Invasive Plant Species' Abundance Similar at Native and Introduced Sites

Many land managers know firsthand the damage invasive plant species can do to natural resources, but no one knows exactly why these species are able to outcompete native plants.

Invasive species are common throughout the world. A long-held theory developed by biologists hypothesizes that invasive plants are more numerous in introduced sites than in their native, or home, range because an ecological change occurs during their invasion that gives them an advantage over native plants. This theory is known as the abundance assumption.

An international team of scientists--called the Nutrient Network--that includes Rebecca McCulley, MS, PhD, a grassland agroecologist with the University of Kentucky College of Agriculture, tested the abundance assumption on 26 invasive plant species at 39 grassland locations on four continents. Their results were contradictory to the abundance assumption, indicating that most species (20 of 26) had similar or lower abundances at the introduced sites than at their home range sites.

McCulley and her lab members contributed two Kentucky sites to the study: a pasture at Spindletop Farm in Lexington and Hall's Prairie, a restored native tallgrass prairie in Logan County. Eight species from both sites were considered invasive and included in the study. For the most part, McCulley's observations about the invasive species in Kentucky fell in line with the international findings. However, two species from Spindletop--Kentucky bluegrass and plantain--were more common there than at their native sites.

"In Kentucky we don't consider some of the species on the list (such as Kentucky bluegrass) to be invasive. They are widespread throughout the state and have proven beneficial to our forage systems," said McCulley. "However, they aren't native to the United States, and some states do consider them to be invasive and problematic."

One species found in Kentucky, Canada thistle, is widely considered an invasive, noxious weed that threatens ecosystems throughout North America. Results from this study indicate this species tends to be less abundant in its invasive range than in its home site worldwide.

"The results suggest that it's relatively unusual for invasive plants to have a population explosion at introduced sites," McCulley said. "Instead, abundance at native sites, in most cases, can predict abundance at introduced sites."

The scientists' findings also held up across diverse climate zones. McCulley said sites in Kentucky, New Zealand, and Switzerland had as many as six shared invasive species, all with similar plant abundances.

These findings might help scientists speculate how new invasive species will behave once introduced to a foreign site.

The Nutrient Network's work was funded by a research coordination network grant from the National Science Foundation's Division of Environmental Biology. The UK College of Agriculture also helped fund McCulley's research.

The Nutrient Network is the first group to bring together international scientists to conduct grassroots level research evaluating humans' impact on ecological systems at nearly 40 grassland sites worldwide.

"The research was simple, but because of the global collaboration, the Nutrient Network will provide a new, global approach for addressing many critical ecological issues," she said. "It will tell us information we need to know about invasive species and changing climates, as well as alterations to nutrient availability."

The group's findings were published in Ecology Letters. This was the network's first published study.

Katie Pratt is an Agriculture Communications Specialist at the University of Kentucky.

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