It was more than 200 years ago that Alexander von Humboldt noticed a striking pattern: Most of the earth's biodiversity is bunched up in the tropical regions—there are more species, medicinal plants, diseases, cultures, languages, you name it, the nearer you get to the equator. As far as it goes, this observation turns out to be something like a law: the warmer the climate, the more biological diversity you have. So, roll the tape forward 200 years and you'd think biologists would have a good theory to explain this. Surprisingly, writes Rob Dunn in Seed magazine, they don't:
No fewer than six prominent explanations for diversity gradients have found support recently in high-profile journals. Among these more popular arguments, one hypothesis holds that diversity is highest in those regions that have had the longest uninterrupted time for new species to form. Unperturbed by glaciations or other major climatic changes, these evolutionary Edens are the cradles of diversity. Or so that argument goes.
A related series of three hypotheses is built on the argument that diversity is highest where speciation rates are highest. And what influences speciation rates? Perhaps they are higher where temperatures are higher or where the geographic areas of biomes are larger, or maybe instead where there is the most plant matter, the most food on which to survive.
Another hypothesis holds that the high productivity of the tropics—itself a function of the local plants' maximal exposure to sunlight—doesn't boost speciation but instead keeps extinction rates down. If there is enough food, no one starves.
Or maybe, one hypothesis suggests, the latitudinal diversity gradient doesn't deserve explanation at all. Instead, species distributions, like pancakes tossed haphazardly on a plate, pile up in the middle latitudes as a consequence of geometry's constraints.
And those are just the high-profile hypotheses; there are another three dozen or so minor theories nestling in the wings. You can find a fuller list here. Maddeningly, the ability to construct detailed evolutionary trees hasn't resolved this question—it's, unexpectedly, made it more complex. I'd guess one big dilemma is it's nearly impossible to test all these theories (running natural experiments in biodiversity isn't anywhere near as straightforward as it is in physics), and the four-billion-year history of life on Earth can be a bear to sort through. Plus, as Dunn notes, the distribution of species across the globe is becoming more homogenous—"pigeons for example, are as common in Mumbai as the are in London"—which is making this question harder and harder to tackle: "More pieces of a fragmented text are lost, and the narrative becomes muddier."
Meanwhile, here's Wikipedia on why anyone should care about cracking the gradient mystery: Not only would it placate a lot of curious scientists, but an answer might help us better understand how invasive species spread, how to better control disease vectors, and how biodiversity might fare on a slowly warming planet. So it's not just all fun and games...