Seed dispersal study shows value of conservation corridors
Ecologists study how wind moves seeds through longleaf pines
Field ecologists go to great lengths to get data. Radio collars and automatic video cameras are among their tools for documenting the natural world.
So when a group of ecologists set out to see how wind moves seeds through isolated patches of habitat carved into a longleaf pine plantation, they came up with a novel way of addressing this question. They twisted colored yarn to create mock seeds that would drift with the wind much like native seeds.
The scientists discovered that both wind and the corridors between the patches of habitat matter to seed dispersal in the longleaf pine forest.
Their experimental "seeds" were dusted with fluorescent powder and inserted into custom-made boxes mounted on poles, then released as the scientists monitored local wind conditions.
That night, the field crew returned for a black-light treasure hunt, locating more than 80 percent of the fake seeds, which glowed under the ultraviolet light.
The paths of these glowing seeds were matched with output from a computer model to produce the first accurate picture of how wind moves seeds through corridors linking two patches of habitat.
The study results are published in a paper in this week's issue of the journal Proceedings of the National Academy of Sciences (PNAS).
Conservation biologists have long discussed building conservation corridors to link isolated patches of protected land.
"Understanding the conservation impact of corridors is at the cutting edge of conservation," says lead paper author Ellen Damschen, a zoologist at the University of Wisconsin-Madison.
Corridors are designed to improve conditions for uncommon native species living in separated habitats.
Small populations in these "islands" of habitat may be killed by storms or disease. They may lack genetic diversity and be prone to inbreeding. And they may be unable to reach new habitat.
"It makes intuitive sense that these connections could foster genetic and biological diversity," says Damschen. "But there has been little scientific evidence for if and how they work."
Most of the studies have involved animals, she adds, even though plants provide the basic energy and structure to land ecosystems.
Wind matters for the movement of seeds and whole organisms, Damschen says. "In many open habitats, more than one-third of plants are wind dispersed, but there are also insects, spiders, pathogens and fungi that move on the wind."
The experiment, supported by the National Science Foundation (NSF) and the U.S. Forest Service, began in 2000 with the creation of eight groups of patches at the Savannah River Site, a large holding of the U.S. Department of Energy. Each set of patches was built at a different orientation to prevailing winds.
"Relatively few researchers have investigated the effects of habitat configuration on wind-dispersed species," says Betsy Von Holle, a program director in NSF's Division of Environmental Biology, which funded the research. "This study demonstrates that influences on wind-dispersed species are more complex than previously thought."
A research group of meteorologists and ecologists found that corridors increased the movement of wind and of their glowing artificial seeds, echoing the results of a computer model developed by Gil Bohrer at The Ohio State University, a paper co-author.
And when Damschen and colleagues counted newly dispersed plants over the 12-year experiment, they found that a corridor linking two patches of land indeed promotes the diversity of plants dispersed by wind - especially if the corridor is oriented roughly parallel to the prevailing winds.
Both the data and the model showed that wind speeds up in certain areas of the patches, and that a strong vertical air movement is present.
"Uplift is important because the wind tends to be faster higher above the ground," Damschen says, "and uplift can lead to long-distance dispersal, which is significant for moving plants around the landscape."
That's why the study matters for conservation biology, Damschen says.
"We predicted that corridors in line with the dominant winds would move more species, and this is what we found. Wind alignment matters for species diversity in conservation areas."
The results are especially relevant to threatened Midwestern ecosystems like grasslands, prairies and savannas, where big bluestem and milkweed are two of many native plants that loft their seeds on the wind.
"In conservation science, it is often assumed that wind-dispersed seeds can go everywhere, but that's not true," Damschen says.
"Wind direction, and the shape of the habitat, control where these seeds go.
"While this adds another factor to consider in management of natural areas, the information is on the table so we can make better decisions about how to achieve management goals."
Other co-authors of the paper are: Dirk Baker of the University of Wisconsin-Madison; Ran Nathan of The Hebrew University of Jerusalem; John Orrock of the University of Wisconsin-Madison; Jay Turner of Washington University in St. Louis; Lars Brudvig of Michigan State University; Nick Haddad of North Carolina State University; Doug Levey of the University of Florida, Gainesville; and Joshua Tewksbury of the University of Washington.
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