One of Wilson’s central arguments in Half-Earth is that we know very little about the planet’s biodiversity. The public and scientists are most familiar with vertebrates—mammals, birds, amphibians, fishes, and reptiles. Wilson suggests that this familiarity is due to these organisms’ size (i.e., most are fairly large) and their importance to human life (21). In contrast, very little is known about the rest of life. Estimates suggest that scientists have only identified about 20% of “Earth’s biodiversity at the species level” (23). To Wilson, this fact is deeply troubling given the rate of extinction, which is around 1,000 times higher today than during the pre-human era. We do not have a complete understanding of the species we are losing. Human activities are “breaking many threads,” and how they impact ecosystems is “still impossible to predict” (106).

By providing a naturalistic portrait throughout the book, Wilson pays tribute to creatures both big and small as well as the various environments in which they live. Among the species Wilson cites is one he finds to be “‘the most bizarre,’ at least as a human would judge it” (128): the Osedax worm (128). These worms feed on lipids found in whale carcasses that sink to the ocean floor.