Invasive species present a great and baffling problem for those concerned with the health and usefulness of our ecosystems, stemming both from a lack of understanding of what traits tend to make a species invasive and an inability to contain them. This paper will seek to explicate the scale of the invasive species problem, discuss the merits of different methods of containment and eradication, and predict how the problem may affect our species in the long run.
Prior to human development (and specifically that of trade), the introduction of a foreign species to an ecosystem was extremely rare, but could have great consequences. When the continents of North and South America collided _____, allowing North American predatory mammals, such as the saber-toothed cat, smilodon, to hunt several species of large South American animals to extinction. Today, however, thanks in large part to our global trade, invasion events are common, and their consequences can still be disastrous.
Often, foreign species are introduced to an ecosystem on purpose with a certain goal in mind. Kudzu, for instance, was brought to the Southern US with the intent of preventing road erosion. Unfortunately, Kudzu turned out to be devastating to the local flora, spreading rapidly at its expense, and capable of swallowing whole fields.
Another common path, or vector, for an invasive species to reach a foreign ecosystem is that of trade. A perfect example of this is the Japanese long-horned beetle, which was introduced to the Eastern US in the 1990's in a shipment of lumber. The beetles live in holes that they burrow into the trees, and the only way to kill them is by cutting the tree down and burning it. As is commonly the case with an invasive species, the effect is made more drastic by the fact that the local species being affected have not evolved to defend themselves from the beetle's assault like the trees in Japan have. In all, the US loses $137 Billion per year from the effects of and costs associated with invasive species.
Another country that has faced great pressures from invasive species is Australia. There, importation of species that were commonplace in Europe has lead to population explosions several times. Perhaps the best known example of this is the introduction of rabbits, which destroy land by eating plants that prevent erosion. Their population eventually reached about 600 million. The list of invasive species in Australia is a long one, and includes toads, camels, water buffalo, goats and deer, as well as several plant species.
There are several methods for controlling invasive species. The simplest is mechanical control, which involves physically removing the species. In the case of plants this may require pulling the roots up, a process which takes long periods of human labor, making it costly. In the case of animals mechanical control usually means hunting. In Australia, camels are sometimes shot from helicopter to control their numbers. Mechanical control is very reliable, but also very costly.
Chemical control is also a commonly used method, but it can be associated with environmental issues. It usually involves spraying herbicide or pesticide. The problem with chemical controls is their tendency to negatively affect the plants or animals living in proximity to the target. There is also the possibility that chemicals used to control invasive species could seep into the water table and pollute human supplies.
The last method of control is biological, in which another new species is released to control the population of the invasive species. Biological control has the potential to take care of the problem without any human effort beyond the initial release. Unfortunately, with this possibility for smashing success comes the chance for unforeseen effects. The extent of these unforeseen affects can be is understood through the efforts that have been made to contain Spotted Knapweed in the US. The plant was introduced in the 1980's and has a tendency to spread rapidly at the expense of local biodiversity. Recently an insect that kills Knapweed by laying eggs in the plant's seeds was released in an attempt to decrease the plant's range. While the insects did this, they also provided a quality source of food held above the snow for mice to access at a time when food would normally be scarce. This has resulted in a significant increase in the mouse population which would usually be controlled by lack of food in the winter. The increased population in turn has lead to an increased risk of hantavirus to humans, and stories like this are not unusual.
In Australia, the Cane Toad was released to control the population of the native cane beetle. Unfortunately, the toads ate not just the cane beetles but also a number of other native species. They also produce a toxin that is poisonous to the majority of predators, which has prevented natural control of their numbers. Having been released in the 1930's the toad's range is still expanding with some toads moving as fast as a kilometer per day. In the case of Lantana, an invasive perennial in Australia, 28 different foreign species have been released in an effort to control it over the last 80 years.
There is no indication that the problem with invasive species is going to get better any time soon. Even with increased awareness, it is extremely difficult to eradicate an invasive species once it has gained a foothold. Future technological improvements such as nanotechnology may allow us clean areas effectively without spending many man-hours in the process, but this is unquestionably many years away. It is also possible that in the long run, the affected ecosystems will stabilize to deal with their new inhabitants. Predators in Australia, for instance, could evolve to deal with the Cane Toad's toxin, but this would take a long time. As much as this would make things easier for us, we cannot count an ecosystems ability to sustain itself without risking total collapse. Thus our path for the future must involve continued control in traditional methods, being mindful to prevent the release of new species and weighing the pros and cons of our control efforts on the rest of the ecosystem.
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