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Invasive species

2007 Schools Wikipedia Selection. Related subjects: Environment

   Lantana Invasion of abandoned citrus plantation; Moshav Sdey Hemed,
   Israel
   Enlarge
   Lantana Invasion of abandoned citrus plantation; Moshav Sdey Hemed,
   Israel

   The term invasive species refers to a subset of those species defined
   as introduced species or non-indigenous species. Invasive species can
   alter ecological relationships among native species and can affect
   ecosystem function, economic value of ecosystems, and human health. A
   species is regarded as invasive if it has been introduced by human
   action to a location, area, or region where it did not previously occur
   naturally (i.e., invasive), becomes capable of establishing a breeding
   population in the new location without further intervention by humans,
   and spreads widely throughout the new location. 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 (Cassey et al.
   2005). The majority of introduced species do not cause significant
   ecological change or environmental harm because they exist primarily in
   habitats already subjected to intensive human alteration; such species
   may not be considered 'invasive'.

   While the majority of the introduced plant species pose neither
   economic nor ecological problems, a few species become invasive and
   damaging to their new habitat. It is still an open question why certain
   plant species become such successful invaders; several theories are
   currently being debated and tested, but there is no one clear, single
   answer, and little reliable predictive power about the nature and
   extent of future invasions.

   The cost of these invasive plant species can be high. Problems caused
   by invasive plants cost billions to the global economy every year,
   mainly from loss of grazing land and reduced crop productivity due to
   non-native weeds. The cost to the United States alone is an estimated
   $137 billion a year in management and missed economic gain.
   Ecologically, they can disrupt ecosystem services and disrupt
   communities by being space-dominant or through impacts on keystone
   native species. At their worst, invasive plants have the ability to
   degrade whole ecosystems, both terrestrial and aquatic.

   There is currently a debate about whether the benefits of introduced
   plant species, both in increasing local species richness and in
   economic advantages, is a sufficient tradeoff for the global decrease
   in species richness and the costs of plant species that become invasive
   (Brown and Sax, 2004). Whatever the ultimate moral and management
   decisions about invasive plants as a whole may be, the phenomenon of
   invasive plants is indisputable, and has proved a fertile area of
   study.

   Invasive plants can be spread in many ways, and introductions can be
   accidental or intentional. Many invasive plants have been spread
   through deliberate introduction because the species was perceived to
   have value in agriculture or ornamental gardening. However, many have
   also been unintentional introductions, either through planting of
   impure seed mixes that contain the invasive species, or by hitching a
   ride on a vehicle or in cargo.

   For an introduced plant to become an invader, it has to 1) arrive, 2)
   survive, and 3) thrive. The plant must find a vector which will bring
   it to a new environment. This new habitat must be a close enough match
   to its native range that it is able to survive and reproduce here
   without human cultivation (Williams and Meffe, 1998). To actually
   become invasive, the introduced plant has to be able to outcompete
   native plants, to reproduce effectively enough to start spreading
   geographically through its new habitat, and to negatively impact the
   ecosystems in its introduced range. Sometimes, this requires a large
   propagule size, or repeated innoculations, or a combination of changing
   circumstances, before a new species will actually take hold in a new
   habitat.

   There are hundreds if not thousands of examples of invasive plant
   species throughout the world. One example is Bromus tectorum ( Drooping
   Brome, downy brome, or cheatgrass), which spreads rapidly after
   burning, and crowds out plants used by grazing animals in large areas
   of Western North America. Because it has low nutritive value to grazing
   animals, it does not substitute either economically or ecologically for
   the displaced native plants. In the southern United States, Pueraria
   lobata ( kudzu) was originally planted to stop roadside erosion, and
   now covered large areas with its leafy vines. It has been known to
   swallow up entire fields and forests if left unchecked. In the marine
   realm, Caulerpa taxifolia (the “killer algae” of the Mediterranean) is
   able to cover vast stretches of shallow ocean floor with a fast-growing
   monoculture of bright-green fronds, which are inedible and
   uninhabitable by all but a very few animal species.

Conditions that lead to invasion

   Scientific literature proposes several mechanisms to explain
   invasiveness. These mechanisms generally fall into two different
   categories: one for mechanisms which focus on the invasive species, and
   the other which focuses on the invaded ecosystem. More likely, it is a
   combination of several mechanisms that cause an invasive situation to
   happen.

Species-based mechanisms

   Species-based characteristics focus on plant competition. While all
   plants are able to compete in some manner in order to survive and
   persist, invasive species appear to have specific traits or
   combinations of specific traits that make them especially good
   competitors. In some cases it can be as simple as having the ability to
   grow and reproduce more rapidly than native species. Other situations
   are more complex, such as allelopathy, which is a common mechanism
   whereby the invader directly or indirectly prevents other plants from
   growing nearby.

Life history

   The life history of an organism describes the different stages of life
   an organism will go through during its lifetime. Such traits are
   tempting to study because life history is a quantifiable trait that
   could lead to very predictive models.

   Several traits have been singled out by researchers as predictors of
   invasive ability in plants. For example, the ability to reproduce both
   asexually (vegetatively) as well as sexually, rapid growth, early
   sexual maturity, high reproductive output, the ability to disperse
   young widely, tolerance of a broad range of environmental conditions,
   and high phenotypic plasticity are all abilities that might aid an
   invasive plant in establishing and proliferating in a new environment.
   In addition, plants that are associated with human habitats, such as
   crop plants (and their weeds), plants valued for ornamental purposes,
   or plants that are spread along roadways or by domestic animals, are
   more likely to find a vector to travel to a new habitat in the first
   place.

   Collecting data on life-history traits goes back to the beginning of
   invasive species study, and still forms a part of current research. The
   majority of studies agree in a general sense on which kind of traits
   mark an invasive species, but there are differences in how invasive
   each trait can make a species. 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 to only have a small subset of the invasive traits, and
   that many of these invasive traits were found in non-invasive species
   as well. This is one of the great difficulties in invasive species
   research: while many generalities can be made about invasive species,
   there are always exceptions to these observations (Kolar and Lodge
   2001, Thebaud et al. 1996, Reichard and Hamilton 1997).

Superior competition

   A common trait of invasive species is great competitive ability, which
   can be stronger against plants in a new habitat than plants in their
   native habitat. There can be huge differences between how an invasive
   species interacts with its native ecosystem, and with the ecosystem it
   is invading. Often, the invading species has a better chance at
   acquiring resources, which can be light, water, space, or nutrients.
   Ecosystems where all available resources are being used to their full
   capacity by native plants 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 instant, equivalent extinction of native
   plants with increased populations of the invader) is not the rule
   (Stolgren 2003, Sax et al. 2002). Invasive plants can coexist with
   native plants for an extended time, and only gradually does the
   superior competitive ability of an invasive species become apparent, as
   its population grows larger and denser, and slowly increases the risk
   of extinction to other species.

   An invasive species might be able to use resources previously
   unavailable to native plants, such as very deep water sources accessed
   by a long taproot, or an ability to live on previously uninhabited soil
   types. For example, barb goatgrass (Aegilops triuncialis), can be found
   in its introduced range in California on serpentine soils, which have
   low water-holding capacity, low nutrients, high Mg/Ca ratio, and
   possible heavy metal toxicity. Few plants have adapted to grow on them,
   which may explain why they have had so few plant invasions. However,
   since goatgrass can take advantage of these soils, it can also use the
   light, water, and space resources that other plants are restricted from
   using (Huenneke et al. 1990). By using these resources, goat grass can
   become so dense as to exclude other species and form monospecific
   swards.

   There are other reasons that an invader might be a superior competitor.
   For example, an invasive plant may be inedible to local herbivores,
   allowing it to flourish unmolested where the native species are
   constantly held in check. The herbivores would find themselves thus
   come into increasing competition with each other over fewer and fewer
   native plants, while the invader is taking the place of the native
   species. As the invader comes to dominate its new habitat, the local
   food webs are changed from the bottom up, since their foundation of
   native plants has been altered. (Petren and Case 1996, Gray 1986).

Facilitation

   Facilitation is the mechanism by which some species can alter their
   environment through chemicals or manipulation of abiotic factors,
   usually to make it more favorable to their growth or reproduction.
   Sometimes, neighboring species may benefit by another’s facilitation,
   but often the facilitation actually benefits the target species to the
   detriment of its neighbors. One such facilitative mechanism is
   allelopathy, also known as chemical competition. In allelopathy, a
   plant will secrete chemicals which make the surrounding soil
   uninhabitable, or at least inhibitory, to other plant species.

   One example of this is the knapweed species 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
   haven't 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 result
   shows how difficult it can be to predict whether a species will be
   invasive just from looking at its behaviour in its native habitat, and
   demonstrates the potential for novel weapons to aid in invasiveness
   (Vivanco et al. 2004, Hierro and Callaway 2003).

   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 height of the 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 (Brooks et al. 2004).

   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
   nooks and crannies 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 (Silver Botts et al. 1996).

Ecosystem based mechanisms

Unfilled niches

   Basic ecology tells us that every species has a role to play in its
   native ecosystem; some are general while others are highly specialized.
   These roles are known as niches. This mechanism describes a situation
   where the invaded ecosystem in question has unfilled niches, which are
   promptly filled by an invader. While there are cases where the invader
   can stay within its niche, these species often bring about other traits
   in them, which cause harm for the rest of the ecosystem, such as lack
   of suitable predators or alleopathic traits.

   While this mechanism sounds reasonable to the casual reader, the data
   itself is much more mixed. Various experiments have shown positive,
   negative and neutral correlations between ecosystems with high
   diversity and invasiveness. From this I would argue that while this
   mechanism may make sense in theory, it does not pan out that well in
   the literature. Perhaps its effect is simply too weak, and is over
   powered by other mechanisms, or perhaps the concept of a missing niche
   is too broad in its application, and needs to be more strictly defined,
   or better yet, combined into one of the other ecosystem based models
   (Dukes 2001).

Ecosystem instability

   Imagine a situation involving a group of people who have mastered a
   game, and are able to beat anyone outside of their group. They know the
   rules inside and out, and are constantly perfecting their strategies to
   win. Few, if any, have a chance against this group. Then suddenly the
   rules suddenly change, and this group is at a loss, since their
   strategies no longer work the way they used to, and other players with
   vastly different strategies are able to come in, and win where they
   could not before. This is how the familiar ecosystem mechanism works.

   The concept of a successful invasive species seems to be a bit odd.
   After all, wouldn't species, which have co-evolved together for years,
   be able to out compete an exotic species? There has been much time for
   them to evolve and become as efficient as possible. This mechanism
   describes a situation where the ecosystem in question has suffered a
   disturbance of some sort, which changes the fundamental nature of the
   ecosystem (Byers 2002). Examples of this type of disturbance can range
   from the loss of an important predator to the eutrophication of an
   aquatic environment. In situations such as these, the specialized
   evolution of the native plant species becomes wasted, and non-native
   species can gain a foothold.

Ecology of invasive species

Ecological circumstances of invasive species

   Pied Currawong
   Enlarge
   Pied Currawong

   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
   longterm rainfall changes or human modifications to the habitat,
   increase in numbers and become invasive. The Pied Currawong of
   south-east Australia is an example: as a result of human changes to the
   landscape, Pied Currawongs increased greatly in range during the 20th
   century and have caused substantial declines in the populations of the
   smaller birds whose nestlings they prey on.

   All species on earth go through periods of increasing and decreasing
   population numbers, in many cases accompanied by expansion and
   contraction of range. A wide variety of circumstances are the cause of
   such changes, and human “alterations” on the landscape are especially
   significant in this regard. Anthropogenic (human derived) 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 because the range expansion
   results in the species occurring where it was not before native. 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
   is sufficient reason to regard a native species as "invasive" requires
   a broad definition of the term—but it seems reasonable to consider that
   some native species in disrupted ecosystems can spread widely and cause
   harm and in that sense become invasive.

   U.S. Executive Order 13112 (1999) defines "invasive species" as "an
   alien species whose introduction does or is likely to cause economic or
   environmental harm or harm to human health" (Council on Environmental
   Quality 1999). Thus, the term is used to imply a sense of actual or
   potential harm, something that may not be true for all introduced
   species.

Traits of invasive species

   Many features have been attributed to invasive species and invaded
   ecosystems, but none are universal and invasive species tend to have a
   suite of traits rather than all of them. Common invasive species traits
   include 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, ability to live off of a wide range of food types, single
   parent reproduction (especially in plants), and, commonly, association
   with humans (Williams and Meffee 1998). The single best predictor of
   invasiveness, however, is whether or not the species has been invasive
   somewhere else (Ewel et al. 1999).

   A study done by Marcel Rejmanek and David Richardson (1996) on invasive
   Pine species in South Africa has yielded interesting results on the
   traits that make species invasive. They found three main traits that
   successfully separated invasive from non-invasive Pine species. These
   are: small seed sizes, early reproduction, and short periods between
   large seed sets. These traits have also been shown to hold for other
   woody invasives.

   Before an invasive species can even put some of the aforementioned
   “invasive traits” to work and thrive, it must survive at a very low
   population density when it may be difficult to find mates or
   cross-fertilize or when random changes in the environment could easily
   wipe out the entire population (Tilman 2004). This is why successful
   invasive species are often associated with humans. Our repeated
   patterns of movement, such as ships sailing to and from ports, or cars
   driving up and down highways, allow for species to have multiple
   opportunities of establishing (also known as a high “propagule
   pressure”) (e.g., Verling et al. 2005).

Traits of invaded ecosystems

   There is even less consensus about what makes an ecosystem invasible,
   and the bottom line is that it is a combination of many factors, some
   of which we probably do not recognize. In 1958, Charles S. Elton
   published his famous book on invasive species in which he argued that
   ecosystems with higher species diversity were less invasible 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 (Stohlgren et al. 1999). In the end, this debate seems largely
   to hinge on the spatial scale at which invasion studies are performed,
   and the issue as to how diversity affects community invasibility
   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 (Levine 2000, Byers and
   Noonberg 2003)
   The brown tree snake (Boiga irregularis)
   Enlarge
   The brown tree snake (Boiga irregularis)

   Invasion is more likely if a potentially invasible ecosystem is similar
   to the ecosystem in which the potential invader evolved (Williams and
   Meffee 1998). 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
   (Stachowicz and Tilman 2005). A great example of this phenomenon is the
   decimation of the native bird populations on Guam by the invasive brown
   tree snake (Fritts and Leasman-Tanner 2001). Alternately, invasible
   ecosystems may lack the natural competitors and predators that keep
   introduced species in check in their native ecosystems, a point which
   is also seen in the Guam example. Lastly, invaded ecosystems have often
   experienced disturbance, usually human-induced (Williams and Meffee
   1998). This disturbance may give invasive species, which are not
   otherwise coevolved with the ecosystem, a chance to establish
   themselves with less competition from more adapted species (Tilman
   2004).

Vectors

   Non-native species have many vectors, including many natural ones, but
   most of the species that we consider "invasive" are associated with
   human activity. Human vectors have been in place for a long time, such
   as in the case of prehistoric humans introducing the Pacific rat
   (Rattus exulans) to Polynesia (Matisoo-Smith et al. 1998). Today,
   non-native species come from horticultural plants either in the form of
   the plants themselves or animals and seeds carried with them, from
   animals and plants released through the pet trade. Invasives also come
   from organisms stowed away on every type of transport vehicle
   imaginable, to name a few unintentional vectors. For example, ballast
   water taken up at sea and released in port is a major source of exotic
   marine life.

   There are also plenty of intentional vectors, in which people introduce
   species for a specific purpose. 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, but eventually came to wipe out
   several native bird species across the continent. One important thing
   to note about such vectors is that many allow for an often key
   component of repeated introductions of a species, providing multiple
   chances to establish.
   Chinese mitten crab (Eriocheir sinensis)
   Enlarge
   Chinese mitten crab (Eriocheir sinensis)

   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.

Impacts of invasive species

Ecological impacts

   Biological species invasions can negatively impact ecological systems
   in a multitude of ways. Worldwide an estimated 80% of endangered
   species could suffer losses due to competition with or predation by
   invasive species (Pimentel et al. 2005). As highly adaptable and
   generalized species are introduced to environments already impacted by
   human activities, native species are put at a distinct disadvantage to
   survive. There are many examples of decreases in biodiversity in such
   areas. For example, Purple loosestrife ( Lythrum salicaria) changed the
   ecology of wetlands by reducing the abundance of native plants and
   endangering several species of ducks and a species of turtle that
   depend on the native plants. Clearly, a primary threat to biodiversity
   is the spread of human activity into once pristine areas.

   Land clearing and human habitation put significant pressure on local
   species and disturbed habitat is often prone to invasions that can have
   adverse effects on local ecosystems, changing ecosystem functions. A
   species of wetland plant known as ʻaeʻae in Hawaiʻi (the indigenous,
   Bacopa monnieri) is regarded as a pest species in artificially
   manipulated waterbird refuges because it quickly covers shallow
   mudflats established for endangered Hawaiian stilt (Himantopus
   mexicanus knudseni), making these undesirable feeding areas for the
   birds. Sometimes, multiple successive introductions of different
   nonnative species can have interactive effects, where 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 Harbour from the East
   Coast of the United States a century ago. It had been found in small
   quantities in the harbour 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 (Grosholz, 2005).

   Invasive plants can arise from human clearing (such as slash-and-burn)
   or cattle grazing actions, such that the altered land is more
   hospitable to the invasive species than the original plant palette. The
   invasive plant can even be a pre-existing species that has attained
   dominance from the disturbance. For example, 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 invasives; 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 ( Tamarisk) in native ecosystems (Mack et al. 2000).
   Invasive species that are closely related with rare native species have
   the potential to hybridize with native species. Harmful effects of
   hybridization have lead to a decline and even extinction of native
   species (Hawkes et al. 2005, Rhymer and Simberloff 1998). For example,
   hybridization with introduced cordgrass, Spartina alterniflora,
   threatens the existence of California cordgrass ( Spartina foliosa) in
   San Francisco Bay (Ayres et al 2004).

Economic impacts

   Economic costs due to invasive species can be separated into direct
   costs due to production loss in agriculture and forestry, and
   management costs of invasive species. Estimated damage and control cost
   of invasive species in the US alone amount to more than $138 billion
   annually (Pimentel et al. 2005). In addition to these costs, economic
   losses can occur due to loss from recreational and tourism revenues
   (Simberloff 2001). 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 (Pimentel et al. 2005). The following examples
   from different sectors of the economy demonstrate the impact of
   biological invasions.

Agriculture

   Agricultural weeds cause an overall reduction in yield. Most 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 cattle or
   non palatable due to thorns and spines (e.g., yellow star thistle,
   Centaurea solstitialis). Forage loss due to invasive weeds on pastures
   amounts to nearly $ 1 billion in the U.S. alone (Pimentel et al. 2005).
   A decline in pollinator services and loss of fruit production has been
   observed due to the infection of honey bees ( Apis mellifera) by the
   invasive varroa mite. Introduced rodents (rats, Rattus rattus and R.
   norvegicus) have become serious pests on farms destroying stored grains
   (Pimentel et al. 2005).

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 US in 1996 and is expected to infect and
   damage millions of acres of hardwood trees. $30 million have already
   been spent in attempts to eradicate this pest and protect millions of
   trees in the affected regions (Pimentel et al. 2005).

   The woolly adelgid inflicts damage on old growth spruce fir forests and
   negatively impacts the Christmas tree industry (Forest Pests: Insects,
   Diseases & Other Damage Agents 2005). The chestnut blight fungus (
   Cryphonectria parasitica) and Dutch elm disease ( Ophiostoma ulmi) are
   two plant pathogens with serious impacts on forest health.

Trade

   The introduction and/or spread of an invasive species can have major
   trade implications. There is the prospect of losing a competitive
   advantage in exports because unaffected countries will either prohibit
   import of goods from affected countries or establish costly
   precautionary measures (Pimentel et al. 2005).

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 2005). Many invasive species have thorns and spikes than can
   prevent easy access by recreationers to hiking trails. Aquatic invasive
   species such as hydrilla ( Hydrilla verticillata) and Eurasian
   watermilfoil ( Myriophyllum spicatum) affect water-based recreation by
   impeding human access, interfering with the operation of boats and
   fishing lines, lowering water quality, and negatively altering aquatic
   ecosystems, including the abundance and diversity of fishes. For
   instance, hydrilla infestations have caused losses estimated at $10
   million in recreation revenues (Pimentel et al. 2005).

Public Health

   Non-indigenous diseases have a great impact on human health and
   contribute substantially to health care costs. An increasing threat of
   exotic diseases exists due to increased transportation and encroachment
   of humans into previously remote ecosystems that can lead to new
   associations between a disease and a human host (e.g., AIDS virus in
   human host; Pimentel et al. 2005). Introduced birds (e.g. pigeons),
   rodents and insects (such as mosquitoes, fleas, lice and tsetse fly)
   can serve as vectors and reservoirs of human diseases. Throughout
   recorded history epidemics of human diseases such as malaria, yellow
   fever, typhus, and plague have been associated with these vectors
   (Elton 1958). A recent example of an introduced disease is the spread
   of the West Nile virus across North America resulting in human deaths
   and in the deaths of many birds, mammals, and reptiles (Lanciotti
   1999). The neurological symptoms of this virus that is transmitted by
   mosquito vectors include fever, weakness and confusion associated with
   encephalitis in humans (Centers for Disease Control 2005). Waterborne
   disease agents such as Cholera bacteria ( Vibrio cholerae) and
   causative agents of harmful algal blooms are often transported via
   ballast water (Hallegraeff 1998). 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
   (Pimentel et al. 2005).

Human Impact

   Humans have the greatest impact on the problem of invasive species.
   Nearly all ecosystem disturbances come from various forms of human
   activity. Such activity can be organized into four categories:
   conservation, utilization, replacement, and removal (Bright 1998).
   These categories describe how we use the land, everything from wildlife
   preserves to parks to agriculture to urbanization. It is within these
   land decisions that many of the mechanisms of invasiveness come into
   play. Take the earlier example of the cheatgrass: when ranchers
   introduced cattle to an area, these heavy animals disturbed the soil
   allowing cheatgrass a foothold in the ecosystem.

   More general scenarios to consider:
     * Horticulture and Agriculture; from seed contamination to
       cultivation of exotic species
     * Development directly causes a keystone species to be extirpated,
       allowing an invader a chance at establishment
     * Pollution runoff can add nutrients into the environment, opening up
       a niche in an ecosystem

The threat to global biodiversity

   The impact on global biodiversity of human introduction of non native
   species that have subsequently become invasive is subjective. Climate
   change and the movement of the continents through the ages have created
   divisions and changes to species over the long history of this planet.
   Limited information on the circumstances and impact at the time makes
   it difficult to directly compare this to the advent of international
   travel for people and goods which has made the introduction of new
   species increasingly easy, and the direct ecological impacts are now
   far more evident and measurable.

   Historically the deliberate introduction of non native species has been
   done with little or no consideration of the impact outside of having a
   favored animal, fish, or plant available locally, or perhaps an
   ill-conceived attempt to control a native pest. In areas with highly
   endemic, specialised and isolated flora and fauna such as Australia,
   New Zealand, Madagascar, the Hawaiian Archipelago, and the Galapagos
   Islands, introduced species that successfully establish themselves in
   habitats utilized by natives compete for limited resources or prey on
   the native species, some of which are unable to adapt to the more
   competitive environment and gradually die out.

   As more adaptable and generalized species are introduced to
   environments impacted adversely by human activities, native species are
   put at a disadvantage to survive in what previously was a unique,
   balanced ecosystem. There are many examples of decreases in
   biodiversity in those areas. One of the primary threats to biodiversity
   is the spread of humanity into what were once isolated areas with land
   clearing and habitation putting significant pressure on local species.
   Agriculture, livestock and fishing can also introduce changes to local
   populations of indigenous species which may result in a previously
   innocuous native species becoming a pest due to a reduction of natural
   predators.

Control of invasive species

   The control of invasive species can involve their eradication or their
   containment within specified area. In both cases, the overarching goal
   is to prevent further spread to un-invaded systems (National Biological
   Information Infrastructure 2004). This type of management can be
   implemented at several scales, from a homeowner working in his or her
   own backyard to large government agencies taking a nation-wide
   approach. The decision to eradicate a species versus contain it can
   depend on several factors, including, but not limited to, the type of
   habitat, characteristics of the organism, the spatial dimensions of the
   spread, time available to dedicate to control, and cost. Many of these
   factors may also play into the decision of which specific control
   technique(s) to utilize. Examples of such techniques are described
   below. Please note that permitting may be required to undertake some of
   these control methods.

Mechanical control

   Mechanical control involves the removal of invasive species by hand or
   with machines. Often, these methods are effective in controlling small
   populations and can be target specific, minimizing harm to non-invasive
   plants and animals (The Nature Conservancy 2005). Mechanical control,
   however, is extremely labor intensive and requires a large time
   investment, as treatments must often be applied several times to ensure
   success (The Nature Conservancy 2005). Commonly implemented control
   methods for plants include hand pulling, mowing, girdling (removal of
   tree bark), and burning (The Nature Conservancy 2005). For invasive
   animal control, techniques such as hunting, trapping, and the
   construction of physical barriers like fences or nets, are used
   (Hengeveld 1989).

Chemical control

   Chemical control uses the application of chemical compounds to prevent
   invasive species spread. This method of control can be very effective
   in both large and small areas, but is often criticized due to the
   possible contamination of land and water resources and a lack of target
   specificity that can result in the killing of desirable plant and
   animal species (National Park Service 2004). Also, the target species
   may develop a resistance to the chemicals over time, rendering this
   method ineffective. Herbicides are chemicals used to control invasive
   plants and, depending on the target species, can be applied directly to
   a plant, in the soil at a plant’s base, or even to the soil before
   seeds develop (The Nature Conservancy 2005). In animals, other
   pesticides are used to restrict growth and reproduction or to kill
   invasive insect pests (National Biological Information Infrastructure
   2004), piscicides, such as rotenone, can be used to manage fishes and
   other aquatic organisms, and terrestrial vertebrates are controlled
   with poisoned baits. Another form of chemical control is the use of
   attractant pheromones to lure mate-seeking insects into traps (Myers et
   al. 2000).

Biological control

   Biological control involves the release of a specific species to
   restrict the spread of the invasive species. With the proper research,
   this method of control can be both environmentally safe and successful.
   However, it can be ineffective if the released species do not survive
   or if their impact on the invasive species is not as great as predicted
   (The Nature Conservancy 2005). Also, the species chosen for release is
   not always a native organism, increasing the possibility of even more
   invasive species. Predatory insects, called weed feeders, can be
   released to control invasive plants. Similarly, plants can be infected
   by disease causing organisms, such as fungi, bacteria, and viruses,
   killing them or reducing their reproductive output (Cornell University
   2005). Invasive animals can be controlled with the release of predatory
   or parasitic organisms (especially in the case of invasive insects) or
   with the transmission of diseases in a similar manner as with plants
   (Cornell University 2005). Additionally, sterile insect or fish males
   of the invasive species can be released so that after mating, a female
   will lay “dead” eggs or eggs that will develop into sterile adults
   (Myers et al. 2000).

Prevention

   Preventing the establishment of invasive species is always the best
   method of control (The Nature Conservancy 2005). Stopping harmful
   species at this stage can be difficult, but it can also be invaluable
   in terms of both the diminishing cost of future control and the
   preservation of our natural systems. Many governments try to limit the
   entry of invasive species into their lands with thorough inspections of
   international shipments, customs checks, and proper quarantine
   regulations. Additionally, the creation of clean (safe organisms
   cleared for entry) and dirty (potentially harmful organisms denied
   entry) lists can be helpful in regulation. The general public can also
   participate in invasive species prevention by educating themselves
   about this problem and by making informed decisions on related issues.

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