How plants invade and become established in a new environment is not well understood. Scientists have made important discoveries but continue to look for answers that can help in the prevention and management of plant invasions. Understanding how both native and nonnative plants function in the environment, along with the basics of the invasion process, provides a foundation from which management strategies may be developed.
PLANT TRAITS AND COMMUMNITY CHARACTERISTICS
The combination of biological traits of a plant species and the characteristics of a plant community may help explain how and why some invasions occur.
Plant species biological traits
- grow rapidly
- produce seeds early in the growing season
- produce massive amounts of seeds or propagate vegetatively
- produce chemicals that prohibit the growth of other plant species
- adapt to a wide range of environmental conditions
Plant community characteristics
- absence of animals and insects that normally feed on the plant in its native range
- disturbance of vegetation and soil caused by natural factors (fire, flooding) and by humans (land clearing, land degradation)
- availability of environmental resources (nutrients, water, sunlight)
- the number, distribution, and species of native plants
THE INVASION PROCESS
Some scientists view the invasion process in three phrases (1, 2). This process is presented conceptually below and may not apply to all invasive plant species in all environments. Not all nonnative plants become invasive. Many that are accidentally or intentionally introduced do not survive in the new environment, and of the ones that do survive, few become invasive.
The number of invading plants in an area is small during the introduction phase and may not be detected by managers. Plant numbers may not increase for a long time. This period is referred to as the “lag time.”
Environmental factors play an important role in whether or not the invading plants survive and become established. A plant community with more resources such as nutrients, water, and sunlight will provide a more suitable habitat for new plants than an area where resources are limited.
After the introduction phase and a lag time, an explosive period of expansion may occur. During the colonization phase, the invading plants are more easily detected by managers.
Expansion during this phase is thought to be more the result of plant species’ biological traits (such as the ability to propagate vegetatively or grow rapidly) than environmental factors.
Diagram of the three phases of the invasion process showing the change in the number of invading plants over time (Adapted from 1).
When an invading plant species establishes a new self-perpetuating population and is integrated into the plant community, it is considered “naturalized.” During this phase, expansion has slowed and the population is relatively stable.
Environmental factors, such as a decrease in resources, may contribute to slowing down the expansion. If resources are made available by changes in the environment, such as land-use activities that alter an adjacent native plant community, the population may continue to expand.
APPLYING MANAGEMENT TO THE INVASION PROCESS
Understanding which phase in the invasion process a plant population is in provides information that is helpful for developing management strategies.
Preventing an invading plant species from moving into a new area is the most effective form of management.
Many invading plant species have a long lag time following their introduction. Early detection and rapid response (EDRR) is key to eradicating the plants before their population expands.
Once a population has reached the colonization phase, it is very difficult to eradicate. Management efforts should focus on controlling and containing the expanding population before it becomes naturalized.
Naturalized populations are often too widespread and difficult to manage. Restoration may be considered for high value areas that are relatively small.
- Cousens R, Mortimer M. 1995. Dynamics of Weed Populations. New York (NY): Cambridge University Press. 332 p.
- Radosevich S. Online textbook chapter 2: Plant population biology and the invasion process. Center for Invasive Plant Management. <http://www.weedcenter.org/textbook/>. Accessed 2007 May 30.
- Hobbs RJ, Humphries SE. 1995. An integrated approach to the ecology and management of plant invasions. Conservation Biology 9(4): 761-770.