Why Do More Species Live Near the Equator


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[MUSIC] "This episode is supported by Prudential" Here in Tambopata Peru, EO Wilson once found more species of ant in a single tree than there are in all of the British Isles. Scientists have seen this pattern all over the world. More species exist near the tropics. Why is that? [MUSIC] In one night here in the Peruvian rainforest, I saw more insects in an hour than I've seen in my entire life, total. It's not just insects. Birds, mammals, plants… tropical rainforests are some of the most biodiverse places on Earth. It's not just the abundance of it, it's how many different species we find in a given area. But why is that? It might seem obvious, or even like a silly question, but the more you think about it, the weirder it gets, because life has shown it can succeed pretty much anywhere, from the top of the highest mountains to the bottom of the ocean . But Earth's most biodiverse places are always regions like this, tropical rainforests. One reason why is maps lie to us. Rectangular projections are distortions of a sphere that make the poles look bigger and tropics look smaller than they really are. When in reality, the tropics contain about 40% of area on Earth. Unsurprisingly, larger areas usually have more species. In less than half a square kilometer in the Amazon, we can find as many tree species as we find in four million square kilometers of temperate forest. Species here are at a higher density. There must be something special about tropical ecosystems. Climate is one factor. When we look at plant fossils and where they are found in different times in Earth’s natural climate history, tropical forests are older than temperate forests, they've had more time to become rich. But while tropical regions avoided the last ice age and don't have cold winters, it's not EASY to survive here. There are dry and wet seasons, there's competition for resources, no matter what kind of organism you are, there's a lot of stuff that wants to eat you. On average the tropics are warm and they get plenty of water. This part of the planet gets more average solar radiation throughout the year. Which means that plants and the animals they support get more energy, they are more productive. But this still only explains why there's *more* life, not why so many *different* kinds of life. If you've got a pie, more people can get a slice if you cut it up into a thousand tiny ones vs just a few. We call these slices niches, the habitat and conditions that one organism needs to flourish, and here in the rainforest, there's a lot of slices. Organisms that live at higher latitudes have to be more adaptable, be able to handle lots of different conditions. One week it might be snowing. The next, flowers and fish are everywhere. Specialization is too risky, you’ve gotta be adaptable. Life is more stable in the tropics. One thing I noticed here in Peru is the sun sets a lot earlier here this time of year than what I'm used to back at home. Here near the equator there's essentially the same number of hours in day and night no matter what month it is. Let's say you're a bird that eats insects, and maybe these bats over here You've got the same number of hours to do your feeding. The birds get the day shift, the bats get the night shift. You get to split that niche evenly. That wouldn't work at higher latitudes and more temperate climates. There's simply just too much change, too much disruption for these species to keep track of. And this might explain a reason why more species coexist near the equator. The tropics are crowded, so the competition for resources is extreme. That competition drives organisms to specialize, like how we see ants or caterpillars that might be able to live on just one single type of plant. But because climate and seasons are more stable, that specialization isn’t as risky. More species, less area. These theories are really good at explaining why there's so many species in the rainforest NOW, but we're still missing the beginning of our story, where those species came from. It's possible that evolution is actually working on overdrive here near the equator, speciation, the creation of new species by various natural forces, actually happens FASTER near the equator. Each generation of living things gathers changes, mutations, some are good, some are bad, some are neither, but it isn't until those changes are passed on to the next generation that natural selection and time can do their thing . The reason that bacteria are so good at adapting is because they reproduce quickly, they have more generations in less time. The same thing happens here in the rainforest. Plants and animals grow up faster, they can have more generations. This drives competition, this is what forces plants and animals to specialize in all of the amazing ways that we've seen. This theory, that evolution happens faster near the equator, finally ties together the ideas of time, area, and energy to explain the origin of biodiversity. There's an idea that says the tropics are so well suited to the creation of new species…that it's like an engine for biodiversity Another idea says the tropics are so rich and productive, and the climate's so stable, that things don't go extinct as fast. More species are born here and species live longer here, the tropics are both a cradle AND a museum. Scientists even think that over many many years, species from places like this go and seed biodiversity throughout the rest of the world. This is why it's so important to protect the rainforest, to preserve life's cradle and museum. To keep it from being cut up, because more area means more species. To keep the climate from changing, to keep life here stable and rich. Chemists know all the elements on the periodic table. Physicists probably know all the subatomic particles that make up matter. But biology still hasn't answered one of its most basic questions: how much life IS there? EO Wilson once wrote that "unlike the rest of science, the study of biodiversity has a time limit". If species begin to go extinct faster than we can describe them, the we might never know how much life Earth has to offer. And now that I've seen something like THIS, I don't want to see that happen. Stay curious.