that is not native to an area and causes ecological and/or economic damage in their new environment. While all invasive species are exotic species (non-native species that are introduced into a new area), many exotic species never become invasive species (for example, most of our vegetable plants that we cultivate in our gardens are essentially exotic species that never become invasive).
However, many exotics species have become invasive around the world, resulting in a great deal of environmental and economic damage.
How species typically become invasive?
An exotic species can become invasive when it is introduced into a new environment that exists in a climate that is very similar to where it is native. Because a potentially invasive organism is already so well-adapted to the climate of its new environment, it can then take advantage of available niches, where it reproduces, invades, and proliferates within local ecosystems because it now has no natural predators to limit its population levels.
Due to the increased levels of global economic trade activity and travel that is occurring around the world today between regions that were once isolated from one another in the past, there has been an increase in exotic species being introduced around the world.
These exotic species introductions can be intentional, such as the Purple Loosestrife in the United States, or unintentional, such as the Emerald Ash Borer in the United States and the Norway Rat throughout most of the world.
Sources of exotic species introductions include:
- the ballast water of ships
- the intentional and unintentional release of exotic species into the environment by the public
- the pet trade
- the horticultural industry
- the aquaculture industry
Why are invasive species a problem?
With no natural predators, invasive species can proliferate and cause a lot of ecological and economic damage where they have invaded.
Invasive species can outcompete native species, kill native plants and trees, destroy habitat, and over-predate native species within a particular area, potentially throwing an entire ecosystem out of balance. Invasive species can threaten the biodiversity of an ecosystem, especially if native species are outcompeted or entirely driven to extinction.
Invasive species can also inflict a great deal of economic damage to local communities in their new environments, such as by decimating local native fish populations and destroying local fisheries, killing economically important tree species for the forestry industry, and causing great damage to crops .
When ecosystems are negatively impacted by invasive species, tourism may also suffer, such as the invasive aquatic species  that threaten the health and vitality of the Great Lake region in the United States.
Invasive species can be particularly destructive on islands, where they can outcompete or over-predate the unique native species that are found nowhere else on Earth. If native island species are decimated, they likely have no other populations that can repopulate the island, potentially driving the entire native species population to extinction.
An example of this is when the Brown Tree Snake (Boiga irregularis) was introduced by accident in Guam by US. military ships after World War II. The Brown Tree Snake reproduced and took over the entire island, over-predating the island’s native birds and lizards .
As our global natural environment continues to undergo damage and development around the world, the disturbed and deteriorating ecosystems are now becoming increasingly vulnerable to invasion by invasive species. In disturbed ecosystems, introduced invasive species can take advantage of niches that have been opened up that were previously occupied by native species.
Examples of invasive species
Examples of invasive species that have become problematic in certain regions of the world include:
- Zebra Mussels
- the Indo-Pacific Lionfish
- Common Buckthorn
- Garlic Mustard
- Asian Carp
- Spiny Water Fleas
- Sea Lampreys
- Chestnut Blight Fungus
- Water Hyacinth
- Japanese Knotweed
- Eurasian Watermillfoil
- the Emerald Ash Borer