Introduction

What's Being Done

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 National Invasive Species Information Center

 

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Submodule 2: Invasive Species Defined

What Makes a Species Invasive

All organisms require a niche…someplace they can live and grow and reproduce. This applies to native species as well as those that have been introduced into a new ecosystem. If introduced organisms do not find a niche to occupy in their new environment, they probably will not become established. If they do find a niche in the wild, which is generally less than 15% of all introduced species, they begin to establish and to influence their new environment. Even then, most of these alien species grow in harmony, side by side, with the native inhabitants. Unfortunately, about 1% of all introduced species wreak havoc on the ecosystems they invade.

 

Why do some species become established while others do not? And why do some grow “peacefully” in their new home while others overtake native species or destroy native habitats?

 

Most of the information scientists have gathered about invasive species has come from studying the aftermath of an infestation, once the invasive species has already become established. It would be much too risky for scientists to conduct an experiment by purposefully introducing a potentially invasive species to study invasiveness; if the experiment got out of hand, the scientists could potentially create an environmental disaster. For this reason, there is not a lot known about how and why infestations are initiated.

 

Once an introduced species becomes established, meaning it has begun to reproduce in the wild, its “invasiveness” depends partly on the characteristics of the habitat being invaded. But invasiveness also depends on how well the introduced organism adapts to its new environment. Oftentimes, invasive species are able to adjust to a wide range of environmental conditions, such as an invasive plant that thrives in either sunny and shaded conditions, or crowded or sparse surroundings. Also, there are some instances of invasive species adapting to their new environment through the process of natural selection.

 

Identifying attributes of an invasive species is difficult at best, and determining which environments are prone to invasion may be even more difficult. In addition, scientists are not sure why native species do not become invasive in their own environments, nor do they understand how some invaders have been able to damage or take over very diverse ecosystems; water hyacinth and European starlings are two examples of the latter. If scientists could identify characteristics common to invasive species or invasion-prone environments, then predictions could potentially be made about which species to prevent from admittance to the country and which ecosystems to diligently protect.

 

There are some characteristics that seem to be more common among invaded areas:

 

• They lack biotic constraints (natural predators or diseases) that the invading organism had to confront in its native environment
• They are likely to present vacant niches that can be exploited by newcomers
• Because they have vacant niches, these areas are generally lacking species diversity
• Areas invaded by plants tend to lack a multi-tiered canopy
• They are more likely to have been disturbed by fire, construction, agriculture, or other means prior to the invasion

 

 

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