Study examines human transport of plant seeds

Article by: Tina Arons
Media Credit: Kat Hilsabeck
Issue date: 10/16/08 Section: La Vida

Experts say plant seeds hitch rides on our shoes like free taxis that take them to places where they may not be welcome.

But how far can they go?

Some may find the answer surprising.

European researchers studied seeds from two wild plants found along a national coastal trail in southern England and they found seeds can ride on shoes for miles.

Matthias C. Wichmann, a lead researcher at the Center for Ecology and Hydrology in Wallingford, England, said in an e-mail that few studies relate to human population density or movement with plant species distributions.

Long distance dispersal has potential for introducing species into new habitats, said Mark McGinley, a professor of biological sciences at Texas Tech.

"If you bring in a new plant, it could compete with the naturally occurring plants," he said, "and it may not be as good of a food source for animals."

Wichmann said studying plant movements may help researchers understand and potentially direct the impact of humans have on seed dispersal, including invasive-plant species and species of conservation interest.

For the study, Wichmann and his colleagues set up experiments in which participants wearing hiking shoes or rubber boots stepped into mud and then into a tray containing a specific number of seeds. Participants walked from about 3 feet to a little more than 3 miles.

Although more than half of seeds fell off their shoes within 16 or 17 feet, researchers found seeds regularly remained attached to shoes after three miles.

Wind generally disperses seeds within an 830-foot radius of their orgin, according to the study.

Scott Holaday, a Tech biological sciences professor, said plant species with seeds that travel away from the parent plant, often called "pioneer" plants, utilize water, wind and animals to travel to new places to germinate.

"They are normally strong competitors with other plants in the area," he said.

Several pioneer plants call West Texas home, Holaday said. Two of the most common are sandburs and goathead, which have sharp, spiny burs.

"These seed dispersal systems have been devised to cling to animals," he said, "and we are just another animal."

Wichmann offered a general rule of thumb: More human movement in an area equates to a greater potential for human-mediated dispersal and all of its associated effects.

Urban sites have higher potential and may often be the first place invasive species appear, but habitat suitability may be limited, he said. National parks and similar habitats often experience high visitor numbers that potentially makes them more susceptible than surrounding areas.

McGinley said human-seed dispersal occurs more easily in places without landscape maintenance.

"Many plants on campus are cut down before they make seeds," he said.

It also depends on the shoes, McGinley said. Footwear with mud in its treads is more prone to transport seeds than flat shoes.

"I think hiking boots would be a classic example," he said.

According to the study, pedestrians may disperse seeds in other ways than walking. They also may facilitate other means of transportation such as cars, boats or airplanes and thus potentially may carry seeds across much longer distances.

"The biggest problem is that we go everywhere now," Holaday said. "The animals of North America don't travel to Europe, but we do. We have the capability of moving things all over the globe, and it's only a matter of whether or not it can grow there."

Source: The Daily Toreador, Texas Tech University