An invasive species is an organism that causes ecological or economic harm in a new environment where it is not native.

Invasive species can harm both the natural resources in an ecosystem as well as threaten human use of these resources. An invasive species can be introduced to a new area via the ballast water of oceangoing ships, intentional and accidental releases of aquaculture species, aquarium specimens or bait, and other means.

Invasive species are capable of causing the extinction of native plants and animals, reducing biodiversity, competing with native organisms for limited resources, and altering habitats. This can result in huge economic impacts and fundamental disruptions of coastal and Great Lakes ecosystems.

“Invasive species”—they may not sound very threatening, but these invaders, large and small, have devastating effects on wildlife.

Invasive species are among the leading threats to native wildlife. Approximately 42 percent of threatened or endangered species are at risk due to invasive species.

Human health and economies are also at risk from invasive species. The impacts of invasive species on our natural ecosystems and economy cost billions of dollars each year. Many of our commercial, agricultural, and recreational activities depend on healthy native ecosystems.

An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment.

An invasive species is defined legally in the USA as “An alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health…‘Alien species’ means, with respect to a particular ecosystem, any species…that is not native to that ecosystem.”

What is an alien species?
  • Species introduced to regions outside of their historic (post-glacial) native range;
  • Species are being transported to new locations at up to 10,000 times greater rate than by natural dispersal
  • Aliens are also called:

Exotic species, non-indigenous species, non-native species, introduced species, and colonizing species.

There is a general agreement now to ‘alien invasive species, meaning non-native species introduced to new areas where they cause problems.

Charles Elton’s (1958) Invasion Concept
  • Species-poor habitats are more vulnerable to invasion than species-rich habitats due to a lack of biotic resistance (e.g. competition, predation, parasitism);
  • Islands sustain higher invasion rates and greater impacts than do continents

Based on the idea that islands are biologically impoverished, and thus less stable and more vulnerable to invasions

Same idea became dominant in ecology (i.e. diversity begets stability) in 1960s-1970s

Habitats disturbed by man are more vulnerable than less disturbed ones;

What influences invasion success?
1) Species Characteristics
  • high fecundity
  • small body size
  • vegetative or asexual reproduction
  • high genetic diversity
  • high phenotypic plasticity
  • broad native range
  • abundant in the native range
  • physiological tolerance
  • habitat generalist
  • human commensal
  • loss of natural enemies
  • invasional meltdown
  • polyphagy

Evidence in support of these ideas is not always supportive and is sometimes contradictory

2) Generalizations Regarding Habitat Invasibility
  • climatically matched
  • disturbed
  • low diversity
  • absence of predators
  • presence of vacant niches
  • low connectance of food web
  • nutrient-rich (plants)
  • Invasive species are a major threat to our environment because they

(1) can change habitats and alter ecosystem function and ecosystem services,

(2) crowd out or replace native species, and

(3) damage human activities, costing the economy millions of dollars. For example, costs to agriculture, forestry, fisheries, and other human activities by introduced species are estimated at $137 billion per year to the U.S. economy alone.

Threats to Biodiversity

Invasive species rank second only to habitat destruction, such as deforestation, as a threat to biodiversity. Almost half of the species in the United States that are at risk of extinction are endangered because of the effects of introduced species alone or because of their impacts combined with other processes. In fact, introduced species are considered a greater threat to native biodiversity than pollution, harvest, and disease combined. Invasive species threaten biodiversity by

(1) causing disease,

(2) acting as predators or parasites,

(3) acting as competitors,

(4) altering habitat, or

(5) hybridizing with local species.


Accidental introduction of the Asian chestnut blight fungus via the nursery trade virtually eliminated American chestnut from over 180 million acres of eastern United States forests in the first half of the 20th century. This extinction caused a whole-scale transformation of the Eastern deciduous forest ecosystem, which was dominated by American chestnut.

The loss of chestnuts was a disaster for many animals that were highly adapted to live in forests dominated by this tree species. For example, ten moth species that could live only on chestnut trees became extinct.


Invasive predators can severely reduce the population sizes of native species, or even drive them extinct, because native prey species may not have evolved defenses against the novel predators:

  • The predatory brown tree snake was introduced to Guam in cargo from the Admiralty Islands. Predation by brown tree snakes eliminated ten of the eleven native bird species endemic to the forests of Guam.
  • The Nile perch, a voracious predator, was introduced to Lake Victoria in Africa as a food fish. Predation from the Nile perch has eliminated over one hundred species of the spectacular native cichlid fishes of Lake Victoria.
  • Invasive herbivores can cause great damage. For example, goats were introduced by sailors to many remote oceanic islands during the age of European seafaring exploration, to provide a source of food when the islands were revisited. Goats introduced to the island of St. Helena in the 16th century eliminated over half the endemic plant species.
  • North American gray squirrels are driving native red squirrels to extinction in Great Britain and Italy. The introduced squirrels forage for nuts more efficiently than the native species, potentially leading to the loss of a native species.
  • Zebra mussels were accidentally brought to the United States from Russia in the ballast of ships. Zebra mussels alter aquatic habitats by filtering large amounts of water, thus reducing densities of planktonic organisms and settling in dense masses over vast areas. At least thirty freshwater mussel species are threatened with extinction by competition from the zebra mussel.

Hybridization occurs when members of two different species mate with one another and produce viable offspring that carry genes from both parents. When an invasive species is much more abundant than a native relative, they may hybridize so often that the invader’s genes “flood” the native species, such that no individuals contain the entire genotype of the native species, thus effectively driving the native species to extinction. It is possible that hybridization is common in such cases because the native species has not experienced selection for reproductive isolating mechanisms to prevent hybridization with the invader.

Of the 26 known animal species in the USA that have gone extinct since being listed under the Endangered Species Act, at least three were wholly or partly lost because of hybridization with invaders. For example, hybridization between Introduced mallards and the native Hawaiian duck and between the rarest European duck (the white-headed duck) and the invasive North American ruddy duck may result in the extinction of the native species.

Invasion Meltdown

Often invading species interact with one another to generate a problem where either species alone would be harmless, a concept known as invasion meltdown. Ornamental fig trees planted in Miami did not spread because they were sterile because they lacked the wasp species required for pollination.

However, in the early 1990s, a wasp species capable of fertilizing the figs independently invaded the region, so now the figs are capable of reproducing and spreading.

Controlling Invasive Species

Strategies used to control invasive species include

(1) keeping potential invaders out,

(2) eradicating potential invaders soon after the invasion,

(3) biological control,

(4) chemical control, and

(5) mechanical control.

What is Biotic homogenization?

Laliberte & Tylianakis (2010) refer to it as a phenomenon that reduces the variability and uniqueness of flora and fauna across regions.

“A gradual increase in compositional similarity among formerly distinct biological communities” (Naaf and Wulf 2010)

“A temporal increase in community similarity” (McKinney & Lockwood 1999).

“Biotic homogenization is the process by which species invasions and extinctions increase the genetic, taxonomic or functional similarity of two or more locations over a specified time interval” (Olden 2008).

“Biotic homogenization is defined as an increase in spatial similarity of a particular biological variable over time” (Olden et al. 2004).

  • Dr. M.B.POTDAR (Professor Shivaji University Kolhapur)

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