Exotic
species are those species that are not indigenous to a particular place or
region but are introduced from other places. They often arrive with human
assistance in a habitat it had not previously occupied, then establishes a
population and spreads autonomously. Most of these
non-native species die under the new, unfamiliar conditions, but as worldwide
movement of people and goods increases, thousands of so-called invasive species
make their way abroad and drive out or infest native species. In
as much as these species contributes to the flora and fauna of different
regions, they are also considered as invasive species that pose conservation
threats to many ecosystems today and have contributed to ecological extinction of different animal and plant
species. Invasive species threaten global
biodiversity and often damage commercial crops.
The great majority of such invasions are by species introduced from elsewhere,
although some native species have become invasive in newly occupied habitats. Invasive
species damage native species in different ways. Some may directly out compete
natives—for nutrients, light, or water in the case of plants, or for food,
shelter, or nesting sites among animals.
Typically, the species that coexist in ecosystems have evolved together for
many generations. These populations have established balanced interaction with
each other that enable all populations in the area to remain relatively stable
and introduction of the exotic species in a community may disrupt the
interaction ecology among organisms and which may become either serious
predators or competitors. Being species that contribute to ecological
extinctions, invasive species can produce a bewildering array of impacts and
impacts often depends on context; the same introduced species can have minimal
effects on native species and ecosystem in one region but can be devastating
somewhere else. Further, the same species can affect native in several
different ways simultaneously. However, a good understand the scope of the
threat posed by biological invasions is to classify the main types of impacts.
Impacts of
Invasive Species
In a
world where a large proportion of native plants and animals are clinging to
survival in increasingly isolated or degraded patches of habitat, invaders are
often the final straw that dooms these native species to eventual extinction.
Ecosystem modification
is the greatest impact of invasive species because such modifications likely to
affect most of the originally resident species. This includes invaders
that alter the local environment to an extent that the site is no longer
suitable for the natives. In one example, the decaying leaves of
purple-flowering musk thistle release into the soil substances that suppress
the growth of competing plants and create bare patches where only thistle seed
can sprout. This is one way in which the physical
structure of the habitat is changed. Species can also modify the entire
ecosystem by overgrowing and shading out native species. For instance, South
American water hyacinth (Eichhomia
crassipes) now covers parts of late
victoria in Africa, many lakes and rivers in the south-eastern united states
and various water bodies in Asia and Australia, often smothering native
submersed vegetation. Vast quantities of rotting water hyacinth and consequent
drops in dissolved oxygen, can also affect many aquatic animal species.
Resource competition
is another serious impact of invasive species on various organisms across
communities. Studies done in Great Britain suggest that the introduced North
American grey squirrel forages for nuts more efficiently than the native red
squirrel leading to the decline of the latter species. The same North American
grey squirrel species has recently invaded the piedmont in Italy and is
spreading, leading to concern that the red squirrel will also decline on the
mainland of Europeans it has in Britain. The house gecko from Southeast Asia
and parts of Africa has invaded many pacific islands, lowering insect’s
populations that serve as food for native lizards, whose populations have
declined in some areas.
While
ecosystem modification and resource competition are greatest impacts of
invasive species, predation is one
of the most dramatic and frequently seen impacts of exotic species on native
species. Probably the most famous cases are of mammalian predators such as the
ship, Norway rat, pacific rat small Indian mongoose and stoat introduced to
islands that formerly lacked such species. In many instances, native bird
species, not having evolved adaptions to such predators, nested on the ground
and were highly susceptible to the invaders. Introduced rats for example have
caused extinction of atleast 37 species and subspecies of island birds
throughout the world.
Herbivory
is another is another impact caused by the invasive species. Introduced
herbivores can devastate the flora of areas lacking similar native species,
especially on islands. Goats (capra aegagrus) introduced to the island of St.
Helena in 1513 are believed to have eliminated at least half of 100 endemic
plant species before botanist had a chance to record them. In some
cases, non-native species may be closely related enough to crossbreed with
native species, thus damaging native populations through genetic invasion.
The
threats to freshwater areas and ecosystems are extensive and varied. Many
plants also threaten freshwater ecosystems. Salt cedar, also known as tamarisk,
is a deep-rooted tree that has dried up springs and ponds and has drastically
narrowed the channels of rivers throughout the West. In the East and Midwest,
exotic waterweeds such as hydrilla, water hyacinth, and Eurasian milfoil have
mushroomed, choking out native organisms and seriously hampering navigation,
fishing, municipal water systems, and other human uses.
Therefore, it would of course be impossible to
unscramble the world's biota and send all non-native plants, animals, and
microorganisms back to their places of origin. Our diets, our parks and
gardens, our very culture would be immeasurably poorer without imported organisms.
The challenge is to protect and make use of the world's rich living heritage
without allowing the small but significant contingent of biological invaders to
cause further economic and ecological harm.
Where
invaders are already established, eradicating them is rarely possible. Instead,
agriculture and forestry officials, wildlife reserve managers, government
agencies, and others responsible for the health and productivity of our lands
must devise on-going programs to minimize the damage the invaders cause and to
control their spread.
Monitoring and eradication
The key to eradicating an introduced
species before it can spread widely is an early warning rapid response system,
and early warning requires an ongoing monitoring program. Because of the great
expense of trained staff, few if any nations adequately monitor consistently
for all sorts of invasions, although for specific habitats (e.g. waters in
ports) or specific groups of species (e.g. fruit fly pests of agriculture)
intensive ongoing monitoring exists in some areas. Probably the most
cost-effective way to improve monitoring is to enlist the citizenry to be on
the lookout for unusual plants or animals and to know what agency to contact
should they see something. Such efforts entail public education and wide
dissemination in popular media and on the web, but they can yield enormous
benefits.
Many introduced species have been
successfully eradicated, usually when they are found early but occasionally
when they have already established widespread populations. The keys to
successful eradication have been as follows;
- Sufficient resources must be available to see the project through to completion; the expense of finding and removing the last few individuals may exceed that of quickly ridding a site of the majority of the population.
- Clear lines of authority must exist so that an individual or agency can compel cooperation. Eradication is, by its nature, an all-or-none operation that can be subverted if a few individuals decide not to cooperate (For instance, by forbidding access to private property, or forbidding the use of a pesticide or herbicide)
- The biology of the target organism must be studied well enough that a weak point in its life cycle is identified, this is helpful because you don’t affect other organisms and their life cycle.
- Should the eradication succeed, there must be a reasonable prospect that reinvasion will not occur fairly quickly.
Maintenance
and management
If eradication is not an option, many available
technologies may limit populations of invasive species so that damage is
minimized. There are three main methods—mechanical
or physical control, chemical control, and biological control. Sometimes these
methods can be combined, especially
mechanical and chemical control. Mechanical means alone—cutting and pulling roots—so long as sufficient labor is available. Sometimes
chemical control alone can keep a pest at low numbers.
Biological
control is an introduction of a particular species to act and control target
invasive species. In addition to the fact that most biological control projects
have not panned out, several biological control agents have attacked non-target
species and even cause extinctions. In general, problems of this sort have been
associated with introduced biological control agents such as generalized
predators that are not specialized to use the specific target host. However,
even species that are restricted to a single genus of host, such as the cactus
moth, can create problems. All these measures are not 100% effective but when
combined they can at least minimize the populations of invasive species.
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