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 by introducing agricultural plants or animals for cultivation. In fact, all of the most important species of agricultural plants and animals are much more widespread 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 it is now cultivated on all of the habitable continents. Rice (Oryza sativa) is native to Southeast Asia, but 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 beyond their natural range 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 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 the 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 such European trees 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 exotic species of shrubs and herbaceous plants. The European settlers also introduced some species of birds and other animals with which they were familiar, such as the starling (Sturnus vulgaris), house sparrow (Passer domesticus), and pigeon or rock dove (Columba livia).