When most people think of catastrophic global extinction events, they imagine giant fireballs from outer space or supervolcanoes spewing lava across the landscape. However, new research reveals that some of the most devastating extinction events in our planet’s history were probably the work of a far less violent culprit: tree roots.
In a new study, researchers sought to investigate the causes of multiple population collapses that occurred during the Devonian period, some 419 to 358 million years ago. It was during this era that the first plants began to colonize the land, although the expansion of these early terrestrial life forms coincided with a series of marine extinction events that wiped out almost 70 percent of the world’s aquatic organisms.
Previous research has suggested that the emergence of the first plants resulted in a significant decrease in phosphorous levels across the terrestrial landscapes of Eurasia. This has led to the theory that early tree roots may have liberated this key nutrient from the landscape by breaking down rocks.
All life on Earth requires phosphorous, and the evolution of roots would therefore have allowed for unprecedented plant growth. At the same time, however, phosphorous released by dead and decaying plant material would have been dumped into the ancient oceans in huge amounts, with profound effects on marine ecosystems.
“Our analysis shows that the evolution of tree roots likely flooded past oceans with excess nutrients, causing massive algae growth,” explained study author Gabriel Filippelli in a statement. “These rapid and destructive algae blooms would have depleted most of the oceans’ oxygen, triggering catastrophic mass extinction events.”
Known as eutrophication, this sequence of events is often observed in lakes and rivers when excess fertilizers or other anthropogenic sources of nutrients pollute the water.
To identify ancient examples of eutrophication, the researchers examined geochemical records from five Devonian lake deposits across Greenland and Scotland. As expected, their results indicated a massive depletion in terrestrial phosphorous at various timepoints throughout the Devonian period. Importantly, the dates of these fluctuations correlated with the ages of woody plant fossils, suggesting that the emergence of rooted trees was indeed the cause of this nutrient export.
Even more significantly, the study authors found that these changes often coincided with mass extinction events. For instance, two notable increases in aquatic phosphorous levels were found to correlate with the two major waves of the Late Devonian extinction, which resulted in the loss of 40 percent of marine families and 60 percent of genera.
A closer look at the data indicated that the export of phosphorous into the ocean occurred on a cyclical basis and matched up with regional climate fluctuations. More specifically, the shift was amplified during wetter periods, as humid conditions allowed for more plant growth and therefore resulted in more phosphorous run-off.
Commenting on the significance of this data, Filippelli explained that “these new insights into the catastrophic results of natural events in the ancient world may serve as a warning about the consequences of similar conditions arising from human activity today.”
The study is published in the journal GSA Bulletin.