Invasive species are also called alien species. These are life forms that are not indigenous or naturally present in a biome or habitat. They can be introduced by natural causes, such as floods, winds or forced migration during natural disasters. Humans introduce invasive species by importing exotic plants along with insects and microbes that come in with any imported goods. When exotic or unusual pets are brought in, then are released or escape into the environment, they can form whole colonies.
The most important problems with invasive species happens when the native species have no ability to fend of predation. When the native life forms have no natural defenses, their populations can be forced to migrate or can be quickly decimated.
Invasive aquatic plants can be introduced by ships that carry them in their bilges and can grow aggressively to clog waterways when they have no natural predators to keep their growth in check. With such over growth, the oxygen levels in the water can become so low that aquatic animals and other submerging life forms cannot get enough oxygen to survive or reproduce.
Feral pigs are a growing problem in the United States. They combine the aggression and biting power of a wolf with the reproductive frequency of a rabbit, will eat anything in sight and have no known natural predators except for humans. When various species of pigs, such as peccaries and feral hogs were introduced for pleasure hunting and other reasons, they replaced the calmer and more manageable domestic pigs and became an overwhelming problem in many places. These pigs are known to cause all forms of damage, from plant and sapling destruction to disruption of soil, wetland water cycles and domestic animal safety.
The Australian Eucalyptus was introduced to California as a promising feedstock. The animals hated Eucalyptus, but the trees were out of control by the time that the problem was revealed. Eucalyptus grows phenomenally, contains high amounts of oil, sheds barks and leaves in huge amounts, and creates one of the worst fire hazards possible, especially in hilly or inaccessible areas. Still, this human introduced invasive species has become one of the most beloved species of tree in the state.
Kudzu was brought in from Japan in the late 1800s, to become "The Plant That Ate The South". Yet this plant has since proved to have so many benefits and uses that it has been a good example of adapting uses for an invasive species in combination with eradication where the species creates problems.
The Pike is a voracious fish that will eat any other type of fish, including Alaskan Salmon, trout and other species that are part of the industry and food supply for whole regions. Despite many states that have made introduction of alien species illegal, the illegal introduction of Pike has occurred in lakes throughout the Western and Northwestern United states, decimating populations of native fish and in one case, requiring repeated poisonings of Lake Davis.
In summary, whether natural or anthropogenic, the introduction of new and invasive, or alien species will continue to increase throughout the world, creating more and more need to either eradicate, to introduce natural predators for that species, or to adapt and to find uses for the species.