By: Luke Morales
Rainforests are Earth’s oldest living ecosystems, consisting of tall, mostly evergreen trees and a high amount of rainfall (Johnson, 2015). Though rainforests only cover 6% of Earth’s surface, they are home to more than half of the world’s plant and animal species, making them extremely dense with flora and fauna (Johnson, 2015). In fact, a 10-square-kilometer patch can hold up to 1,500 flowering plants, 750 species of trees, 400 species of birds, and 150 species of butterflies (Johnson, 2015). A 2021 study published in Science traces the roots of modern tropical rainforests to the Chicxulub asteroid, which wiped out Earth’s dinosaurs 66 million years ago. One of these rainforests birthed was the Amazon.
The Amazon rainforest is the largest remaining tropical rainforest in the world, and it is home to several million species of insects, plants, birds, and other forms of life—many of which are still unrecorded by scientists (Britannica, 2019). In addition to being one of the most diverse places on Earth and housing many endangered species, the Amazon rainforest is critical to the health of the planet in other ways. For example, it acts as a carbon sink: it absorbs large amounts of carbon dioxide from the atmosphere to help keep Earth’s carbon concentrations in balance (National Geographic Society, 2020). In conjunction with producing about 20% of our oxygen and absorbing solar radiation, the carbon-draining rainforest helps to regulate temperatures and reduce the impact of greenhouse gas emissions worldwide, stabilizing Earth’s climate (National Geographic Society, 2020).
Modern-day rainforests are characterized by their closed canopies and their four distinct but interdependent layers: emergent, canopy, understory, and forest floor (Johnson, 2015). Prior to the Chicxulub impact, the composition and structure of rainforests differed drastically. For example, pre-Chicxulub rainforests had relatively open canopies and were subject to lower angiosperm—flowering plants—abundance and a minor presence of conifers (Carvalho et al., 2021). This implies that these rainforests may have been accompanied by slower rates of carbon fixation, transpiration, and nutrient cycling (Carvalho et al., 2021). These factors also contributed to the lower levels of plant diversity in pre-Chicxulub rainforests when compared with that of modern-day rainforests (Carvalho et al., 2021).
So, how did an asteroid that wiped out nearly 75% of all terrestrial life give life to these now-vital ecosystems? According to Mónica Carvalho, a Colombian paleobotanist, and her colleagues, the widespread extinction and decrease in plant diversity following Chicxulub’s impact allowed for angiosperms to dominate the suddenly less-populated forests over the subsequent six million years (Carvalho et al., 2021). The scientists came to this conclusion by analyzing thousands of fossil pollen samples and leaf fossils recovered from Colombia (Carvalho et al., 2021). They proposed three non-mutually exclusive explanations for why the regrowing rainforests didn’t revert to their pre-Chicxulub composition and structure.
First was the disturbance of large herbivores: with the absence of sustained trampling and extensive feeding by large herbivores, plants were unable to maintain an open canopy and now had to compete for sunlight, creating a wider variety of growth strategies (Carvalho et al., 2021). Another explanation involves soil nutrients: pre-Chicxulub forests grew on strongly weathered and infertile soils due to the extensive and stable, humid lowland environments (Carvalho et al., 2021). The ashfall from the Chicxulub impact, however, added minerals to the soil, stimulating forest productivity and the diversification of plant species (Carvalho et al., 2021). A third explanation relates to selective extinction: species with narrow ecological ranges are more susceptible to mass extinction events, leaving species with wide ecological ranges to prosper. The near extinction of conifer trees, for example, may have released resources that modern, versatile plant species could have used to grow and further diversify (Carvalho et al., 2021).
It is unimaginable that one event can have such extensive effects on ecosystems around the world. Though there is still much to learn about pre- and post-Chicxulub environments, this study uncovers the possibility that a single historical phenomenon has the potential to alter continents and species. The Chicxulub impact is one such phenomenon, with profound environmental effects on the entirety of our planet, giving birth to one of the world’s most important ecosystems and resulting in what scientists call the most diverse biome on Earth.
Luke Morales (he/him/his) is a junior pursuing a major in English writing, a minor in Portuguese, and a certificate in Latin American Studies. He is very active in university life, working as a Resident Assistant, Teaching Assistant, and Social Media Coordinator for the Luso-Brazilian Student Association. When writing nonfiction, Luke likes to write about things relating to literature; diversity, equity and inclusion; and current events. His favorite hobbies include reading, writing, and playing videogames. He plans to pursue a career in authoring fiction, creating and sharing multicultural literature with focus on the LGBTQ+ community
Carvalho, M. R., Jaramillo, C., de la Parra, F., Caballero-Rodríguez, D., Herrera, F., Wing, S., Turner, B. L., D’Apolito, C., Romero-Báez, M., Narváez, P., Martínez, C., Gutierrez, M., Labandeira, C., Bayona, G., Rueda, M., Paez-Reyes, M., Cárdenas, D., Duque, Á. Crowley, J.L., ... Silvestro, D. (2021). Extinction at the end-Cretaceous and the origin of modern Neotropical rainforests. Science, 372(6537), 63-68. https://www.doi.org/10.1126/science.abf1969
Johnson, H. J. (2015, May 11). Rainforest. National Geographic. https://www.nationalgeographic.org/encyclopedia/rain-forest/
National Geographic Society. (2020, March 26). Carbon sources and sinks. National Geographic. https://www.nationalgeographic.org/encyclopedia/carbon-sources-and-sinks/
The Editors of Encyclopaedia Britannica. (2019, December 30). Amazon rainforest. Britannica. https://www.britannica.com/place/Amazon-Rainforest