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What is an Invasive Species?


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Any species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health. These invasive species aggressively compete with and displace the associated flora and fauna communities.

This definition is based on the definition of invasive species as written in Executive Order 13112 and clarified in "Invasive Species Definition Clarification and Guidance White Paper", submitted by the Definitions Subcommittee of the Invasive Species Advisory Committee (ISAC) and approved by ISAC April 27, 2006. Accordingly, only a small portion of non-native species are invasive as many nonnative species do not cause harm.

Where Do They Come From?

Many invasive species which are a problem in the United States originated in areas of South America, Asia, Europe, Australia, and Africa that have similar climates and comparable food sources.

How Did They Get Here?

Some species were introduced accidently in packing material, as crop contaminants, in shipping containers, etc. Additionally, many species have been brought here deliberately as ornamental plants, exotic pets, etc. and they became invasive years or decades later.

Why Do Species Become Invasive?

As mentioned above, 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 species will become invasive is difficult. Researchers are still trying to discover why a species can suddenly become an invasive problem immediately upon introduction or only 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 introduced repeatedly 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 invasive plants 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 species 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 an introduced 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 species populations is especially important for species which have been shown to be an invasive problem in other regions, states, or countries.

There are some examples of invasives that became so after few or even a single introduction, such as kudzu bug. Kudzu bug quickly became established after either a gravid female or eggs from one female came over from Japan to Georgia in 2009. This invasive insect quickly spread across the southeastern US and was found in five states by 2012.

The graph below was developed (based on figures in Priorities for Australia's Biosecurity System by Craik W, Palmer D, & Sheldrake R. 2017 and Geburzi JC & McCarthy ML 2017) to include a few different concepts about invasive species spread. As the area occupied increases, the goal changes from eradication to containment and then eventually to protection. The protection stage is when the spread is so wide that only specific, high-value areas will have management activities. The two bars under the graph describe the stages of invasion and the economic returns. The economic returns illustrates the amount of money that is returned based on the money put in at each step. In prevention, every dollar spent returns $100 whereas every dollar spent in the Protection phase only returns upto $5.

Why Should I care about Invasive Species?

Below are several links which each address the question "Why should I care about invasive species?" from different perspectives.