The term “biodiversity”—short for biological diversity—was first used in the 1980s by scientists to refer to the richness of biological variation on Earth or within a particular region. In their book Saving Nature’s Legacy, ecologists Reed Noss and Allen Cooperrider define biodiversity as

the variety of life and its processes. It includes the variety of living organisms, the genetic differences among them, the…ecosystems in which they occur, and the ecological and evolutionary processes that keep them functioning, yet ever changing and adapting.

As this definition suggests, biodiversity exists on several levels. Perhaps the most common definition of the term refers to the variety of different species on the planet or in a given habitat. Approximately 1.7 million species of plants, animals, fungi, microbes, and other forms of life have been identified and named by biologists, but estimates of the...