"Invasive species" is a phrase with several
definitions. The first definition expresses the phrase in terms of
non-indigenous species (e.g.
plants or animals) that adversely affect the habitats they invade
economically, environmentally or ecologically. It has been used in
this sense by government organizations as well as conservation
groups such as the
IUCN
(International Union for Conservation of Nature).
The second definition broadens the boundaries to include both
native and
non-native species that heavily
colonize a particular habitat.
The third definition is an expansion of the first and defines an
invasive species as a
widespread non-indigenous species.
This last definition is arguably too broad as not all
non-indigenous species necessarily have an adverse effect
on their adopted environment. An example of this broader use would
include the claim that the common goldfish (
Carassius auratus) is invasive.
Although it is common outside its range globally, it almost never
appears in harmful densities.
Because of the ambiguity of its definition, the phrase
invasive
species is often criticized as an imprecise term within the
field of
ecology. This article concerns the
first two definitions; for the third, see
introduced species.
Conditions that lead to invasion
Scientists propose several mechanisms to explain invasive species,
including species-based mechanisms and ecosystem-based mechanisms.
It is most likely a combination of several mechanisms that cause an
invasive situation to occur, since most introduced plants and
animals do not become invasive.
Species-based mechanisms
Species-based characteristics focus on competition. While all
species compete to survive, invasive species appear to have
specific traits or combinations of specific traits that allow them
to outcompete
native species.
Sometimes they just have the ability to grow and reproduce more
rapidly than native species; other times it's more complex,
involving a
multiplex of traits and
interactions.
Studies seem to indicate that certain traits mark a species as
potentially invasive. One study found that of a list of invasive
and noninvasive species, 86% of the invasive species could be
identified from the traits alone. Another study found that invasive
species tended only to have a small subset of the invasive traits
and that many of these invasive traits were found in non-invasive
species as well indicating that invasiveness involves complex
interaction not easily categorized. Common invasive species traits
include:
- The ability to reproduce both asexually as well as sexually
- Fast growth
- Rapid reproduction
- High dispersal ability
- Phenotypic plasticity (the
ability to alter one’s growth form to suit current conditions)
- Tolerance of a wide range of environmental conditions (generalist)
- Ability to live off of a wide range of food types
(generalist)
- Association with humans
- Other successful invasions
Typically an introduced species must survive at low population
densities before it becomes invasive in a new location. At low
population densities, it can be difficult for the introduced
species to reproduce and maintain itself in a new location, so a
species might be transported to a location a number of times before
it become established. Repeated patterns of human movement from one
location to another, such as ships sailing to and from ports or
cars driving up and down highways, allow for species to have
multiple opportunities for establishment (also known as a high
propagule
pressure).
An introduced species might become invasive if it can out-compete
native species for resources such as
nutrients, light, physical space, water or food. If
these species evolved under great
competition or
predation, the new environment may allow them to
proliferate quickly. Ecosystems in which all available resources
are being used to their fullest capacity by native species can be
modeled as
zero-sum systems, where any gain
for the invader is a loss for the native. However, such
unilateral competitive superiority (and
extinction of native species with increased populations of the
invader) is not the rule. Invasive species often coexist with
native species for an extended time, and gradually the superior
competitive ability of an invasive species becomes apparent as its
population grows larger and denser and it adapts to its new
location.
An invasive species might be able to use resources previously
unavailable to native species, such as deep water sources accessed
by a long
taproot, or an ability to live on
previously uninhabited soil types.
For example, Barbed Goatgrass
(Aegilops triuncialis)
was introduced to California
on serpentine soils,
which have low water-retention, low nutrient levels, a high Mg/Ca
ratio, and possible heavy
metal toxicity. Plant populations on these soils tend to
show low density, but goatgrass can form dense stands on these
soils crowding out native species that have not adapted well to
growing on serpentine soils.
Facilitation is the
mechanism by which some species can alter their environment using
chemicals or manipulating
abiotic factors,
allowing the species to thrive while making the environment less
favorable to other species with which it competes. One such
facilitative mechanism is
allelopathy, also known as
chemical
competition or
interference competition. In
allelopathy a plant will secrete chemicals which make the
surrounding soil uninhabitable, or at least inhibitory, to
competing species.
One example of this is the
knapweed
Centaurea diffusa. This
Eastern European weed has spread its
way through the western United States. Experiments show that
8-Hydroxyquinoline, a chemical
produced at the root of
C. diffusa, has a negative effect
only on plants that have not co-evolved with
C. diffusa.
Such co-evolved native plants have also evolved defenses, and
C. diffusa does not appear in its native habitat to be an
overwhelmingly successful competitor. This shows how difficult it
can be to predict if a species will be invasive just from looking
at its behavior in its native habitat, and demonstrates the
potential for novel weapons to aid in invasiveness.
Changes in fire regimes are another form of facilitation.
Bromus tectorum, originally
from Eurasia, is highly fire-adapted. It not only spreads rapidly
after burning, but actually increases the frequency and intensity
(heat) of fires, by providing large amounts of dry
detritus during the dry fire season in western
North America. In areas where it is widespread, it has altered the
local fire regime so much that native plants cannot survive the
frequent fires, allowing
B. tectorum to further extend and
maintain dominance in its introduced range.
Facilitation also occurs when one species physically modifies a
habitat and that modification is advantageous to other species. For
example,
zebra mussels increase
habitat complexity on lake floors providing crevases in which
invertebrates live. This increase in
complexity, together with the nutrition provided by the waste
products of mussel
filter-feeding
increases the density and diversity of
benthic invertebrate communities.
Ecosystem-based mechanisms
In
ecosystems, the amount of available
resources and the extent to which those resources are utilized by
organisms determines the effects of additional species on the
ecosystem. In stable ecosystems, equilibrium exists in the
utilization of available resources. These mechanisms describe a
situation in which the ecosystem has suffered a disturbance which
changes the fundamental nature of the ecosystem.When changes occur
in an ecosystem, like
forest fires in an
area, normal succession would favor certain native
grasses and
forbs. With the
introduction of a species that can multiply and spread faster than
the native species, the balance is changed and the resources that
would have been used by the native species are now utilized by an
invader. This impacts the ecosystem and changes its composition of
organisms and their use of available resources.
Nitrogen and
phosphorus
are often the limiting factors in these situations.
Every species has a role to play in its native ecosystem; some
species fill large and varied roles while others are highly
specialized. These roles are known as
niche. Some invading species are able
to fill niches that are not utilized by native species, and they
also can create niches that did not exist.
When changes occur to ecosystems, conditions change that impact the
dynamics of species interaction and niche development. This can
cause once rare species to replace other species, because they now
can utilize greater available resources that did not exist before,
an example would be the
edge effect. The
changes can favor the expansion of a species that would not have
been able to colonize areas and niches that did not exist
before.
Ecology
Although an invasive species is often defined as an introduced
species that has spread widely and causes harm, some species native
to a particular area can, under the influence of natural events
such as long-term rainfall changes or human modifications to the
habitat, increase in numbers and become invasive.
All species go through changes in population numbers, in many cases
accompanied by expansion or contraction of range. Human landscape
alterations are especially significant. This
anthropogenic alteration of an environment may
enable the expansion of a species into a geographical area where it
had not been seen before and thus that species could be described
as invasive. In essence, one must define "native" with care, as it
refers to some natural geographic range of a species, and is not
coincident with human political boundaries. Whether noticed
increases in population numbers and expanding geographical ranges
is sufficient reason to regard a native species as "invasive"
requires a broad definition of the term but some native species in
disrupted
ecosystems can spread widely
and cause harm and in that sense become invasive. For example, the
Monterey Cypress is an endangered
endemic naturally occurring only in two small stands in California.
They are being exterminated as exotic invasive species less than
from their native home.
Traits of invaded ecosystems
In 1958, Charles S. Elton argued that ecosystems with higher
species diversity were less
subject to invasive species because of fewer available niches.
Since then, other ecologists have pointed to highly diverse, but
heavily invaded ecosystems and have argued that ecosystems with
high species diversity seem to be more susceptible to invasion.
This debate seems largely to hinge on the
spatial scale at which invasion studies are
performed, and the issue of how diversity affects community
susceptibility to invasion remains unresolved. Small-scale studies
tend to show a negative relationship between diversity and
invasion, while large-scale studies tend to show a positive
relationship. The latter result may be an artifact of invasive or
non-native species capitalizing on increased resource availability
and weaker overall species interactions that are more common when
larger samples are considered.
Invasion is more likely if an ecosystem is similar to the one in
which the potential invader evolved. Island ecosystems may be prone
to invasion because their species are “naïve” and have faced few
strong competitors and predators throughout their existence, or
because their distance from colonizing species populations makes
them more likely to have “open” niches.
An example of this
phenomenon is the decimation of the native bird populations on
Guam
by the invasive brown
tree snake. Alternately, invaded ecosystems may lack the
natural competitors and predators that keep introduced species in
check in their native ecosystems, a point that is also seen in the
Guam example. Lastly, invaded ecosystems have often experienced
disturbance, usually human-induced. This disturbance may give
invasive species, which are not otherwise co-evolved with the
ecosystem, a chance to establish themselves with less competition
from more adapted species.
Vectors
Non-native species have many
vector, including many biogenic
ones, but most species considered "invasive" are associated with
human activity. Natural
range
extensions are common in many species, but the rate and magnitude
of human-mediated extensions in these species tend to be much
larger than natural extensions, and the distances that species can
travel to colonize are also often much greater with human
agency.
One of the earliest human influenced introductions involves
prehistoric humans introducing the
Pacific
rat (
Rattus exulans) to Polynesia. Today, non-native
species come from
horticultural plants
either in the form of the plants themselves or animals and seeds
carried with them, and from animals and plants released through the
pet trade. Invasive species also come from organisms stowed away on
every type of transport vehicle. For example,
ballast water
taken up at sea and released in port is a major source of exotic
marine life.
The invasive freshwater zebra mussels, native to the Black
, Caspian
and Azov
seas
, were probably transported to the Great Lakes
via ballast water from a transoceanic vessel. The arrival of
invasive
propagules to a new site is a
function of the site's invasibility.
Species have also been introduced intentionally. For example, to
feel more "at home", American colonists formed "Acclimation
Societies" that repeatedly released birds that were native to
Europe until they finally established along the east coast of
North America.
Economics play a major role in exotic species introduction. The
scarcity and demand for the valuable
Chinese mitten crab is one explanation
for the possible intentional release of the species in foreign
waters.
Impact
Ecological impacts
Biological species invasions alter ecosystems in a multitude of
ways. Worldwide, an estimated 80% of endangered species could
suffer losses by competition with, or predation by, invasive
species. Pimentel also reports that introduced species, such as
corn, wheat, rice, cattle, and poultry, provide more than 98% of
the U.S. food system at a value of approximately $800 billion
per year. As highly adaptable and generalized species are
introduced to environments already impacted by human activities,
some native species may be put at a disadvantage to survive while
other species survival is enhanced.
Land clearing and human habitation put significant pressure on
local species. This disturbed habitat is prone to invasions that
can have adverse effects on local ecosystems, changing ecosystem
functions. A species of wetland plant known as ae ae in Hawai i
(the indigenous
Bacopa
monnieri) is regarded as a pest species in artificially
manipulated water bird refuges because it quickly covers shallow
mudflats established for endangered Hawaiian stilt (
Himantopus mexicanus knudseni),
making these undesirable feeding areas for the birds.
Multiple successive introductions of different nonnative species
can have interactive effects; the introduction of a second
non-native species can enable the first invasive species to
flourish. Examples of this are the introductions of the
amethyst gem clam (
Gemma gemma)
and the European green crab (
Carcinus maenas).
The gem clam was
introduced into California's Bodega Harbor
from the East Coast of the United States a century
ago. It had been found in small quantities in the harbor but
had never displaced the native clam species (
Nutricola
spp.). In the mid 1990s, the introduction of the European green
crab, found to prey preferentially on the native clams, resulted in
a decline of the native clams and an increase of the introduced
clam populations.
In the
Waterberg region of
South Africa, cattle grazing over the past six
centuries has allowed invasive scrub and small trees to displace
much of the original
grassland, resulting
in a massive reduction in
forage for native
bovids and other grazers. Since the 1970s large scale efforts have
been underway to reduce invasive species; partial success has led
to re-establishment of many species that had dwindled or left the
region. Examples of these species are
giraffe,
Blue
Wildebeest,
impala,
kudu and
White Rhino.
Invasive species can change the functions of ecosystems. For
example invasive plants can alter the fire regime (cheatgrass,
Bromus tectorum), nutrient
cycling (smooth cordgrass
Spartina alterniflora), and
hydrology (
Tamarix) in native
ecosystems. Invasive species that are closely related with rare
native species have the potential to hybridize with the native
species. Harmful effects of hybridization have led to a decline and
even extinction of native species. For example, hybridization with
introduced cordgrass,
Spartina alterniflora, threatens the
existence of California cordgrass (
Spartina foliosa) in San Francisco
Bay.
Genetic pollution
Natural, wild
species can be threatened with
extinction through the process of
genetic pollution.
Genetic pollution is uncontrolled
hybridization and
introgression which leads to homogenization or
replacement of local
genotypes as a result
of either a numerical or
fitness
advantage of the introduced species. Genetic pollution can bring
about a form of extinction either through purposeful introduction
or through habitat modification, bringing previously isolated
species into contact. These phenomena can be especially detrimental
for rare species coming into contact with more abundant ones where
the abundant ones can interbreed with them, creating hybrids and
swamping the entire rarer gene pool, thus driving the native
species to extinction. Attention has to be focused on the extent of
this problem, it is not always apparent from
morphological observations alone. Some
degree of
gene flow may be a normal,
evolutionarily constructive process, and all constellations of
genes and genotypes cannot be preserved.
However, hybridization with or without introgression may,
nevertheless, threaten a rare species' existence.
Economic impacts
Benefits
Often overlooked, economic benefits from so-called "invasive"
species should also be accounted. The wide range of benefits from
many "invasives" is both well-documented and under-reported.
Asian oysters, for example, are better
at filtering out water pollutants than native oysters. They also
grow faster and withstand disease better than natives. Biologists
are currently considering releasing the mollusk in the
Chesapeake Bay to help restore oyster stocks
and clean up the bay's pollution. A recent study by the
Johns Hopkins School of
Public Health found the Asian oyster could significantly
benefit the bay's deteriorating water quality.
Costs
Economic costs from invasive species can be separated into direct
costs through production loss in agriculture and forestry, and
management costs of invasive species. Estimated damage and control
cost of invasive species in the U.S. alone amount to more than
$138 billion annually. In addition to these costs, economic
losses can occur through loss of recreational and tourism revenues.
Economic costs of invasions, when calculated as production loss and
management costs, are low because they do not usually consider
environmental damages. If monetary values could be assigned to the
extinction of species, loss in biodiversity, and loss of ecosystem
services, costs from impacts of invasive species would drastically
increase. The following examples from different sectors of the
economy demonstrate the impact of biological invasions.
Economic Opportunities
For many invasive species there are commercial benefits, either
existent or capable of being developed. For instance,
Silver Carp and
Common
Carp where heavy metals are not excessive in their flesh can be
harvested for human food and exported to markets already familiar
with the product, or into
pet foods, or
mink food. Numerous vegetative 'invasives'
like
Water Hyacinth can, when in
sufficient quantities to be harvestable, be turned into
methane digesters if no other better use
can be determined. The depletion or exploitation of any unwanted
species is dependent on officials who recognize the need for a
solution. Commercial enterprises need assurances that the
exploitation can continue long enough for a reasonable profit to be
generated and that taxation of the 'resource' is given a
sufficiently long period of grace that an enterprise is attracted
to the proposition.
Agriculture
Weeds cause an overall reduction in yield, though they often
provide essential nutrients for subsistence farmers. Weeds can have
other useful purposes: some deep-rooted weeds can "mine" nutrients
from the subsoil and bring them to the topsoil, while others
provide habitat for beneficial insects and/or provide alternative
foods for pest species. Many weed species are accidental
introductions with crop seeds and imported plant material. Many
introduced weeds in pastures compete with native forage plants, are
toxic (e.g., Leafy Spurge,
Euphorbia
esula) to young cattle (older animals will avoid them) or
non-palatable because of thorns and spines (e.g., Yellow
Starthistle,
Centaurea
solstitialis). Forage loss from invasive weeds on pastures
amounts to nearly $1 billion in the U.S. alone. A decline in
pollinator services and loss of fruit production has been observed
to cause the infection of honey bees (
Apis mellifera another invasive
species to the Americas) by the invasive
varroa mite. Introduced rodents (rats,
Rattus rattus and
R. norvegicus) have
become serious pests on farms destroying stored grains.
In many cases, one could consider the over-abundant invasive plant
species as a ready source of
biomass in the
perspective of
biogas production. See
Eichhornia crassipes.
Forestry
The unintentional introduction of forest pest species and plant
pathogens can change
forest ecology
and negatively impact timber industry. The Asian long-horned beetle
(
Anoplophora
glabripennis) was first introduced into the U.S. in 1996
and is expected to infect and damage millions of acres of hardwood
trees. Thirty million dollars have already been spent in attempts
to eradicate this pest and protect millions of trees in the
affected regions.
The
woolly adelgid inflicts damage on
old growth spruce fir forests and negatively impacts the
Christmas tree industry. The
chestnut blight fungus (
Cryphonectria parasitica) and Dutch elm
disease (
Ophiostoma
novo-ulmi) are two plant pathogens with serious impacts on
forest health.
Tourism and recreation
Invasive species can have impacts on recreational activities such
as fishing, hunting, hiking, wildlife viewing, and water-based
recreation. They negatively affect a wide array of environmental
attributes that are important to support recreation, including but
not limited to water quality and quantity, plant and animal
diversity, and species abundance. Eiswerth goes on to say that
"very little research has been performed to estimate the
corresponding economic losses at spatial scales such as regions,
states, and watersheds." Eurasian Watermilfoil (
Myriophyllum spicatum) in parts of the US,
fill lakes with plants making fishing and boating difficult.
Health impacts
An increasing threat of exotic diseases exists because of increased
transportation and encroachment of humans into previously remote
ecosystems. This can lead to new associations between a disease and
a human host (e.g.,
AIDS virus). Introduced
birds (e.g. pigeons), rodents and insects (e.g. mosquitoes, fleas,
lice and
tsetse fly) can serve as vectors
and reservoirs of human diseases. The introduced Chinese mitten
crabs are carriers of the
Asian lung
fluke. Throughout recorded history epidemics of human diseases
such as
malaria,
yellow fever,
typhus, and
bubonic plague have been associated
with these vectors. A recent example of an introduced disease is
the spread of the
West Nile virus
across North America resulting in the deaths of humans, birds,
mammals, and reptiles. Waterborne disease agents, such as
Cholera bacteria (
Vibrio cholerae), and causative agents
of
harmful algal blooms are
often transported via ballast water. The full range of impacts of
invasive species and their control goes beyond immediate effects
and can have long term public health implications. For instance,
pesticides applied to treat a particular pest species could pollute
soil and surface water.
Threat to global biodiversity
Biotic invasion is one of the five top drivers for global
biodiversity loss and is increasing because of tourism and
globalization. It poses a particular risk to
inadequately regulated
fresh water
systems, though
quarantines and
ballast water rules have improved the
situation.
Scientific definition
| Stage |
Characteristic |
| 0 |
Propagules residing in a donor region |
| I |
Traveling |
| II |
Introduced |
| III |
Localized and numerically rare |
| IVa |
Widespread but rare |
| IVb |
Localized but dominant |
| V |
Widespread and dominant |
In an attempt to avoid the ambiguous, subjective, and pejorative
vocabulary that so often accompanies discussion of invasive species
even in scientific papers, Colautti and MacIsaac have proposed a
new nomenclature system based on
biogeography rather than on
taxa.
By removing taxonomy, human health, and economic factors from
consideration, this model focuses only on ecological factors. The
model evaluates individual populations, and not entire species.
This model does not attribute detrimentality to invasive species
and beneficiality to native species. It merely classifies a species
in a particular location based on its growth patterns in that
particular microenvironment. This model could be applied equally to
indigenous and to non-native species.
See also
References
Further reading
Chronological order of publication (oldest
first)
External links
Top topics
Wikis
Quiz
Facts
Map
Search