Invasive Alien Plant Species Threaten Native Plants and Butterflies

Colorado State University researchers have observed that native plant communities in Rocky Mountain National Park that are being invaded by alien plant species also seem to attract many nectar-drinking butterflies.

While the link remains speculative, Sara Simonson, a research associated with Colorado State’s Natural Resources Ecology Laboratory, said, "We were surprised to find that the areas richest in butterflies and native plants were the same areas invaded by weedy, alien plant species."

Simonson presented the findings to the 85th annual meeting of the Ecological Society of America in Snowbird, Utah, this week.

Simonson said that many factors could account for the success of invasive alien plant species (hereafter called "invasive species") in the richest habitats: moisture, light, interactions with other species and so on. But the possibility cannot be ignored that nectar-drinking butterflies are drawn to the flowers of invasive species and, in the process of feeding, pollinate the invasive species.

"At clusters of alien thistles, we observed high concentrations of insects (principally butterflies and bees)," Simonson said. "We believe they are attracted to the fragrant flowers, but we don’t know what the actual effects are."

Most nectar-feeding butterflies observed on the flowers of invasive species are generalists (feeding from a variety of plant species), but some are more specialized, including rare, threatened or endangered species. A variety of invasive species produce flowers that are attractive to insects, she said.

The principal examples of invasive species in the study area included Canada thistle, musk thistle and leafy spurge.

Counts were conducted in montane habitats between 8,000 and 9,000 feet on the eastern slope of Rocky Mountain National Park June through August of 1996-1998.

Simonson worked with co-investigators Geneva Chong and Tom Stohlgren. Simonson and Chong are research associates at the Natural Resource Ecology Laboratory, an interdisciplinary research unit in Colorado State’s College of Natural Sciences, and Stohlgren is a senior scientist in the laboratory and Colorado State affiliate faculty member. Both Stohlgren and Chong are ecologists with the U.S. Geological Survey.

Simonson discussed several possible outcomes to the preliminary findings:

• If butterflies are attracted to flowers of invasive species, the plants could be pollinated more often.
• With increased pollination, the invasive species could have a reproductive advantage and crowd out native plant species.
• If insects favor invasive species, native plant species may receive inadequate pollination.
• These circumstances could in turn affect the butterflies themselves.

While most butterflies will feed from a wide variety of nectar-producing plants, in the larval stage many feed only on a few species of native plants. Loss of native plants could mean less opportunity for the butterflies to reproduce successfully. This could cause particular problems for rare or endangered butterfly species.

To help resource managers make decisions for control of invasive plant species, Simonson said that further research is needed to determine how effective generalist species are in pollinating invasive species, what effects this has on neighboring native plant species and the potential implications for butterflies and other native pollinators.