Biological invasions by non-native species
constitute one of the leading threats to natural ecosystems
and biodiversity, and they also impose an enormous cost on
agriculture, forestry, fisheries, and other human enterprises,
as well as on human health. The ways in which non-native
species affect native species and ecosystems are numerous and
usually irreversible. The impacts are sometimes massive but
often subtle. Natural barriers such as oceans, mountains,
rivers, and deserts that allowed the intricate co-evolution of
species and the development of unique ecosystems have been
breached over the past five centuries, and especially during
the twentieth century, by rapidly accelerating human trade and
travel. Planes, ships, and other forms of modern transport
have allowed both deliberate and inadvertent movement of
species between different parts of the globe, often resulting
in unexpected and sometimes disastrous consequences.
Introduced species often consume or prey on
native ones, overgrow them, infect or vector diseases to them,
compete with them, attack them, or hybridise with them.
Invaders can change whole ecosystems by altering hydrology,
fire regimes, nutrient cycling, and other ecosystem processes.
Often the same species that threaten biodiversity also cause
grave damage to various natural resource industries. The zebra
mussel (Dreissena polymorpha), Lantana camara,
kudzu (Pueraria lobata), Brazilian pepper (Schinus
terebinthifolius), and rats (Rattus spp.) are all
economic as well as ecological catastrophes. Invasive
non-native species are taxonomically diverse, though certain
groups (e.g., mammals, plants, and insects) have produced
particularly large numbers of damaging invaders. Thousands of
species have been extinguished or are at risk from invasive
aliens, especially on islands but also on continents. Many
native ecosystems have been irretrievably lost to invasion.
Weeds cause agricultural production losses of at least 25% and
also degrade catchment areas, near-shore marine systems, and
freshwater ecosystems. Chemicals used to manage weeds can
further degrade ecosystems. Ballast water carries invasives
that clog water pipes, foul propellers, and damage fisheries.
Imported pests of livestock and forests reduce yields
drastically. Further, environmental destruction, including
habitat fragmentation, and global climate change are extending
the range of many invaders.
Not all non-native species are harmful. In
many areas, the great majority of crop plants are introduced,
as are many animals used for food. Some productive forest
industries and fisheries are based on introduced species. And
introductions for biological control of invasive pests have
often resulted in huge savings in pesticide use and crop loss.
However, many of the worst introduced pests were deliberately
introduced. Horticultural varieties and zoological novelties
have become invasive and destructive; fishes introduced for
human consumption have extirpated many native species, and
even biological control introductions have occasionally gone
awry. The rapid development of the science of invasion
biology, as well as growing technologies for detecting
unintentionally introduced invaders and managing established
invasive species, can provide major advances in the war
against invasive exotic species, so long as the public and
policymakers are aware of them.