Why Plants Become Invasive?

Just to be clear, not all exotics are invasive. Exotic or non-native species are considered to be invasive only when they cause harm. Of our agricultural crops 98% are not native to the regions where they are grown. Predicting which plants will become invasive is difficult. Scientists are still trying to discover why a plant can suddenly become an invasive problem after many years. The two lists here show that many of the characteristics we prize in an ornamental plant are very similar to the characteristics which increase the likelihood of a non-native plant becoming an invasive problem.

Invasive Habitat generalist Out-competes other plants Reproduces easily Abundant seeds Bird-dispersed seeds Not affected by native pests/diseases

Ornamental Hardy-easy to grow Requires little attention, "care free" Easy to propagate Abundant flowers Fruits to attract birds Disease and pest resistant

If a non-native plant is used over a long period of time, it stands a better chance of naturalizing. Extensive use in landscaping also increases the likelihood that a plant will naturalize, which is the first step to becoming an invasive problem. Many invasives start on disturbed soils and habitat disturbances such as construction, grading, plowing and mowing which can increase the vulnerability of any habitat to invasive species. Areas such as rights-of-way, fencerows, old fields, ditches and roadsides all have a high rate of disturbance.

Some non-native plants were used here for many years before they became an invasive problem. This phenomenon has been termed “The Lag Phase”. This refers to the period of time after a plant has been introduced, escapes, and naturalizes but before the population reaches the point at which it begins to increase rapidly. It may take decades before a plant becomes an invasive problem. This is one reason it is important to track populations of non-native plants which have escaped and naturalized, even though they may not actually be invasive at this time. Tracking non-native plant populations is especially important for species which have been shown to be an invasive problem in other regions, states or countries.

The graph shown above represents the lag phase of Chinese privet in the South. Control measures would have been more effective and much less costly had they been started in 1960 as the privet began its rapid spread. Early Detection & Rapid Response programs attempt to identify problem species in the early stages of infestation, so that control measures can be taken before their populations become so large that control is difficult or impossible.