number of individuals to survive. If its numbers fall too low, life’s cruel lottery will finish it off. Perhaps environments with more energy can support a greater quantity of life and so allow more species to keep their numbers on the right side of the threshold for survival.
But the idea that energy promotes diversity by allowing greater numbers of organisms to live together also has its problems. Tropical seas and soils are both low in nutrients and rich in species. More resources do not necessarily mean more diversity. Adding nutrients to lakes, rivers, or soil, in the form of fertilizer or sewage, leads to a drop in the number of species. A few fast-growing species, good at sucking up abundant nutrients, come to dominate. On the other hand, this might be because few of the local species are adapted to such conditions. Over evolutionary timescales, one might expect diversity to rise as more species evolved to deal with the richer environment.
And in reality it is the quality, not the quantity, of tropical life that is outstanding. As Al Gentry found, and Brian Enquist and Karl Niklas confirmed, there aren’t more trees, or a greater mass of wood, in a hectare of Amazonian rain forest than a hectare of Alaskan conifer woods—just vastly more species. Other researchers have found that the same goes for North American birds and butterflies. There is a slight trend toward increased numbers of individuals at lower latitudes, but the number of species rises far more quickly.
Perhaps, then, it’s the quality of the climate, not the quantity of energy, that’s important. As well as being warmer and wetter, the climate in the tropics is, as Humboldt noted, more stable, lacking the seasonal fluctuations in temperature found in temperate regions.
This stability might encourage diversity by letting species be more specialized, making their niches narrower and allowing a finer division of natural resources. If an animal in a temperate forest lives on fruit, or leaves, or insects, it has a problem. These food sources disappear for some of the year. The animal must either broaden its diet, hibernate, or migrate. But in the tropics it can eat leaves, fruit, or bugs all year round, leaving more resources for other species. Perhaps stability promotes