Why species diversity increases toward the tropics

Why are there so many more kinds of animals in the tropics? Picture the variety of frogs, insects, and snakes in a tropical region compared with life near a pole. The opening makes that observation more precise: compare the same kind of habitat at different latitudes, and species numbers rise as we move toward the equator. Researchers think undiscovered tropical species would make the difference even larger.

I can imagine explanations already. Maybe warmth helps. Maybe more sunlight supports more life. But those are guesses. Let's see which explanation holds up.

The time theory says tropical species have had more uninterrupted time to emerge. Ice ages repeatedly disrupted other regions, while the tropics escaped those interruptions. Okay, that could explain why one region accumulated more species. But then comes “However”: some temperate regions experienced less disruption than others, and arctic conditions weren't interrupted. So the history of disruption doesn't neatly match the difference we're trying to explain. We have a proposed explanation and a reason to question it.

The species-energy hypothesis takes the sunlight idea and breaks it into steps: more solar energy, more growth and reproduction, more living matter, and more species. Slow down enough to distinguish those last two. “Biomass” means the amount of living matter, not the number of different kinds of living things.

Now the author challenges the connections. Rapid reproduction needn't leave much biomass if organisms also die rapidly. And lots of biomass needn't mean lots of species. The proposed link between more energy, larger populations, and less local extinction is also untested. This is our second explanation, and the author isn't satisfied with it either.

The third explanation focuses on a stable climate and reliable resources. If your food supply is dependable, a narrow diet is less of a problem. Different species can also share resources more easily. That helps us understand how several species could live together. But where did those species come from in the first place? Paragraph 4 says local processes do not create a region’s supply of species. Keeping existing species together doesn't explain why a tropical region has so many different species available to begin with.

Then we finally get an endorsement: the fourth hypothesis is “most plausible.” That matters. We aren't finishing with four equally attractive possibilities.

This explanation concerns how quickly new species form and how quickly species disappear. If new species emerge faster in the tropics and extinction doesn't cancel that advantage, the tropical total builds up over time. We don't need to assume identical extinction rates everywhere. We need the extra species being formed to survive.

The final paragraph explains how that could happen. Imagine a small group becoming separated from the rest of its population. Over time, it adapts differently to its local conditions and eventually can no longer interbreed with the original population. Now we have a new species. But that process requires survival: a group that dies out first won't get there. Tropical subgroups are more likely than arctic ones to survive long enough.

That's the answer to the question we started with. We can return for the details of each theory when needed, but our basic map should be clear: an observed pattern, three explanations with problems, and a fourth explanation the author favors.