Research Communications

What triggers mass extinctions? Study shows how invasive species stop new life 


ATHENS, Ohio (Jan. 3, 2011) – An influx of invasive species can stop the dominant natural process of new species formation and trigger mass extinction events, according to Ohio University research 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 ecosystem, which is struggling with biodiversity loss, could meet a similar fate.

Although Earth has experienced five major mass extinction events, scientists have observed that the environmental crash during the Late Devonian 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 interval as a mass extinction, but it’s actually a biodiversity crisis,” said study author Alycia Stigall, an associate professor of geological sciences and a member of the university’s Ohio Center for Ecology and Evolutionary Studies.
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Alycia Stigall. Photo credit: Rick Fatica, Ohio University. (Download a high-resolution version of this photo.)


Stigall’s research suggests that the typical method by which new species originate, vicariance, was absent during this ancient phase of Earth’s history, and could be to blame for the mass extinction.

Vicariance occurs when an ancestral 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 of living organisms moves to a new location.

In a departure from previous studies, Stigall used phylogenetic analysis, which draws on an understanding of the tree of evolutionary relationships to examine how individual speciation events occurred. She focused on three mainly North American clades: one bivalve subgenus, Leptodesma (Leiopteria), and two brachiopod clades, Floweria and Schizophoria (Schizophoria), as well as a subclass, Archaeostraca, a clade of predatory crustaceans.

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Stigall studied (clockwise from top left) a crustacean, a bivalve and two types of brachiopods. (Download a high-resolution image of the fossils here.)

These small, shelled marine animals were some of the most common inhabitants of the Late Devonian oceans, which featured the most extensive reef system in Earth’s history. The seas teemed with huge predatory fish such as Dunkleosteus, as well as 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, Stigall explained. 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 this time, Stigall noted, 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. It just stops in its tracks,” said Stigall, whose research was funded by the National Science Foundation, the American Chemical Society’s Petroleum Research Fund and Ohio University.

Of the four clades Stigall studied, most lost substantial diversity during the Late Devonian event, 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.

Stigall’s study is relevant for the current biodiversity crisis, she argued, as human activity has introduced a high number of invasive species into new ecosystems—and has destroyed natural habitats—across the globe. 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 of these invasive species around the planet will take a long time to recover from, as the high level of invasions has suppressed the speciation rate substantially,” Stigall said.

Stigall acknowledged that it may be too late to curb habitat loss and the spread of invasive species, but noted that those concerned about maintaining Earth’s dynamic ecosystems might focus efforts and resources on protection of new species generation.

“The more we know about this process,” she said, “the more we may understand how to best preserve biodiversity.”

Contacts: Alycia Stigall, (740) 593-0393, stigall@ohio.edu; Director of Research Communications Andrea Gibson, (740) 597-2166, gibsona@ohio.edu.