Going, going, gone? In this essay, a scientist argues that we're headed for another mass extinction like the one that wiped out the dinosaurs.

• Author: Pearson, Richard
• Position: OPINION - Essay

Nearly 20,000 species of animals and plants around the globe are considered high risks for extinction in the wild. They're all on the Red List, the most authoritative compilation of living things at risk, maintained by the International Union for Conservation of Nature.

This should keep us awake at night.

A study in the journal Nature in 2011 concluded that if all species listed as threatened on the Red List were lost over the coming century and that rate of extinction continued, we would be on track to lose three quarters or more of all species within a few centuries.

We know from the fossil record that such rapid loss of so many species has previously occurred only five times in the past 540 million years. The last mass extinction, around 65 million years ago, wiped out the dinosaurs.

And the Red List provides just a tiny insight into the real number of species in trouble. The vast majority of living things that share our planet remain undiscovered or have been so poorly studied that we have no idea whether their populations are healthy or approaching their demise. Less than 4 percent of the roughly 1.7 million species known to exist have been evaluated. And for every known species, there are most likely at least two others--possibly many more--that have not yet been discovered, classified, and given a formal name by scientists. Just recently, for instance, a new species of leopard frog was found in ponds and marshes in New York City. So we have no idea how many undiscovered species are at risk or already lost.

We often forget how dependent we are on other species. Ecosystems of multiple species that interact with one another and their physical environments are essential for human survival.

The Benefits of Biodiversity

These ecosystems provide food, fresh water, and raw materials for construction and fuel; they regulate climate and air quality; buffer against natural hazards like floods and storms; maintain soil fertility; and pollinate crops. The genetic diversity of the planet's many different life forms provides the ingredients for new medicines and new commercial crops and livestock, including those that are better suited to conditions under a changed climate.

Do we need to protect so many species? Or can we rely on ecosystems with a smaller number of parts? A recent study of grassland ecosystems shed new light on these questions. Seventeen grasslands with different numbers of species were created and then studied over many years. The...