The riches of the natural world are not spread evenly across the globe. Some places, such as the tropical Andes in South America, are simply stacked with unique species of plants and animals, many found no place else on Earth. So-called biodiversity “hotspots” are thought to cover just 2.3 percent of the planet’s surface, mostly in the tropics, yet they account for half of all known plant species and 77 percent of land vertebrates.
