(CN) — Scientists may have solved a 200-year-old mystery: Why is biodiversity — that is, a wide variety of different species — greater in the tropics, near the equator, where the earth is the hottest?
"One of the largest-scale and longest-known patterns in ecology is the fact that we have more species in the tropics than at the poles," said Erin Saupe, an associate professor of paleobiology at the University of Oxford. "Scientists have been trying to understand why we have more species in the tropics for centuries."
The answer appears to be a familiar one: climate change.
Saupe is the lead author of a new paper, published Wednesday in the journal "Nature," that argues the biodiversity bonanza began 15 million years ago, when cooling of the earth accelerated.
"Life hasn’t been always distributed in the same places as it is today, and there have been dramatic changes over earth history," said Saupe. "And climate has dictated those changes, prompted those changes. That will continue into the future."
Saupe and two other researchers spent two and a half years compiling a database of nearly half a million entries on planktonic foraminifera. The unicellular marine plankton have been around some 170 million years and appear in oceans all over the world, including near the north and south poles. Their hard outer shells mean they've left behind an exceptional fossil record going back more than 60 million years, making them ideal for this kind of research, enabling the researchers to note when and where on earth they appeared.
What Saupe and her team found was that 20 to 40 million years ago, the diversity of planktonic foraminifera was lower than it is today at the equator, and was far more evenly distributed. That began to change around 30 million years ago, and the change accelerated at around 15 million years, as the rate at which earth was cooling went up a gear. The team's hypothesis is that this cooling gave the ocean's temperature a wider range at different depths near the tropics.
"At low latitudes, you have a lot more temperatures represented in the water column," said Saupe, "high temperatures at the surface, colder temperature lower down. That wasn’t always the case." That, she hypothesizes, gave species different environments to live in, and evolve into. Cooling sea temperatures at higher latitude may have caused other species to die out, making biodiversity at the tropics even greater by comparison.
Exactly why the earth cooled 15 million years ago is another mystery for another day.
Though the earth was a very different place back then — there were no humans, and far less ice — it's hard not to think about what this shift might mean for us today.
"There’s ample evidence to suggest that if anthropogenic warming continues, then the tropics are going to get very warm, and that will potentially exceed species thermal tolerance," said Saupe. "To lose species is sad, obviously, in terms of the beauty and wonder and appreciation that biodiversity brings to the world. But also, we rely on biodiversity for our medicine, our food, our infrastructure, our resources. If we start to lose species, that might have knock-on effects for our ability to persist on this earth."
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