[PCA] How Invasive Species Stop New Life

Mon Jan 3 10:22:16 CST 2011

What Triggers Mass Extinctions? Study Shows How Invasive Species Stop New
Life

ScienceDaily (Dec. 31, 2010) - An influx of invasive species can stop the
dominant natural process of new species formation and trigger mass
extinction events, according to research results published December 29 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 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 PLoS ONE 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--vicariance--was absent during this ancient phase of Earth's
history, and could be to blame for the mass extinction.

Vicariance 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.

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 one bivalve, Leptodesma (Leiopteria), and two brachiopods,
Floweria and Schizophoria (Schizophoria), as well as a predatory crustacean,
Archaeostraca. These small, shelled marine animals 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 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 this 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 Stigall 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, Stigall said, as
human activity has introduced a high number of invasive species into new
ecosystems.

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.

Maintaining Earth's ecosystems, she suggests, would be helped by focusing
efforts and resources on protection of new species generation. "The more we
know about this process," Stigall said, "the more we will understand how to
best preserve biodiversity."

The research was also funded by the American Chemical Society and Ohio
University.

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily
staff) from materials provided by  <http://www.nsf.gov> National Science
Foundation.

  _____  

Journal Reference:

Anna Stepanova, Alycia L. Stigall. Invasive Species and Biodiversity Crises:
Testing the Link in the Late Devonian. PLoS ONE, 2010; 5 (12): e15584 DOI: 

Disclaimer: Views expressed in this article do not necessarily reflect those
of ScienceDaily or its staff.

The ocean in Devonian times. A 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.
(Credit: University of Michigan Museum of Paleontology)

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