Betsy Von Holle talks with

Betsy Von Holle talks with and answers a few questions about this month’s Emerging Research Front paper in the field of Environment/Ecology.

Why do you think your paper is highly cited?

Prior to our study, scientists believed that a primary reason for success of nonnative species in a given habitat was the diversity of the community of the recipient habitat. This is the idea of “biotic resistance” to invasion, which was first introduced by Charles Elton in 1958 and was the most popular hypothesis tested regarding invasion success into natural areas when we initiated our study in the 1990s.

We decided to test multiple hypotheses simultaneously to see if the reason for success for nonnative species was the resident diversity, the environmental conditions of the site, or the number of individuals introduced to that site, which is known as propagule pressure. We found that propagule pressure overwhelmed biotic and environmental resistance to invasion. This was a surprising result, but a number of other authors working in other systems have since found similar results.

Does it describe a new discovery, methodology, or synthesis of knowledge?

“I am intrigued by how species are responding to climate change.”

This experiment was the first to simultaneously test three different hypotheses that had been suggested in the literature to be important to the success of nonnative species in a natural area: resident diversity, environmental conditions, and propagule pressure.

Would you summarize the significance of your paper in layman’s terms?

We found that biological invasion into our natural area was best predicted by the number of nonnative species and number of individuals of each introduced into that site, rather than the level of resistance produced by the native community members, which can be thought of the competitors, predators or parasites “ganging up” on that new species, or whether the environmental conditions are best suited to those nonnative invaders. Subsequent research by several scientists confirms that our result is relevant to other systems.

How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes that you’ve encountered along the way?

I first started studying biological invasions by working as an intern at the Smithsonian, studying the number of aquatic species being introduced through the ballast water of cargo ships. At that time huge volumes of water full of nonnative organisms from ports from other countries were being pumped into our ports. It was at this point that I realized how important propagule pressure could be to invasive success.

It was a real challenge to find the perfect site for this experiment. We needed to find a site that had a rich native community, which was regularly disturbed. We looked for sites throughout the southeastern US and found a naturally flooding stream in southwestern Virginia.

Where do you see your research leading in the future?

I am intrigued by how species are responding to climate change. Climate change has been different in subtropical and tropical systems as compared to mid- to northern latitudes, and so I’m interested in gauging how subtropical and tropical species respond to climate change, given that their environmental tolerances are lower and that the environmental conditions are different than in the heavily studied northern latitudes.

Do you foresee any social or political implications for your research?

We found that propagule pressure, or the number of individuals introduced into an ecosystem, was the overwhelming determinant of nonnative species success in natural areas, greater than the resistance of the native community to the invader, or even the environmental conditions of that habitat.

The policy implication of this finding is that as we increase our imports of organisms from other countries, both accidental and intentional introductions will increase over time. Our work also suggests that management of introduced species should target propagule pressure—that is, measures to cut down on the number of individuals of potentially invasive species traveling by various routes, such as ballast water.

Betsy Von Holle, Ph.D.
Assistant Professor
University of Central Florida
Department of Biology

Article: Ecological resistance to biological invasion overwhelmed by propagule pressure
Authors: Von Holle, B;Simberloff, D
Journal: ECOLOGY, 86 (12): 3212-3218, DEC 2005
Addresses: Harvard Univ, POB 68, Petersham, MA 01366 USA.
Univ Tennessee, Dept Ecol & Evolutionary Biol, Knoxville, TN 37996 USA.

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