Invasive species play role in mass extinctions

The arrival of invasive species can prevent the formation of new species and help to trigger mass extinctions, according to a study published Thursday.
The Great Barrier Reef off the Australian coast. A mass extinction in the Late Devonian period 375 million years ago led to the disappearance of the world's reef systems for 100 million years.

The arrival of invasive species can prevent the formation of new species and help to trigger mass extinctions, according to a study published Thursday.

That conclusion is based on the study of one of the five major mass extinctions that have occurred in Earth's history — the Late Devonian collapse of marine life that occurred 378 million to 375 million years ago.

That mass extinction was unique in that very few new species arose in that period.

"We refer to the Late Devonian as a mass extinction, but it was actually a biodiversity crisis," said Ohio University scientist Alycia Stigall, author of the paper which appears in the journal PLoS ONE.

Stigall studied four types of small-shelled marine animals — one bivalve, two brachiopods, and a predatory crustacean.

Seas teeming with marine life

These were all common in Late Devonian oceans at a time when the world's waters were teeming with extensive reef systems, and a tremendous variety of marine life ranging from huge predatory fish like the 10-metre-long Dunkleosteus to small trilobites.

But this was a time when sea levels rose, allowing some species to access new environments. Some of those species were so numerous and so dominant that new species found it difficult to gain a foothold in the newly competitive oceans.

"The main mode of speciation that occurs in the geological record is shut down during the Devonian," Stigall said in a statement. "It just stops in its tracks."

Typically, new species arise by a process know as vicariance — where new species arise when some members of a population move to a new area or when a population becomes geographically separated by the formation of new mountains or rivers.

During the Late Devonian, vicariance was largely absent and could be to blame for the mass extinction, Stigall argues.

Three of the four species she studied lost much of their diversity during the Late Devonian and one of the brachiopods became extinct.

The marine ecosystem of that era saw a complete collapse, with reefs disappearing from the world's oceans for 100 million years.

Lessons for today?

Stigall believes her study has relevance for the current biodiversity crisis, as human activity has brought many invasive species into new surroundings.   

"If the Late Devonian is an accurate analog for modern ecosystems, modern human-mediated species invasions should result in a similar long term diversity decline due to preferential survival of broadly adapted invasive species, extinction of geographically restricted ecological specialists, and suppression of vicariant speciation of new species," she writes.

Her research was funded by the National Science Foundation, the American Chemical Society and Ohio University.