Probing Question: Are invasive plants always a threat?

If you’ve ever enjoyed the sight of a field of flowering purple loosestrife, cease and desist immediately. You are siding with a foreign enemy. While it may be pretty to gaze upon, the highly invasive Lythrum salicaria is on the National Park Service’s “Least Wanted” list for ruthlessly choking out indigenous grasses, sedges and flowering plants that are crucial sources of nutrition and habitat for wildlife in American wetlands.

field of purple flowers
Flickr user Shelly & Dave

Flowering purple loosestrife

Across the country, gardeners and ecologists are waging hand-to-hand combat with “noxious weeds” such as kudzu, Japanese honeysuckle and mile-a-minute, some of which are capable of causing serious damage to crops, habitats, livestock and people.

Are all invasive plants a threat or is there a way to live peaceably with them?

“A relatively small percentage of introduced species are truly problematic and a threat to native biodiversity,” says Tomás A. Carlo, Penn State assistant professor of biology. “Most introduced species become ‘naturalized’ and fully integrated into their new communities.”

Notes Carlo, “The label ‘invasive’ refers to plants that experience rapid population growth in new environments, have few natural controls such as insects and diseases, and can displace other higher value species from growing in the same areas. The term applies to both native and non-native species, but it is considered as more problematic, ecologically speaking, when the invasive species was introduced by humans.”

While purple loosestrife (thought to have first come to our shores as stowaways in shiploads of grain brought by European settlers) meets the criteria of an invasive plant, other foreign weeds such as chicory, dock, and dandelion pose no threat to the biodiversity of native species, says Carlo. “More than 90 percent of all introduced plants do not become invasive, and when they do so, the invasive species usually are contained by limited spatial and temporal boundaries (i.e., they are a problem in a particular time in a particular place and for a particular reason).

Says Carlo, “Because humans have altered the ecosystems so much by destroying or altering the native communities, and then by bringing so many species from abroad, most ecosystems are now composed of native and introduced species living side by side by a process of natural self-organization.”

In some instances, he notes, introduced species actually help the native species to come back to areas that have been destroyed by human activities. For example, the African Tulip Tree (Spathodea campanulata) has helped to bring back native-dominated forests to the island of Puerto Rico by competing with grasses in man-made pastures and facilitating the establishment of native tree species under their shade. Now 57 percent of the land surface of Puerto Rico is covered by native-dominated forests, while in the early 20th century forest cover amounted to just 5 percent of the land surface.

drawing illustrating cattle lying dead in a field with famers looking on, 1907.

Wikimedia Commons

“The ecological function of a species cannot be predicted by whether it has been present in an ecosystem for a hundred, a thousand, or even a million years,” Carlo stresses. “In fact, biological invasions are an important and vital natural process that all species experience at some point or another in their long ecological and evolutionary history.” The problem, he adds, is that “humans have quadrupled invasions and their potential negative effects by destroying and disturbing native plant communities worldwide, and at the same time bringing new species across ecosystems that have been out of touch for millions of years.”

Given the severe damage some plants can cause, should we focus on wiping them out? “Eradication is a double-edge sword,” says Carlo. “It is fundamentally a dangerous activity, but it is sometimes necessary to protect unique populations and species. It should only be implemented when strictly necessary, and never for the sake of a ‘nativism’ ideology that has no relationship to the ecological reality of a functioning ecosystem.”

Prejudice can exist in natural communities, just as in human ones, Carlo emphasizes. “In general, people should not feel that nature is damaged by the presence of introduced species, especially in systems that have been severely impacted by human activities. We need to more actively promote the love and care for living systems without entering in prejudiced ideologies. Introduced species are nature too!”

Tomás Carlo, Ph.D., is assistant professor of biology at Penn State. He can be reached at tac17@psu.edu.

Last Updated January 11, 2012