Arlington (Bernd F. Laeschke – December 2010): Mass extinction events can be triggered by an influx of invasive species. They can stop the dominant natural process of new species formation, according to research results published in the journal PLoS ONE. The study of the collapse of Earth's marine life 378 to 375 million years ago suggests that the planet's current ecosystems, which are struggling with biodiversity loss, could meet a similar fate.
Although Earth has experienced five major mass extinction events, the environmental crash during the Late Devonian time period was unlike any other in the planet's history. The actual number of extinctions wasn't higher than the natural rate of species loss, but very few new species arose. "We refer to the Late Devonian as a mass extinction, but it was actually a biodiversity crisis," said Alycia Stigall, a scientist at Ohio University and author of the paper.
"This research significantly contributes to our understanding of species invasions from a deep-time perspective," said Lisa Boush, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research. "The knowledge is critical to determining the cause and extent of mass extinctions through time, especially the five biggest biodiversity crises in the history of life on Earth. It provides an important perspective on our current biodiversity crises."
The research suggests that the typical method by which new species originate was absent during this ancient phase of Earth's history, and could be to blame for the mass extinction. It occurs when a population becomes geographically divided by a natural, long-term event, such as the formation of a mountain range or a new river channel, and evolves into different species. New species also can originate through dispersal, which occurs when a subset of a population moves to a new location.
Small, shelled marine animals called Leptodesma (Leiopteria), Floweria and Schizophoria (Schizophoria), and Archaeostraca were some of the most common inhabitants of the Late Devonian oceans, which had the most extensive reef system in Earth's history. The seas also teemed with huge predatory fish such as Dunkleosteus, and smaller life forms such as trilobites and crinoids (sea lilies).
The first forests and terrestrial ecosystems appeared during this time; amphibians began to walk on land. As sea levels rose and the continents closed in to form connected land masses, however, some species gained access to environments they hadn't inhabited before. The hardiest of these invasive species that could thrive on a variety of food sources and in new climates became dominant, wiping out more locally adapted species.
The invasive species were so prolific at the time that it became difficult for many new species to arise. "The main mode of speciation that occurs in the geological record is shut down during the Devonian," said Stigall. "It just stops in its tracks."
Of the species studied, most lost substantial diversity during the Late Devonian, and one, Floweria, became extinct. The entire marine ecosystem suffered a major collapse. Reef-forming corals were decimated and reefs did not appear on Earth again for 100 million years. The giant fishes, trilobites, sponges and brachiopods also declined dramatically, while organisms on land had much higher survival rates.
The study is relevant for the current biodiversity crisis, as human activity has introduced a high number of invasive species into new ecosystems, Stigall says. In addition, the modern extinction rate exceeds the rate of ancient extinction events, including the event that wiped out the dinosaurs 65 million years ago.
"Even if you can stop habitat loss, the fact that we've moved all these invasive species around the planet will take a long time to recover from because the high level of invasions has suppressed the speciation rate substantially," Stigall said.