Biological invasions are called as the rapid spread and establishment of a species once itis introduced to a new niche.These species have the ability to replace the other species fromtheir niche and can lower the biodiversity. Due to these abilities they pose a major threat tobiodiversity and to global economy.Alien invaders are the a major cause of species extinctions, according to a new study, butnot everyone accepts this . They are blamed for the extinction of large number of species inthis world. For species that are completely endangered or threatened in the wild, if we try todraw up the list, we can identify one or more contributing factors. A minority of species areconfounded: cats, rats and goats are amongst the most common culprits, along with microbslike the amphibian-killing chytrid fungus and the avian malaria parasite. 86% of the extinctionsascribed to invasive species was occurred on islands, where native species have small populationsand are poorly adapted for predators or environmental changes.In Australia and other islands the extinction of the native species is majorly affected byinvasions. Examples can be the introduced European red foxes that had adverse impacts onnative species, and were suspected in the decline of the desert rat kangaroo, and feral cats aresaid to have involvement in the extinction of several ground-dwelling birds in Australia .TheWWFs Living Planet Index points out that invasive species as the underlying threat for only5% of the vertebrate species listed .Exploitation, , habitat degradation and climate change areall rated as major hazards.An introduced species might become invasive if it can outcompete native species for resourcessuch as nutrients, light, physical space, water, or food. If these species evolved under greatcompetition or predation, then the new environment may host fewer able competitors, allowingthe invader to breed quickly. Ecosystems in which are being used to their full potential by nativespecies can be modeled as zero-sum systems in which any gain for the invader is a loss for thenative. 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 growslarger and denser and it adapts to its new location.Over the past 500 years, we know of 77 mammal species (out of about 5,000) and 140 birdspecies (out of about 10,000) that have gone totally extinct. There may be a handful more we donot 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. Bluebuck antelope, Algerian gazelle, Omilteme cottontail rabbit, Labrador duck, Carolina parakeet,slender-billed grackle, passenger pigeon, Colombian grebe and Atitlan grebe. Of course theextinctions in the islands are not only caused by the climate change or deforestation but also bythe introduction of non-native species. There would be more species to be extinct of course. Byfar the greatest cause is invasive species, especially on islands. Hawaii has lost about 70 speciesof bird since contact with Captain Cook ten times as many as all the worlds continents combined.1

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