Characteristics that make plants and animals valuable or attractive to humans are, in a sense, preadaptations for their dispersal to new geographical regions. Crop, forestry, and horticultural plants, domestic animals and exotic wildlife, aquacultural fish and shellfish, species prized by the pet and aquarium trade—all have gained access to new geographical areas because of their value or attractiveness to humans.
European colonization of many world regions led to the deliberate introduction of many plants and animals. Many plants were introduced to North America and other world regions from Europe in early Colonial times because of their agricultural, medicinal, or herbal properties. Numerous grasses and legumes were widely introduced, also beginning in Colonial times, because of their real or supposed value as forages. Intro ductions for forestry and horticulture become important somewhat later and have continued to the present. In the eastern United States, between 61 and 68% of plants, depending on location, that were introduced before 1900 and have become thoroughly naturalized were deliberately introduced (Mack and Erneberg 2002).These percentages are likely to be low because the manner of introduction of 31-37% of naturalized species could not be determined with certainty. For Hawaii, about 57% of naturalized species were deliberately introduced (Wester 1992). Since 1900, the percentage of naturalized plants that were deliberately introduced has tended to be even higher in many areas, including the United States,Aus-tralia, and New Zealand (Mack and Erneberg 2002).
Many species of trees have been introduced to new regions for timber production, fuelwood, creation of windbreaks, stabilization of stream banks, or wetland reclamation. Australian eucalypts have been introduced throughout the world for timber and fuelwood production and for use as windbreaks. Russian olive (Eleagnus angustifolia) and Siberian elm (Ulmus pumila), together with other species, were planted extensively in central North America as windbreaks and landscaping trees. Several varieties of tamarisk (Tamarix spp.) were used for stabilization of stream banks in the southwestern United States in the early twentieth century. In Florida, Australian melaleucas were planted in hopes that they would help reclaim marshland by increasing transpiration of water. Russian olives, tamarisks, and melaleucas have become very serious invaders in their areas of introduction in North America.
In terms of number of alien species, horticultural attractiveness accounts for more introductions of invasive woody plants in the United States than any other perceived value (Reichard and White 2001). In Australia, similarly, 65% of all plants that have become naturalized were introduced for horticulture. Deliberate introduction for horticultural and related use is responsible for establishment of many plants that reproduce vegetatively (Pysek 1997).
Animals have also been deliberately transported to new areas. Domestic livestock, including cattle, horses, sheep, goats, and rabbits, have been introduced to many oceanic islands or have become feral after their introduction for husbandry.Various game and fur-bearing animals have been released on islands or new continental areas or have escaped from farms. Game birds and songbirds have been introduced to new geographical areas for sport and pleasure. Game fish have been freely translocated to new waters, both within and between continents. Fish and invertebrates introduced for aquaculture or aquarium display have escaped or been released into natural waters and established wild populations.The ecological impacts of many of these introductions are substantial (Cox 1999).We shall examine the role of some in extirpation and extinction of native species in chapter 16.
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