In the 2012 computer-animated film The Lorax (based on the book of the same name by Dr. Seuss and now streaming on Peacock!) the residents of Thneedville live in a world without trees. Unbridled greed leads to deforestation and pollution, and the outside world becomes a barren wasteland. In the end, the characters restore balance to nature by heeding the words of the titular Lorax.
Today, trees and other plants are at the mercy of animals, and so need an advocate like the Lorax. In the Devonian, however, the power structure was flipped and the world needed someone to speak for everyone else or else the trees would destroy us all.
According to a recent study published in the Geological Association of America, plants may have been responsible for a series of extinction events that occurred between 419 and 358 million years ago. During the Devonian, animal life hadn’t yet made its way onto the land, but plants had, and things were about to get weird. Over the course of the Devonian, the world underwent a series of extinction events — including one of five great mass extinctions — that devastated marine ecosystems. By the time all was said and done, more than two-thirds of all species on the planet had been wiped out.
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Scientists proposed that the extinction of marine species may have been directly caused by the evolution and expansion of early terrestrial plants. The idea was that as plants spread farther on land, they developed root systems that loosened the soil and released nutrients that had previously been locked in by land. Essentially, once the roots developed, the oceans experienced an incredible influx of nutrients from the land.
You might think that extra nutrients in the water would be a net good, but plants have placed their green thumbs so firmly on the ecosystem’s shell that it nearly collapsed completely. In modern times, when nutrient supplies increase dramatically – either by natural or anthropogenic means – we see massive algal blooms that kill thousands or millions of fish in a relatively short time. The algae releases toxins into the water and as it decays and is eaten by bacteria, it depletes the water column’s oxygen and literally suffocates any animals unhappy to live near it. The process is known as eutrophication, and if it had happened on a global scale, it would very quickly have become very difficult to survive, perhaps even destroying entire species or ecosystems. At least that was the hypothesis, but researchers needed hard evidence.
Researchers at Purdue and the University of Southampton assumed that if such a nutrient spike were to occur, there would have to be some sign in the geological records that a nutrient spike was higher than background levels. They looked at geochemical data from ancient lake deposits in Greenland and northern Scotland and found exactly the signal they were looking for.
The geologic record reveals elevated nutrient levels, particularly phosphorus, at the same time plants evolved and expanded in the Devonian. Elevated nutrient levels also coincided with fossil evidence of trees, including the first species with deep root systems. In two cases, the identified nutrient influxes also corresponded to marine extinction events, including the Late Devonian mass extinction event.
The discovery could also have solved the mystery of why the Devonian had so many extinction events. Researchers found that there was a periodic nature to plant expansion at that time, and that cycle appears to be related to the wet-dry climate cycle. These fits and starts of plant expansion meant the world would go through periods of higher and lower nutrient release, triggering extinction events over and over again until things stabilized.
This surge in plant growth and evolution would produce incredible amounts of oxygen for our planet, not to mention the global carbon reserves that modern humans exploit for things like coal and oil. But before that, they redistributed food resources so dramatically that life on Earth almost ended.
Suddenly pulling weeds doesn’t feel so bad.
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