diversity in general. Some other highly variable environments, such as estuaries, which alternate between saltwater and fresh water twice daily, have relatively low numbers of species.
But while the weather in the tropics might be more agreeable, the biological environment is as cutthroat as anywhere else, perhaps more so. This too has been suggested as the cause of abundant tropical diversity. Freed from the stress of coping with the inclement or unpredictable, tropical species might instead evolve to exploit each other by becoming better competitors, or predators, or parasites. If tropical species faced fiercer struggles with their enemies, or with others of their own kind, than temperate ones, they might compete less with their neighbors over resources—the force that drives species out. Thus might more tropical species coexist.
But although a stable environment or more intense biological interactions could enhance diversity, they can only be secondary factors. The problem is that nastier predators or narrower niches are consequences of the number of species in a place as much as they are causes. There is evidence that competition, predation, and parasitism are stronger in more diverse ecological communities. But this doesn’t explain why there are more species there to begin with. So such arguments also carry the risk of circularity. There is evidence that diversity can beget diversity: A study by Brent Emerson and Niclas Kolm of the plants and insects of the Canary Islands and the Hawaiian archipelago found that, on islands with more species, new species are more likely to evolve. Islands with the most species also had the most endemic species, which are more likely to have evolved on that island. So the twists and turns in the struggle for life may well make the gradient in diversity steeper. But for them to have their effect, that gradient needs to be there in the first place.
A recent theory, however, suggests that this gradient might appear regardless of temperature, rainfall, or anything else. In the early 1970s, Robert Colwell, then a grad student at the University of Michigan, began looking for a model of a uniform world, to show what it is about