Invasive species are defined as, "a non-native (alien) species whose introduction to an area causes harm (environmental, economic or health)". Species get introduced to new regions usually due to human activity, either intentionally or accidentally.
Some particularly well known examples of invasive species:
The poisonous Cane Toad (Bufo marinus) was deliberately introduced to Australia in 1935 as a biological control for sugarcane beetle pests. Well, the toads didnít help the beetle problem and are now a pest themselves. Theyíve spread throughout the northeast of the continent and are expanding their range every year. These toads prey on smaller animals and poison the larger animals that could otherwise control their numbers. Due to the isolated and unique ecosystems of Australia, invasive species are a particularly severe problem there. Besides the Cane Toad, wild cats and rabbits have also had a devastating effect on the Australian landscape.
These mollusks (Dreissena species) are becoming a wide-spread problem throughout the Great Lakes region and other North American river systems. They originated in Europe and hitchhiked to North America in boat ballast water. Zebra mussels are filter-feeders and have greatly reduced the amount of zooplankton and algae living in effected bodies of water. This may result in more attractive clear water, but it is wreaking havoc on the aquatic food chain. Though their ecological impact is significant, these mussels are better known for their economic damage. Thick layers of shellfish clog intake pipes, piers and boat propellers.
Comb jellies (Mnemiopsis leidyi) are native to the Atlantic coastlines of North and South America. These tiny jellyfish relatives are less than a centimetre in length (on average) and yet manage to have a large impact on areas where they have been introduced. The most severe case is in the Black Sea, where comb jellies arrived in the late 70s, likely in ship ballast water. Most of the native fish in the Black Sea have been wiped out due to these plankton-eating jellies. Recently, another species of comb jelly (Beroe ovata) has arrived in the Black Sea. This second jelly is a predator of the first, and scientists are hoping that some balance will be restored to the ecosystem. Only time will tell.
This is another example of a deliberate species introduction that got out of hand. The Nile Perch (Lates niloticus) is a fresh-water fish native to the Nile region of Africa. Growing up to 6 ft in length, these fish have great commercial and food value. This is why they were brought in 1954 to Lake Victoria, the second largest freshwater lake in the world. The perch thrived, and subsequently eliminated hundreds of other native species (mostly cichlids). Today, about 80% of the fish in Lake Victoria are Nile Perch. Though it has decimated the natural fish populations, the Nile Perch has been quite the commercial success after all.
Even plants can be invasive problems. Purple Loosestrife ( Lythrum salicaria) is a very attractive plant, and hard to picture as a pest plant. It came from Europe as a medicinal herb, and possibly as seed in ship ballast. Loosestrife has thrived and spread throughout US and Canadian wetlands. These plants produce a huge number of seeds, and can clog a wetland area within a few seasons. They have thick underground roots, which make it very difficult to clear the plants out of an infected area.
Asian Longhorn Beetle
Anoplophora glabripennis is a large beetle native to China, that came to North America in wood product shipments, likely in the early 90s. The larvae bore deeply into the trunks of many deciduous trees (particularly maple, willow and poplar) causing considerable damage to the trees. Over a period of 3-5 years, the trees will eventually die. Asian Longhorn Beetles are just as comfortable living in urban environments as in wooded areas, and they are well established in New York, Chicago and Toronto.
Even with these few examples, you can see how a momentís carelessness can have devastating results.