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Tropical rainforests are known for being super biologically diverse – they’re full of different species, from colorful birds and insects to plants and fungi. We haven’t even come close to cataloguing everything that’s there.

Hosted by: Olivia Gordon
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Sources:
http://www.ucmp.berkeley.edu/paleozoic/paleozoic.php
http://www.brown.edu/Research/Sax_Research_Lab/Documents/PDFs/Evolution%20and%20latinitudinal%20diversity.pdf
http://www2.hawaii.edu/~khayes/Journal_Club/spring2007/Hillebrand_2004_Amer_nat.pdf
https://people.clas.ufl.edu/gillooly/files/25_Allen_etal_2006b.pdf
[Intro music]

Tropical rain forests are known for being super biologically diverse.

They're full of different species From colorful birds and insects to plants and fungi. We haven't even come close to cataloging everything that's there.

If your a biologist heading for the arctic tundra though... well... I hope you love lichen. This pattern of lots of biodiversity at the equator, but less as you move towards the poles is called the, latitudinal diversity gradient.

It's true both the northern and southern hemispheres, warm and cold blooded animals, land and ocean ecosystems, and all up and down the food chain. According to the fossil record, this gradient has existed to some degree since the Paleozoic Era around 540 million years ago. When life was first colonizing land.

Scientists have proposed a lot of different explanations over the years, but they can be grouped into two main hypotheses. First up is the, Time and Area hypothesis. The main idea is that on the scale of geologic time, Tropical habits are older and have more land area than temperate and polar habits.

Which means there's been more time and space for lots of species to accumulate. In the early Cenozoic Era, about 66 million years ago, after the downfall of dinosaurs, a tropical-like climate extended as far north as what's now London. The warm climate of the region around the equator has stayed pretty much undisturbed since then.

Which has left plenty of time to accumulate diversity. Over lots of generations organisms in one species can build up random DNA mutations that happen to help them survive better in their environment. Like a fur color that helps baby animals hide from predators, or a sweeter nectar that helps a plant attract more pollinators.

Eventually, with enough of these helpful traits called adaptations, one species can split into two, then more. All of them thriving in different ways. Temperate latitudes, on the other hand, dealt with multiple waves of glaciers during the Pleistocene epoch around 2 million years ago.

Which made survival hard and limited life's ability to get a foothold. In fact, a lot of modern day temperate species are actually descended from tropical species, which colonized temperate areas as glaciers retreated. The Time and Area hypothesis gets some support from the fact that there does seem to be a trend of bigger and older ecosystems around the world having more diversity, but there are problems with this idea too.

For instance, the fossil record shows that relatively few of today's tropical species actually date back to the beginning of the Cenozoic Era. Even in the tropics, some estimates say the average species only sticks around for 15 million years or so, before going extinct. So the latitudinal diversity gradient can't just be the result of tropical ecosystems being stable for a long time.

Something else must be going on too. That is where the Diversification Rate hypothesis comes in. Some scientists think that new species actually arise more quickly and go extinct more slowly in the tropics.

Studies from the world of paleontology mostly support the idea that diversification rates are faster in the tropics. But we're not exactly sure how this happens. It's possible that the stable mild climate allows species to specialize.

If they don't have to expend a ton of energy to survive things like extreme weather or lack of nutrients, they can adapt to specialized niches in their environment. Or maybe there's lots of interaction between species speeding up co-evolution, the process where two species affect the course of each other's evolution. When one species has to share its environment with another, they compete to survive.

So over lots of generations both species will keep coming up with new adaptations, and maybe even spin off new species in the process. And then there's the idea that higher temperatures, like those near the equator, actually affect an organism's metabolism. This might speed up the pace of evolution by promoting shorter generation times and faster DNA mutation rates, leading to more organisms with different traits and possibly new species.

The details here are still a bit murky, but there's studies showing that controlling for other factors like how big their habitats are, some tiny organisms called protists evolve faster into more species in tropical waters than in arctic waters. Any way you look at it, biologists and ecologists have been speculating about this pattern for a very long time. The real story probably involves some combination of these ideas, or something we haven't thought of yet.

For now though the latitudinal diversity gradient remains one of the great unsolved questions of natural history. Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show you can go to Patreon.com/scishow and don't forget to go to youtube.com/scishow, and subscribe.