Biological invasions are called as the rapid spread and establishment of a species once it is introduced to a new niche.These species have the ability to replace the other species from their niche and can lower the biodiversity. Due to these abilities they pose a major threat to biodiversity and to global economy.par Alien invaders are the a major cause of species extinctions, according to a new study, but not everyone accepts this .
They are blamed for the extinction of large number of species in this world. For species that are completely endangered or threatened in the wild, if we try to draw up the list, we can identify one or more contributing factors. A minority of species are confounded: cats, rats and goats are amongst the most common culprits, along with microbs like the amphibian-killing chytrid fungus and the avian malaria parasite. 86\% of the extinctions ascribed to invasive species was occurred on islands, where native species have small populations and are poorly adapted for predators or environmental changes.par In Australia and other islands the extinction of the native species is majorly affected by invasions. Examples can be the imported European red foxes that had adverse impacts on native species, and were suspected in the decline of desert rat kangaroo, and feral cats are said to have some role in the extinction of several ground-dwelling birds in Australia .The WWF’s Living Planet Index points out that invasive species as the underlying threat for only 5\% of the vertebrate species listed .
Exploitation, , habitat degradation and climate change are all rated as major hazards.par An introduced species might become invasive if it can outmatch native species for resources such as nutrients, sunlight, physical space, water, or food. If these species evolved under great competition or predation, then the new environment may host fewer able competitors, allowing the invader to breed quickly. Ecosystems in which are being used to their full potential by native species can be modeled as zero-sum systems in which any gain for the invader is a loss for the native. 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.
par Over the past 500 years, we know of 77 mammal species (out of about 5,000) and 140 bird species (out of about 10,000) that have gone totally extinct. There may be a handful more we do not know about, and there are plenty more on the list. Of those 217 species of bird and mammal, almost all lived on islands if you count Australia as an island and just nine on continents. Blue buck antelope, Algerian gazelle, Omilteme cottontail rabbit, Labrador duck, Carolina parakeet, slender-billed grackle, passenger pigeon, Colombian grebe and Atitlan grebe. Of course the extinctions in the islands are not only caused by the climate change or deforestation but also by the introduction of non-native species. There would be more species to be extinct of course.By far the greatest cause is invasive species, especially on islands. Hawaii has lost about 70 species of bird since contact with Captain Cook ten times as many as all the world’s continents combined.
The cause is man-made, but it’s not because we killed them or destroyed their habitat. It’s the rats, cats, goats, pigs, mosquitoes and avian malaria we brought with us that did the damage on Hawaii and throughout the Caribbean, the south Atlantic, the Indian Ocean and the rest of the Pacific. The dodo disappeared from Mauritius not because sailors ate them but because of predation by monkeys, pigs, rats and the like animals. The ‘Tristan albatross’ is an endangered species on ‘Gough’ Island because its chicks are eaten alive by introduced mice.