If you were to go back and look at the United States 1,000 years ago, the number and types of animal and plant species you would find would be quite different than they are today. The change started with the arrival of Europeans to the North American continent. Some of the species they brought with themselves initially included farm animals like horses, chickens, cattle, pigs and even honey bees. None of these animals existed in North American prior to the arrival of Europeans.
And none of these species would be considered to be particularly invasive either. That's because none of these species spreads with any special rapidity. And none of them, with the possible exception of feral pigs in places like Texas, is particularly hard to control. In fact, cows and chickens have some trouble surviving without human help.
One of the first invasive species that the Europeans brought with them are brown rats, which arrived in the United States sometime in the middle of the 1700s. And rats fit the definition as an invasive species. Rats are a non-native species that spreads rapidly, causes a great deal of damage and are extremely hard to control. There could be as many as 100 million brown rats living in New York City alone. There really isn't a way to do a definitive census, so no one knows for sure.
Amazingly, there are invasive species infesting the United States today that are even more troublesome than brown rats. Let's look at three of the most interesting to get a sense of how invasive a species can be.
One of the most surprising is the Asian Carp. You may have never heard of this one if you don't live along certain rivers in the Midwest, but if you do, the spread of the Asian Carp is spectacular. There are millions of them, and they have a propensity toward leaping out of the water as boats go by in numbers that boggle the mind. You can find videos of Asian Carp on YouTube that show stunning numbers of fish, like this one:
The problem is that the fish are big (20 pounds or more is common), they eat a lot and they reproduce rapidly, so they kill off all native fish. One possibility is to harvest them for food, but people in the United States do not tend to associate any kind of carp with fine dining:
Another invasive species is the quagga mussel – a small freshwater shell fish about the size of a quarter. To get an idea of the problem quagga mussels create, take a look at the size of Lake Michigan on a map of the United States. Compare Lake Michigan's size to the size of a state like of Massachusetts or Connecticut. Now imagine the floor of Lake Michigan – the whole thing – covered in a dense layer of quagga mussels. The mussels filter all of the beneficial food algae out of the water, excrete it as feces and in doing so promote the growth of other forms of algae that can create a stinking mess on shore. At the same time, with the beneficial algae gone, the number of fish in the lake has plummeted. Mussels like the quagga have a tendency to clog pipes for things like municipal water systems as well.
It is also possible for an invasive species to be a plant, and kudzu is a great example. If you live in the southeastern United States, you cannot miss kudzu because it is a vine that is very hard to stop. In the summer, kudzu vines grow as much as a foot per day. You can almost see Kudzu growing if you sit and watch it. Because it grows so fast and has very dense foliage, Kudzu can grow right over trees and smother them. Kudzu will also grow over things like houses, power lines, etc. and cause a great deal of damage. It has very deep, bulbous roots that make kudzu very difficult to eliminate from a piece of land.
There are many other invasive species in the United States including things like starlings, fire ants and even pythons that are proliferating in the Everglades. Efforts to control them have so far proven to be fairly futile. In each case we hope that science can figure out some way to control their spread, or that the market place to create some kind of widespread demand for them so they come under predatory pressure from humans.