The same asteroid impact that wiped off the dinosaurs likely allowed the rainforests of today to evolve, a new paper explains.
The end of the Cretaceous Period is marked by a hugely violent event: the extinction of the dinosaurs at the hands of Chicxulub. That story has been threaded and re-threaded, but now, researchers are exploring a new possible consequence of the impact. According to a new paper, this event allowed the lush forests that currently cover Earth’s tropical regions to form.
Chaos is a ladder
“Our study follows a simple question: How do tropical rainforests evolve?” Monica Carvalho, botanist and lead author of the study, said in a press release. “The lesson learned here is that under rapid disturbances —geologically speaking— tropical ecosystems do not just bounce back; they are replaced, and the process takes a really long time.”
The team, working at the Smithsonian Tropical Research Institute (STRI) in Panama, wanted to understand how tropical rainforests behave after significant ecological disturbances. The Chicxulub impact presented an ideal case study for this scenario; it is estimated that the event wiped out around 45% of all the plants in the area it hit (today’s Mexico), and had a profound effect on the entire globe beyond that.
For the study, the researchers analyzed plant fossils recovered from several sites in Central and South America. All in all, they looked at over 50,000 fossil pollen specimens and 6,000 fossilized leaves, from both before and after the impact.
A comparison between these showed that the impact spurred a transition from relatively sparse, conifer-dominated rainforests into the dense, tall canopies we’re used to seeing today. The massive destruction of plant life opened up whole ecological niches, which were further filled with other lineages that we see growing around tropical trees today, such as lianas and orchids.
An interesting effect of this transition was a shift towards taller, more stratified rainforests. Pre-impact rainforests was quite open, meaning light could reach all the way down to the forest floor. Trees didn’t have to compete for light, so they didn’t need to grow so tall. Today, rainforest canopies are very crowded and block most incoming light from reaching the forest floor. Thus, modern rainforests show significant vertical stratification as various species of plants scramble to grow as tall as they can and capture as much light as possible. This competition, however, also breeds evolution — so today’s rainforests are much more biodiverse than their previous incarnation.
But how exactly does an asteroid impact help forests evolve? The team believes that, apart from the impact opening up ecological niches, the absence of dinosaurs also helped. These animals likely kept the rainforests of yore ‘open’ by feeding on plants and physically destroying them as they moved about. The ash generated by the impact also likely helped in fertilizing tropical soils, which favored fast-growing plants. Finally, this event also broke the hegemony of conifers, allowing flowering plants to take over.
The paper “Extinction at the end-Cretaceous and the origin of modern Neotropical rainforests” has been published in the journal Science.
