our own irregular world any theory needs to explain. Where would species be found if every part of the world had the same climate? The obvious assumption is that they would spread out evenly and that no one continent or ocean would harbor more biodiversity than anywhere else. But Colwell found that even in a world without tropics more species would live around the equator.
Colwell based his model on the geometry of species’ homelands. Every species has its range: a stretch of land or water where it lives and beyond which it rarely strays, thanks to barriers such as seas, mountains, or deserts. This is why that subset of British bird-watchers known as twitchers get so excited when a hapless red-eyed vireo is blown across the Atlantic. Most species have small ranges. The giant onion, Allium pskemense, is restricted to the western part of the Pskem mountains in China, and bonobo chimpanzees are found only in one corner of the Congo Basin. A few species, such as barn owls, which are found on every continent except Antarctica, have spread themselves very widely. Humans, a close relative of bonobos, live where even barn owls don’t go. Colwell took the distribution of range sizes—a few large and lots of small—and spread them out at random across a continent.
He saw that more species would be found in the middle of such a random continent because this is where most species ranges would overlap. And if species were spread at random over our world, more should end up at the equator, because this is where the middles of most oceans and continents are found. Colwell explains the effect by comparing the world to a pencil case. Take a pencil case and fill it with pencils of different lengths, from short stubs to long ones that barely fit in the box. The long ones represent species with big ranges, the stubby ones those with small. Shake the pencil case, so that the pencils are spread randomly along its length, but do not reach beyond the box, just as a land-dwelling species cannot live at sea. Then slice cross-sections through the box, and see how many pencils (species) are caught in each slice. The middle slices should contain more, not because the middle of the box is more pencil-friendly, but because any pencil longer than half the length of the box will inevitably be found in the middle. Species, he argues, should distribute themselves in a similar way, even without different environments.