact. Similarly, fisheries are sustainable only if annual catch is no larger than the capacity of fish populations to grow and produce. By changing the way that catch is distributed among fishermen, the harvest of fish along the Pacific coast of the United States has become more predictable (as has the income of fishermen) and stocks of commercial species are increasing. Expanding marine reserves are also proving successful in rebuilding fish populations. At the same time, decisions by both fishermen and consumers are shifting the targets of commercial fishing away from vulnerable species and toward those that can be harvested sustainably.

It can be difficult to quantify sustainable yields for agriculture, as the cost of producing food includes the costs of fuel and fertilizer and, where episodic floods or droughts influence food supply, sustainable yield may vary from year to year. Despite these difficulties, the broader target of sustainable development—reducing our ecological footprint—has begun to influence agricultural practice and consumer choices. The task is formidable, but the goal is important: to ensure that meeting the needs of humans does minimal damage to Earth’s biodiversity.

Without question, there will be fewer species when the 21st century ends than when it began. Lost species mean lost opportunities for the discovery of novel compounds for medical research. Diminished biodiversity may make communities less productive and less resilient to fires, hurricanes, or other environmental events. There is a pressing need to feed the world, and there are also many good reasons to conserve the biodiversity that has evolved over 4 billion years. Balancing these two imperatives will entail creative approaches to both science and policy, including thoughtful ways of using land that serve both people and conservation.

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