You may notice that when you're visiting the United States or going to Australia for your vacation that customs has very strict regulations about what type of food, plants, and animals you are allowed to bring with you. If you're wondering why that is, the reason is that nations need to protect their natural environments from the threat of invasive species. This article will explain what invasive species are and why they are such a danger.

An invasive species is any type of living thing, whether it be animal, plant, bug, or fungus, that is not native to a particular area. When a species is not native, it needs to shoehorn its way into the local biosphere, which can be akin to slicing your way through a crowd. Invasive species often take over the food supply or habitats of native species, pushing them out and putting them in danger of extinction if measures are not taken to curb the invasion.

Invasive species can cause big problems for biologists and local wildlife and can even change the composition of the local biosphere forever. Because invasive species often don't have any natural predators in the new area, they may reproduce so rapidly that it is impossible to stop them before the takeover is complete. This happened with zebra mussels in the Great Lakes, which filtered the murky waters and made them clear, which was great for the value of properties but bad for fish trying to hide.

Similar invasive species horror stories occurred in Europe in the Dark Ages, when the spread of the invasive black rat species through ships coming from tropical Asia caused the Black Death. It was also carried by rabbits, who were brought over in the 18th Century and ravaged crops and wildlife, necessitating nation-long fences to be built to curb their spread.

Invasive species are almost always introduced to an area where they don't belong by human action. It could be direct action, like the person who deliberately introduced rabbits to Australia, or an accident, like tracking mussels into the Lakes on the hull of your ship. For this reason, you may be asked to clean your shoes and let customs inspect your crates of abstract art for hidden plant or animal matter. Additionally, you should always declare any organic matter and let them decide if it's dangerous.