Biology Reference
In-Depth Information
species” to refer to those species transported and released outside their native
ranges by the activities of humans, whether done intentionally or not. The move-
ment of such a species by humans is referred to as an “introduction”. Not all intro-
duced species become established, but many do. Such established populations are
often referred to as “alien”, “naturalized”, “non-native”, “non-indigenous”, “feral”,
or “exotic”, but I will confine myself to the first two terms.
Human-mediated dispersal of species is not necessarily a
qualitatively
different
phenomenon than dispersal by other means, such as attaching to a bear's fur or a
waterbird's foot. However, the temporal and spatial scales at which humans are
homogenizing the world's biota are of a far greater magnitude than previously seen
in Earth's history. As one example, Loope (1998) estimated that prior to human
arrival, the rate of new species establishment in the Hawaiian Islands was approxi-
mately 1 species/35,000 years. Now it is on the order of 20-30 species/year
(Beardsley, 1962, 1979; Miller and Holt, 1992), an approximately million-fold rate
increase. Similar changes have occurred on other oceanic islands and in marine and
freshwater systems (Ricciardi, 2007), although with perhaps not so extreme a rate
increase as in Hawaii. Establishment rates on continents seem to be lower but are
already far above historical rates and appear to be increasing. From a spatial perspec-
tive, species are now being mixed among continents that have not been connected
for 250 million years. As well, species having limited mobility - such that they
would not previously travel even to locations a short distance away - are now spread
around the world by human activity. This overwhelming increase in rate and areal
extent of alien-species introductions has had profound effects on native species and
ecosystems throughout the globe. Hence, restricting use of the term “alien” to those
species introduced by humans provides a very practical distinction for scientific and
management purposes.
Invasive species are that subset of alien species having a demonstrated negative
effect on native ecosystems, species, or human values and concerns. Invasive species
are often referred to as either “weeds” or “pests” as well, and if impacts are largely
incurred by natural ecosystems the species may be termed an “environmental pest”.
The distinction between alien and invasive species may be made clearer by a few
examples. Corn (
Zea mays
) is an alien species everywhere on Earth outside of
southern Mexico, but it is invasive nowhere because it fails to establish outside the
artificial ecological conditions imposed by agriculture. Many alien species - including
most important crop species - are like this, growing only where deliberately
planted, or living in sparse numbers in the wild, to all appearances having no delete-
rious effects on native or human ecosystems. But invasive aliens - such as brown
treesnakes, gypsy moths, cheatgrass, or bubonic plague - are another matter
entirely. They spread throughout areas to which they are introduced and cause
tremendous harm to wildlife, agriculture, or human health. Escaping one or more
forms of ecological constraint allows them to achieve unregulated population
growth, forming the ecological equivalent to cancerous cell proliferation within an
organism. The process by which an alien species establishes, expands its geo-
graphic range and numbers, and exerts ecological or economic impacts in a new
locality is referred to as “invasion”.
