We've known this for centuries, but the explanation is not simple.
By Martin McGuigan
It's one of the oldest questions tackled by naturalists: Why do tropical regions have so much biodiversity compared with other areas of the globe?
"The nearer we approach the tropics, the greater the increase in the variety of structure, grace of form, and mixture of colors, as also in perpetual youth and vigor of organic life," German naturalist Alexander von Humboldt wrote in 1807. In tropical regions, there are more species of plants, animals and fungi in any given area, and that concentration declines as you move farther from the equator.
This phenomenon is known as the latitudinal diversity gradient. But what causes it?
The first hypothesis relies on energy. There is more sunlight in the tropics and, when combined with rainfall and soil nutrients, this leads to more plant growth. "Half the year is in darkness as you go into the Arctic Circle or the Antarctic Circle," Dobson said. "There's no energy coming in to sustain life." The abundance of plants, therefore, means more animals can survive and reproduce.
From an evolutionary perspective, the abundance of plant growth leads to a greater diversity of animals. "If you can explain plant diversity, then you have more things to eat the plants and either be specialists or generalists, and then more things to eat the herbivores and either be specialists or generalists," Dobson said. This level of interaction across the food web — with plants and fungi, herbivorous animals, and predators — leads to a "higher rate of speciation," the point at which a new species appears to be distinct from its evolutionary ancestors.