Introduced Species | Research & Encyclopedia Articles

Introduced Species

Some species of plants, animals, and microorganisms have been spread by humans over much wider ranges than they occupied naturally. Some of these introductions have been deliberate and were intended to improve conditions for some human activity, for example, in agriculture, or to achieve aesthetics that were not naturally available in some place. Other introductions have been accidental, as when plants were introduced with soil transported as ships' ballast or insects were transported with timber or food. Most deliberate or accidental introductions have not proven to be successful, because the immigrant species were unable to sustain themselves without the active intervention of humans. (In other words, the introduced species did not become naturalized.) Some introduced species have become very troublesome pests, causing great economic damages or severe losses of natural values.

Deliberate introductions

The most common reason for deliberate introductions of species beyond their natural range has been to improve the prospects for agricultural productivity. Usually this is done to provide agricultural species of plants or animals that would not otherwise be available for cultivation. In fact, all of the most important species of agricultural plants and animals are much more widespread on Earth today than they were prior to their domestication and extensive cultivation by humans. Wheat (Triticum aestivum), for example, was originally native only to a small region of the Middle East, but it now occurs virtually anywhere that conditions are suitable for its cultivation. Corn or maize (Zea mays) originated in a small area in Central America, but this species is now cultivated on all of the habitable continents. Rice (Oryza sativa) is native to Southeast Asia, but it is now very widespread under cultivation. The domestic cow (Bos taurus) was native to Eurasia, but it now occurs worldwide. The turkey (Meleagris gallopavo) is native to North America, but it now occurs much more extensively. There are many other examples of plant and animal species that have been widely introduced because they are useful as agricultural crops.

Other species have been widely introduced because they are useful in improving soil fertility for agriculture or sometimes for forestry. For example, various species of nitrogen-fixing legumes such as clovers (Trifolium spp.) and alfalfa (Medicago sativa) have been extensively introduced from their native Eurasia to improve the fertility of agricultural soils in far-flung places. In other cases, species of earthworms (such as the European nightcrawler, Lumbricus terrestris) have been widely introduced because these animals help to humify organic matter and are thereby useful in aerating soil and improving its structural quality. There have also been introductions of beneficial microorganisms for similar reasons, as when mycorrhizal fungi are inoculated into soil or directly onto tree roots. When their roots are infected with a suitable root mycorrhiza, plants gain significant advantages in obtaining nutrients, especially phosphorus, from the soil in which they are growing.

In some cases, species of animals have been introduced to improve the prospects for hunting or fishing. For example, Eurasian gamebirds such as ring-necked pheasant (Phasianus colchicus) and gray or Hungarian partridge (Perdix perdix) have been widely introduced in North America, as have various species of deer in New Zealand, especially red deer (Cervus elaphus). Species of sportfish have also been widely introduced. For example, various species of Pacific salmon (Oncorhynchus spp.) and common carp (Cyprinus carpio) have been introduced to the Great Lakes to establish fisheries.

Species of plants and animals have also been widely introduced in order to gain aesthetic benefits. For example, whenever people of European cultures discovered and colonized new lands, they introduced many species with which they were familiar in their home countries but were initially absent in their new places of residence. Mostly, this was done to make the colonists feel more comfortable in their new homes. For example, parts of eastern North America, especially cities, have been widely planted with European trees such as Norway maple (Acer platanoides), linden (Tilia cordata), horse chestnut (Aesculus hippocastanum), Scots pine (Pinus sylvestris), Norway spruce (Picea abies), as well as with many species of shrubs and herbaceous plants. The European settlers also introduced some species of birds and other animals with which they were familiar and comfortable, such as the starling (Sturnus vulgaris), house sparrow (Passer domesticus), and pigeon or rock dove (Columba livia).

Accidental introductions

Humans have also accidentally introduced many species to novel locations, and where the habitat was suitable these species became naturalized. For example, when cargo ships do not have a full load of goods they must carry some other heavy material as ballast, which is important in maintaining stability of the vessel in rough seas. The early sailing ships often used soil as ballast, and after a trans-oceanic passage this soil was usually dumped near the port and replaced with goods to be transported elsewhere. In North America, many of the familiar European weeds and soil invertebrates probably arrived with solid ships' ballast, as is the case for water horehound (Lycopus europaeus), an early introduction to North America at the port of New York. Ships have used water as ballast since the late nineteenth century, and many aquatic species have become widely distributed by this source. This is how two major pests, the zebra mussel (Dreissena polymorpha) and spiny water flea (Bythothrepes cederstroemii), were introduced to the Great Lakes from European waters.

An important means by which many agricultural weeds became widely introduced is through the contamination of agricultural seedgrains with their seeds. This was especially important prior to the twentieth century when the technologies available for cleaning seeds intended for planting were not very efficient.

Introduced species as an environmental problem

In most places of the world, introduced species have caused important environmental degradations. There are so many examples of this phenomenon that in total they represent a critical component of the global environmental crisis. A few selected examples can be used to illustrate problems associated with introduced species.

Several European weeds are toxic to cattle if ingested in large quantities, and when these plants become abundant in pastures they represent a significant management problem and economic loss. Some examples of toxic introduced weeds of pastures in North America are common St. John's wort (Hypericum perforatum), ragwort (Senecio jacobaea), and common milkweed (Asclepias syriaca).

Some introduced species become extremely invasive, penetrating natural habitats and dominating them, to the exclusion of native species. Purple loosestrife (Lythrum salicaria), originally introduced in North America as a garden ornamental, is becoming extensively dominant in wetlands, causing major degradations of their value as habitat for other species of plants and animals. In Florida, several introduced species of shrubs and trees are similarly degrading habitats, as is the case of the bottlebrush tree (Melaleuca quinquinerva) and Australian oak (Casuarina equisetifolia). In Australia, the prickly pear cactus (Opuntia spp.) was imported from North America for use as an ornamental plant and as a living fence, but it became a serious weed of rangelands and other open habitats. The cactus has since been controlled by the deliberate introduction of a moth (Cactoblastis cactorum) whose larvae feed voraciously on its tissues.

Some introduced insects have become troublesome pests in forests, as is the case of the gypsy moth (Lymantria dispar), introduced in 1869 to North America from Europe, and a defoliator of many tree species. Similarly, the introduced elm bark beetle (Scolytus multistriatus) has been a factor in the spread of Dutch elm disease, caused by an introduced fungus (Ceratocystis ulmi) that is deadly to North American species of elm trees (especially Ulmus americana). Another introduced fungus (Endothia parasitica) causes chestnut blight, a disease that has eliminated the once abundant American chestnut (Castanea dentata) as a canopy tree in deciduous forests of eastern North America.

Other introduced species have caused problems because they are widely feeding predators and herbivores. Vulnerable animals in many places, especially isolated oceanic islands, have been decimated by introduced predators such as mongooses (family Viverridae), domestic cats (Felis catus), and domestic dogs (Canis familiaris), by omnivores such as pigs (Sus scrofa) and rats (Rattus spp.), and by herbivores such as sheep (Ovis aries) and goats (Capra hircus). The recent deliberate introduction of the predatory Nile perch (Lates niloticus) to Lake Victoria, Africa's largest and the world's second largest lake, has recently caused a tragic mass extinction of native fishes. Until recently, Lake Victoria supported an extremely diverse community of more than 400 species of fish, mostly cichlids (family Cichlidae), with 90% of those species occurring nowhere else. About one-half of the endemic cichlid species are now extinct in Lake Victoria because of predation by the Nile perch, although some species survive in aquaria, and a few are still in the lake.

Ecologically, it is reasonable to consider humans and their symbiotic associates (that is, the many species of plants, animals, and microorganisms with which humans have intimate, mutually beneficial, and essential relationships) as the ultimate in invasive species. Humans, in fact, are widely self-introduced.