Weed Surprises Scientists Studying Population Extinction

Released: 25-Mar-2005 4:20 PM EST
Source Newsroom: University of Vermont
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Citations Proceedings of the National Academy of Sciences "Extinction Dynamics in Experimental Metapopulations" (28-Feb-2005)

Newswise — When Jane Molofsky arrived as a botany faculty member at the University of Vermont in 1995, she brought more from North Carolina's Duke University than her Ph.D. " something that she worked with for four years and even kept safe during her ensuing two years as visiting research fellow at Princeton University.

Molofsky brought to Vermont several hundred seeds of a weed whose life cycle is just two and a half months and whose seeds practically explode from the plant and scatter a good distance.

"They're not a nasty weed. They're not invasive," Molofsky defends herself.

She's right.

Pennsylvania bittercress is a North American native whose delicate, white, four-petal flowers begin blooming this month of March in the low wet woodlands in places around the 35th parallel such as Missouri and Kentucky. As the plant's range stretches farther north, its bloom gradually reaches into July. The young plants of this mustard relative can be eaten raw or cooked as greens. But that's certainly not what Molofsky had in mind.

"I knew I was going to experiment with them some day," says the UVM associate professor.

Molofsky carefully arranged pots of Cardamine pensylvanica under lights in a portion of the "growth chambers" that cover nearly 250 square feet of the Marsh Life Science building basement level. Here she set out to test the general tenet that local populations that are connected to each other persist longer than do isolated ones.

"We looked at how migration could prevent extinction," says Molofsky, "What is interesting is that in an experimental situation we know what causes extinction, whereas in a wild population it could be several factors."

Plants, as well as animals migrate, and to mimic migration, Molofsky varied the distances of her experimental populations to see how far they could successfully throw their seeds and start new generations of plants. If she expected anything, it was that the further the distance, the greater the chance of extinction.

But after three years and 16 generations of the bittercress, Molofsky's National Science Foundation-funded research turned up a few surprises.

"What we found was pretty cool, actually. The relation between extinction and migration is nonlinear," Molofsky says. "That means, it's like Goldilocks strategy " only instead of too hot and too cold " patches too close together exchange many migrants and form one big population. Patches too far apart don't receive enough migrants to sustain populations. But those at a middle distance are, well, just right.

"At a certain migration distance there's a sudden steep threshold; when you cross that threshold, the chance of extinction dramatically increases," she explains.

This experiment relates to the field situations, because conservation biologists have long argued that it was important to create "corridors" among local populations to help migration of endangered species. This study reinforces that theory, but shows that accurate measurements of migration rates are imperative to stay below the threshold distance.

Molofsky claims that this study, which is outside her main research areas of invasive species biology and plant community organization, is of little interest to anyone besides population ecologists.

The prestigious journal Proceedings of the National Academy of the United States of America (PNAS) begs to differ. PNAS published the work of Molofsky and her colleague Jean-Baptiste Ferdy on Feb. 28.

"When you see extinctions in the natural world, you don't know why they occurred. When we study them in the lab we can understand why populations go extinct," says Molofsky. Studies such as Molofkly's inform conservation biologists' management techniques as they try to increase endangered species' chances for successful survival.

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Link to PNAS research:
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