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In Brief: The textbook definition of an Invasive Species generally runs along the lines of:
“An Invasive Species is a non-native (or ‘exotic’) plant or animal that is detrimental to the native members of its adopted habitat.”
Like all textbook definitions however, this only sketches the outline of the issue at hand. If we want to color the picture in, shade it, and eventually stick it to the refrigerator we’ll need to look a little closer at the subject.
Native / Exotic / Invasive / ect: Unfortunately, as with many topics, the nomenclature of Invasive Species work is rather important to achieving a solid understanding of the state of things. Native species are those which have a long evolutionary history in a given habitat. More importantly they generally predate the arrival of humans to a location.
Exotic species are those which do not have that long evolutionary history in an area, they are transplants from another habitat entirely; generally relocated — either purposefully or accidentally — by humans. As a rule however, the vast majority of exotic species do not thrive in their adopted homes, perhaps in the location they are native to they relied on specialized pollinators or seed transporters in order to spread, or perhaps their new environment lacks enough food for them to compete successfully.
On the other hand some exotics do happen to be well adapted to novel habitats, biologists call these species Invasive, a subcategory of Exotic.
All Invasive Species are Exotic, not all Exotic Species are Invasive.
As mentioned above, Invasive species fit in just fine in their new habitats. More than fine in fact. They fit in superbly. Perhaps due to a lack of predators, a lack of herbivores, or a surfeit of nutrients, species categorized as invasive are able to out-compete their native neighbors with ease.
Harmful Repercussions: After the definitions have been laid out, the next step towards understanding the topic is explaining exactly what is meant by “detrimental.” Unfortunately for brevity, Invasive Species effect their new homes in a wide variety of ways. Lets look first at the most common.
In cases where Invasive plant species have distinct advantages over their native counterparts one possible end-stage is that the Invasive plant in question will simply overrun its native neighbors, creating a monoculture stand. This can cause problems up and down the food chain as plants and animals that relied on a diverse community of plants are stressed by the removal of the area’s diversity. Plants like Japanese Barberry, Kudzu, Honeysuckle, and Fennel are often guilty of this. All four are capable of developing large thickets where nothing else is able to grow, effectively eliminating most of an area’s plant community.