Life with latitude: The great puzzle of biogeography

Mention the tropics and you’ll likely imagine vast rainforests teeming with wildlife. With over 300 species of tree per hectare in Panama (there’s less than half of that in the whole of Canada!) and more than 1800 species of birds in Colombia, it’s clear the tropics are bursting at the seams with plant and animal life.

Yet look to the north or south and—whilst the natural world is still breathtaking—you can’t help but notice that the closer to the poles you approach, the less species there are about. Why this occurs is a puzzle to modern science, but there is no shortage of weird and wonderful theories attempting to explain the great mystery of the latitudinal diversity gradient.

The Area Hypothesis

heatmap of wildlife diversity across the globe
Source: Mannion et al. (2014), based on work by Clinton Jenkins.

The tropics are the largest biome (regions of the world with similar climate, animals and plants), and a bigger area can support more species. Plus, if you have a larger territory then there is less chance of a species going extinct. Over time, this might result in many more species in the tropics than the smaller biomes closer to the poles.

The Energy Hypothesis

Photo by Gabby Pangilinan
Photo by Gabby Pangilinan

It’s hotter in the tropics because of greater solar energy. With more intense levels of sunlight, comes greater photosynthesis. This in turn might support more individuals. Finally, when a species is composed of a higher number of individuals, then there is a lower chance of going extinct. Over time, this too might result in more species in the tropics than elsewhere.

The Harsh Climate Hypothesis

Photo by Ryan Pohanic

Where would you rather live? At the North Pole or on a tropical beach? Maybe it’s the same for many of the world’s species. Perhaps it’s just harder for species to physically tolerate the conditions at higher latitudes. Though recently, scientists have poked a lot of holes in this theory.

The Historical Disturbance Hypothesis

Photo by Ahmed Radwan
Photo by Ahmed Radwan

If you were to look back over time, then higher latitudes are a rather risky place to live. Every few hundred thousand years, billions of tons of ice come thundering down from the poles as we move in and out of ice ages. Some scientists think that this regular disturbance means that species have had less chance to evolve and adapt in temperate areas, compared to the relatively tranquil and consistent tropics.

Perhaps the answer lies in one of these theories, perhaps it is several of them working together. Or maybe the cause is something else entirely. All that we can say with any confidence is that the answer is still out there awaiting discovery. In the meantime, there’s plenty of amazing biodiversity to keep us wondering.