By
definition, a species is considered "introduced" when its transport into an area outside of its native range is human mediated. Introductions by humans can be described as either intentional or accidental. Intentional introductions have been motivated by individuals or groups who either: (1) believe that the newly introduced species will be in some way beneficial to humans in its new location or, (2) species are introduced intentionally but with no regard to the potential impact. Unintentional or accidental introductions are most often a byproduct of human movements and are thus unbound to human motivations. Subsequent range expansion of introduced species may or may not involve human activity. '' introduced worldwide from its place of origin (
Mesopotamia)
Intentional introductions Species that humans intentionally transport to new regions can subsequently become successfully established in two ways. In the first case, organisms are purposely released for establishment in the wild. It is sometimes difficult to predict whether a species will become established upon release, and if not initially successful, humans have made repeated introductions to improve the probability that the species will survive and eventually reproduce in the wild. In these cases, it is clear that the introduction is directly facilitated by human desires. In the second case, species intentionally transported into a new region may escape from captive or cultivated populations and subsequently establish independent breeding populations. Escaped organisms are included in this category because their initial transport to a new region is human motivated. The widespread phenomena of intentional introduction has also been described as
biological globalization.
Positive Introductions Although most introduced species have negative impacts on the ecosystems they enter into, there are still some species that have affected the ecosystem in a positive way. For example, in New Hampshire invasive plants can provide some benefits to some species. Invasive species such as autumn olive, oriental bittersweet, and honeysuckle produce fruit that is used by a handful of fruit-eating bird species. The invasive plants can also be a source of pollen and nectar for many insects, such as bees. These invasive plants were able to help their ecosystem thriving, and increase the native animal's chances of survival. Several introduced exotic trees served as nest sites for resident waterbird species in Udaipur city, India.
Motivations for intentional introductions Economic Perhaps the most common motivation for introducing a species into a new place is that of economic gain. Non-native species can become such a common part of an environment, culture, and even diet that little thought is given to their geographic origin. For example,
soybeans,
kiwi fruit,
wheat,
honey bees, and all
livestock except the
American bison and the
turkey are non-native species to North America. Collectively, non-native crops and livestock account for 98% of US food. These and other benefits from non-natives are so vast that, according to the Congressional Research Service, they probably exceed the costs. Other examples of species introduced for the purposes of benefiting
agriculture,
aquaculture or other economic activities are widespread.
Eurasian carp was first introduced to the United States as a potential food source. The
apple snail was released in Southeast Asia with the intent that it be used as a protein source, and subsequently to places like Hawaii to establish a food industry. In Alaska,
foxes were introduced to many islands to create new populations for the fur trade. About twenty species of African and European
dung beetles have established themselves in Australia after deliberate introduction by the
Australian Dung Beetle Project in an effort to reduce the impact of livestock manure. The timber industry promoted the introduction of Monterey pine (
Pinus radiata) from California to Australia and New Zealand as a commercial timber crop. These examples represent only a small subsample of species that have been moved by humans for economic interests. The rise in the use of
genetically modified organisms has added another potential economic advantage to introducing new/modified species into different environments. Companies such as
Monsanto that earn much of their profit through the selling of genetically modified seeds has added to the controversy surrounding introduced species. The effect of genetically modified organisms varies from organism to organism and is still being researched today, however, the rise of genetically modified organisms has added complexity to the conversations surrounding introduced species.
Human enjoyment Introductions have also been important in supporting recreation activities or otherwise increasing human enjoyment. Numerous fish and
game animals have been introduced for the purposes of sport fishing and hunting. The introduced amphibian (
Ambystoma tigrinum) that threatens the endemic California salamander (
A. californiense) was introduced to California as a source of bait for fishermen.
Pet animals have also been frequently transported into new areas by humans, and their escapes have resulted in several introductions, such as feral
cats,
parrots, and
pond slider.
Lophura nycthemera (
silver pheasant), a native of East Asia, has been introduced into parts of Europe for ornamental reasons. Many plants have been introduced with the intent of aesthetically improving public recreation areas or private properties. The introduced
Norway maple for example occupies a prominent status in many of Canada's parks. The transport of
ornamental plants for
landscaping use has and continues to be a source of many introductions. Some of these species have escaped horticultural control and become invasive. Notable examples include
water hyacinth,
salt cedar, and
purple loosestrife. In other cases, species have been translocated for reasons of "cultural nostalgia", which refers to instances in which humans who have migrated to new regions have intentionally brought with them familiar organisms. Famous examples include the introduction of
common starlings to North America by the American
Eugene Schieffelin, a lover of the works of Shakespeare and the chairman of the
American Acclimatization Society, who, it is rumoured, wanted to introduce all of the birds mentioned in Shakespeare's plays into the United States. He deliberately released eighty starlings into Central Park in New York City in 1890, and another forty in 1891. Yet another prominent example of an introduced species that became invasive is the
European rabbit in Australia.
Thomas Austin, a British landowner, had rabbits released on his estate in
Victoria because he missed hunting them. A more recent example is the introduction of the
common wall lizard (
Podarcis muralis) to North America by a
Cincinnati boy, George Rau, around 1950 after a family vacation to Italy.
Addressing environmental problems Intentional introductions have also been undertaken with the aim of ameliorating environmental problems. A number of fast spreading plants such as
kudzu have been introduced as a means of
erosion control. Other species have been introduced as
biological control agents to control
invasive species. This involves the purposeful introduction of a
natural enemy of the target species with the intention of reducing its numbers or controlling its spread. A special case of introduction is the reintroduction of a species that has become locally endangered or extinct, done in the interests of conservation. Examples of successful reintroductions include wolves to
Yellowstone National Park in the U.S., and the
red kite to parts of England and Scotland. Introductions or translocations of species have also been proposed in the interest of
genetic conservation, which advocates the introduction of new individuals into genetically depauperate populations of endangered or threatened species.
Unintentional introductions Unintentional introductions occur when species are transported by human vectors. Increasing rates of human travel are providing accelerating opportunities for species to be accidentally transported into areas in which they are not considered native. For example, three species of
rat (the black, Norway and Polynesian) have spread to most of the world as hitchhikers on ships, and
arachnids such as
scorpions and exotic
spiders are sometimes transported to areas far beyond their native range by riding in shipments of tropical fruit. This was seen during the introduction of
Steatoda nobilis (Noble false widow) worldwide through
banana shipments. Further there are numerous examples of marine organisms being transported in
ballast water, among them the
comb jelly Mnemiopsis leidyi, the bacterium
Vibrio cholerae, or the
zebra mussel. The Mediterranean and Black Seas, with their high volume shipping from exotic sources, are most impacted by this problem. Busy harbors are all potential hotspots as well: over 200 species have been introduced to the
San Francisco Bay in this manner making it the most heavily invaded estuary in the world. There is also the accidental release of the
Africanized honey bees (AHB), known colloquially as "killer bees") or
Africanized bee to Brazil in 1957 and the
Asian carp to the United States. The insect commonly known as the
brown marmorated stink bug (
Halyomorpha halys) was introduced accidentally in Pennsylvania. Another form of unintentional introductions is when an intentionally introduced plant carries a parasite or herbivore with it. Some become invasive, for example, the
oleander aphid, accidentally introduced with the ornamental plant,
oleander. Yet another unintentional pathway of introduction is during the delivery of
humanitarian aid in the aftermath of natural disasters. This occurred during relief efforts for
Hurricane Maria in Dominica, it was found that the
common green iguana, the
Cuban tree frog, and potentially the
Venezuela snouted tree frog were introduced with the former two becoming established. However the small percentage of introduced species that become invasive can produce profound ecological changes. In North America,
Harmonia axyridis has become the most abundant
lady beetle and probably accounts for more observations than all the native
lady beetles put together. Some unintentional introductions have been labelled "self-introductions". These include cases where invasive species bring in their own natural enemies. Such "self-introductions" may help to control the host invaders, or may become pests themselves. == Introduced plants ==