By Leah Crane

Today’s tropical rainforests came about because of the huge asteroid strike thought to have wiped out the dinosaurs.

Before the asteroid hit the Yucatán peninsula in what is now Mexico, South America’s rainforests were made up of vastly different greenery than the abundance of flowering plants they now contain.

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“If you returned to the day before the meteorite fall, the forest would have an open canopy with a lot of ferns, many conifers and dinosaurs,” says Carlos Jaramillo at the Smithsonian Tropical Research Institute in Panama. “The forest we have today is the product of one event 66 million years ago.”

Jaramillo and his colleagues analysed tens of thousands of samples of fossilised pollen and leaves found in northern South America that dated to the part of the Cretaceous period just before the asteroid hit, and just after the impact, in the Palaeocene epoch.

They found that plant diversity declined by 45 per cent after the impact and took 6 million years to recover. Insect bites on fossilised leaves showed that insect diversity also took a nosedive.

The rainforests of South America changed in the aftermath of the catastrophe. Most of the cone-bearing plants and ferns disappeared, and the rainforests became dominated by flowering plants called angiosperms. A thick canopy allowed only a little light to reach the ground.

“I think the number one lesson here is unpredictability,” says Ellen Currano at the University of Wyoming. “When you have these major perturbations, they change the rules of the whole ecosystem.”

Jaramillo and his colleagues suggest there are several reasons why the asteroid may have caused this major change. For one, the impact probably killed most of the large, herbivorous dinosaurs that once trampled down and ate the lower levels of the forests.

Plus, the ash that settled out of the sky after the impact may have served as fertiliser, creating a nutrient-rich soil that favoured fast-growing angiosperms over other plants. Angiosperms also appear to have been more ecologically diverse before the impact, which would have made it easier for some of them to bounce back afterwards.

“We love the way it ended up, this incredibly diverse, really structurally complex forest, but right now, we are living through a mass extinction caused by humans and, again, whole ecosystems are being set on a different path,” says Bonnie Jacobs at Southern Methodist University in Texas.

“In the case of the rainforest, we might like the final product, but all those animals that were alive in the Cretaceous did not,” she says.

Understanding how this major event shaped the rainforests can help us put into perspective how these biodiversity hotspots are reacting to deforestation today and how long they could take to recover, says Jaramillo.

“At some of the places we studied, I could see right in front of my eyes how this forest that has taken 66 million years to build was gone in a day, and the rate of deforestation is staggering,” he says. “We know from this study that it takes a long time to build these diverse forests back: you can’t chop down the forest and think, ‘Oh, tomorrow I’ll plant more trees.’ ”

Journal reference: Science, DOI: 10.1126/science.abf1969

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