Introduced Species

In most places of the world, introduced species have caused important ecological damage. 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 eaten 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 degradation 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 on its tissues.

Some introduced insects have become troublesome pests in forests, as is the case of the gypsy moth (Lymantria dispar), a defoliator of many tree species introduced to North America in 1869 from Europe. Similarly, the introduced elm bark beetle (Scolytus multistriatus) has been a key 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 wide-feeding predators or 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 captivity, 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 relationships) as the ultimate in invasive species. Humans are, in fact, widely self-introduced.