Invasive woody plants are a major problem in forests worldwide. Invasive trees and shrubs are often fast-growing with light, wind-dispersed seed or bird-carried fruit. They are able to quickly colonize disturbed sites and may prevent native species from germinating or growing. Here in central Kentucky, almost every forest patch near urban areas is so choked with bush honeysuckle that native species are not able to regenerate.
Buy why are invasive species so prevalent? Conventional wisdom is that North America was a biological island before European settlement, and species introduced from Europe and Asia are able to establish themselves here and create conditions that allow them to perpetuate themselves.
An alternative view is that human activities do two things – bring new species into an area, and create the ecological conditions for those species to thrive. Sometimes this is easy to see. Royal paulownia (Paulownia tomentosa) and tree-of-heaven (Ailanthus altissima) are invasive Asian trees in eastern North America, but they are almost never found in mature forests. Instead, they quickly colonize areas that are disturbed by fire, plowing, or other human activities.
Joshua King and Walter Tschinkel of Florida State University have done experiments with the exotic fire ant, Solenopsis invicta, comparing the success of this invasive species in disturbed (mowed or plowed) and undisturbed forest habitat. They found that plowing reduced the abundance and diversity of native ants and allowed fire ants to become established. In the undisturbed forest, fire ants were unable to invade the habitat of native ants.
These experiments show that fire ants are ‘passengers’ rather than ‘drivers’ of ecological change. King and Tschinkel say that fire ants and other invasive species should be described as ‘disturbance specialists’.
If their results can be generalized, land managers may want to pay more attention to disturbance than to invasive species. Invasive species have proved difficult or impossible to control in many habitats. It may be better for land managers to focus on minimizing the kinds of disturbances that favor disturbance specialists.
Pictures:
- Paulownia tomentosa, an invasive tree, Pennsylvania Department of Conservation and Natural Resources – Forestry Archive, , Bugwood.org
- Red imported fire ant, Solenopsis invicta, Russ Ottens, University of Georgia, Bugwood.org
Articles
- Fire-Ant Invasions Are Ecological Karma, Brandon Keim, Wired Science
- King, JR, WR Tschinkel 2008 Experimental evidence that human impacts drive fire ant invasions and ecological change. PNAS DOI: 10.1073/pnas.0809423105
{ 2 comments… read them below or add one }
Tom, I’m wondering how might this apply to those trying to switch to organic farming? Many tell me they have used herbicides in the past to remove or control invasive plant species.
Carrie -
Organic farming presents a lot of challenges for control of invasives, but the answer lies in good management of natives. Native species can often outcompete invasives with the right conditions. For example, undisturbed native grasslands that are periodically burned are remarkably resistant to invasion. As this post discusses, we need to learn to avoid the conditions that allow invasives to be successful. Unfortunately, most of our land management does the opposite.
For organic farmers, the solution probably lies in good cultivation practices, and maintaining ground cover all the time. It is unfortunate that no-till is usually synonymous with herbicide use, but it doesn’t have to be, as Rodale has shown.