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

This is not just a Kentucky problem, as invasive species are common throughout the world. A long-held theory, developed by biologists, hypothesizes that invasive plants are more numerous in introduced sites compared to 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 that included University of Kentucky College of Agriculture’s Rebecca McCulley tested the abundance assumption on 26 invasive plant species at 39 grassland locations on four continents. Their results found that most species (20 of 26) had similar or lower abundances at the introduced sites compared to their home range sites.

McCulley and her lab members contributed two 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, the invasive species from Kentucky fell in line with the international findings. However, two species from Spindletop, Kentucky bluegrass and plantain, were more common here than their native sites.

Different viewpoints

McCulley added that some species in the study, like Kentucky bluegrass, aren’t necessarily considered invasive by everyone, but were chosen because they were considered invasive by enough people to make a country’s invasive species list.

“In Kentucky, we don’t consider some of the species on the list to be invasive. They are widespread throughout the state and have proven beneficial to our forage systems,” said McCulley, a grassland agroecologist. “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.”

In addition, the scientists’ findings held up across diverse climate zones. McCulley said sites in Kentucky, New Zealand and Switzerland had as many as six shared species, all with similar plant abundances.

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

The group, known as the Nutrient Network, 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.