Biological diversity, or biodiversity, refers to the summed variation of life on Earth (Chapter 47). This includes genetic and population variation within species; the number of species in local communities, biomes, or the planet as a whole; and the variations in species composition among ecosystems across the planet. We don’t know how many species exist on our planet, although the number certainly runs into the millions and may exceed 10 million. Whether local or global, patterns of biodiversity reflect all of the processes discussed in earlier chapters: natural selection, functional biology, ecological interactions among species, and the distribution of physical environments. Increasingly, biodiversity also reflects human activities.

As discussed earlier in this chapter, habitat destruction, overexploitation, and invasive species all pose threats to biodiversity, and in the 21st century, global change threatens biodiversity as well. Earth history is punctuated by mass extinctions, but unlike the great extinctions of the past, current threats come not from meteorites or massive volcanism, but from us. Importantly, as human activities lie at the heart of threats to biodiversity, humans can make choices that will help us to preserve and protect the natural world. Conservation biology addresses the challenge of sustaining biodiversity in a changing world crowded with people.