Invasive species are non-native species of animals, plants, microorganisms or pathogens that usurp the habitat of other species, forcing the native species to decline in population or to disappear from their natural environment. Invasive species are introduced either accidentally or intentionally by human beings or their activities. It is important to differentiate invasive species from introduced species. Introduced species, which have been introduced to an area, are not considered a threat to the new environment, whereas invasive species are always considered to be pests and harmful to the ecosystem in some way.

Invasive species tend to be highly competitive, highly adaptive and highly successful at reproducing. The lack of natural predators that normally keep an invasive species population in check is a key factor in a particular species' success, which in turn is devastating to the ecosystem that they come to inhabit. Many of the invasive species that have become common problems for people or the environment were introduced by way of ships. Unwittingly to humans, these species hitched a ride on the bottom of ships, in cargo or in ballast water. Some invasive species were introduced by explorers and travelers as pets or garden plants, or for other purposes such as pest control. In some instances, invasive species were introduced to control other invasive species.

It may come as a surprise to many that certain animals, such as the domestic cat, are considered to be invasive species. The domestic cat, native to northeast Africa, was introduced from Egypt to countries worldwide thousands of years ago. Although it is often a much loved house pet, the domestic cat is also an avid hunter that is very detrimental to native wildlife.

The domestic cat is an invasive species so common that it inhabits all but a few islands worldwide. The domestic cat was first introduced to non-native areas of the world to control rodent populations. To this day, the cat is still beneficial in keeping other invasive species under control, as well as an important companion for many people. However, the domestic cat is capable of spreading disease to wildlife and humans, and it is responsible for the extinction of many species of birds, which is why it is now considered an invasive species.

While the domestic cat arguably provides some benefits for humans and helps to control other invasive species populations, many other invasive species only cause harm or destruction to the ecosystem into which they are introduced. It is estimated that half of all the species on the endangered species list - 57% of animals and 39% of plants - are declining at least in part due to invasive species. Invasive species cost the United States an estimated 128 billion US dollars (USD) annually and 400 billion USD worldwide due to economic losses such as damage to crops. Much of this money is also spent on the prevention of invasions, education about the consequences of invasive species and control measures.