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IntroductionInvasive species means any species that become establised in the place other than its native place or actually native range and have tendency to spread there in terms of number. However, it is seen in many case that just after introduction of Invasive species the ratio of native to invasive species changes dramatically but after a certain interval of time again the ratio comes into some stabilised form, number of native plant resembling to pre-introduction period or very less disturbed, for example in the case of fire ant Solenopsis invicta, an invasive species in the southern USA and native ant where If ecologists had studied S. invicta only just after it arrived, they would have concluded that it had devastating impacts on the native insect fauna. If they had studied it 12 years later, they would have concluded that it was a benign presence in the insect community. So the second part of definition i.e., it spreads in terms of number is technically not right and it depends upon the time duration of invasion of exotic species.Causes of the InvasionInvasion starts with introduction of exotic species. There are many reasons which lead to introduction of new exotic species out of one of the major is done by the humans like globalization that resulted in greater trade, transport, travel and tourism so alongwith the traders, transporters, tourists or as a gift new exotic species is introduced in the area. Also new species is introduced for decoration or they are alsointroduced people like and want to have them, New species may be introduced in parks for conservation purpose, to control pest or weeds, due to adultration or mistake. Besides human caused introduction of exotic species in case of movable animals especially birds they visit the new place as the their native place is not having the suitable condition so temporarily they also invade.After introduction the target is to get stabilised in the new environment to become the invasive species which is explained by either “driver model” or “passanger model”. But many time invasive species adopt a model of “Backseat driver” where the species first act as passanger and later it become driver when they reach sufficient numbers to exert competitive pressure on the native plants or they aquire additional changes to outcompete the native for resources.According to driver model, invasive species directly cause the changes observed in the ecosystem. They have unique growth characteristic, such as nitrogen-fixation where no native nitrogen fixing native species occur or the ability to alter disturbance patterns, fast growth, rapid reproduction, high dispersal ability, phenotype plasticity, ecological competence etc which allow them to enter a community, dominate numerically and reduce native plant species abundance without any “assistance” from accompanying environmental change.While If invadeing species acts as a passenger, it benefits from other changes occurring in an ecosystem, such as pollution, mismanaged grazing, increased wildfire, altered flooding regimes, etc. A passenger may be associated with decreased native plant diversity, but it is not the direct cause of the decreaseand it is increased due to the some ecosystem changes that led to reduced diversity. They take the advantage of the the oppurtunity to stabilise and spread.Means in the passanger model, which is more observed in nature, the invasive species the secondary reason which eliminate the native species while in driver model it is the primary reason.However, more concrete experiment or mathematical test or anything is not yet designed so that we can say that invasive species act as driver or passanger and also the coclusion that exotic species are really a threat to native primarily. These are based on observation and simple tests which have many limitations and assumptions because it neglects other forces.Also there are many examples that exotic species is actually threat to certain species earlier but it becomes threat to other species also due to alteration in ecosystem like sequestration of light, space and nutrients. Even the industrialisation, untreated or incompletely treated water effluent,   also favor the growth and development of species for example release of Arsenic rich waste or nutrient nutrient rich waste lead to growth of Eichhornia in water body.Many times humans purposefully helps in the stabilization of exotic species especially in their critical time by nurturing it in artificial condition. Prior successful invasions may also help the next invansion to occur.ConsequencesToday ecologists, conservation and biologists widely believe that invasions by exotic species are a leading cause of recent species extinctions but many study now also shows that biological invasion is not the primary cause of extinction of native species and extinction of the native species is due to the additive or synergistic effect of multiple reasons like habitat alteration, decline or extinction of native plants by itself, the proliferation of exotic plant species, urbanisation, cattle grazing. Out of these biggest factor towards native species factor is cattle grazing.  it is not possible to quantify or assess general patterns of threats to endangered and threatened species and causes of extinction, because data on threats and causes of extinction are limited, scattered, also there is not proper test for it. However, now IUCN, US National Invasive Species Council and  Convention on Biological Diversity are trying to collect all data, to identify all the factor responsible for the species extinction and looking more mathematically and scientifically. Few scientists are assesing on the basis of “threat score”, correlation, regression. Biological invasion is one of the five  major environmental issue which need serious attention as non-native species are increasing hence ecological imbalance is likely to occur.Besides species extinction there are other damages that occur due to invasive species like economy damage, direct and indirect impact on human and other living health, shift in species composition, shift in Hardy-Weinberg equilibrium, accumulation of materials, loss of diversity, climate change, poor evolution and adaptation, change in genetics and population size of individual species, diversity and structure of community and biogeochemical cycle, financial interest etc.There are few factors which affect ecological effects of invaders are:i) functional distinveness of invader- how much its characteristics such as nitrogen fixation, flammability, phenology, chemical defenses and diet differ from those of native species of the community.ii) Relative number of the invaderiii) Time since invasioniv) Changes in invader species and biotic components of communityv) Cumulative changes in the abiotic components of environment that is invadedvi) Interaction between invading species and other factors rthat control the ecosystem.RefrencesStrayer, D. L., Eviner, V. T., Jeschke, J. M., & Pace, M. L. (2006). Understanding the long-term effects of species invasions. Trends in Ecology & Evolution, 21(11), 645–651. http://sci-hub.tw/10.1016/j.tree.2006.07.007Molnar, J. L., Gamboa, R. L., Revenga, C., & Spalding, M. D. (2008). Assessing the global threat of invasive species to marine biodiversity. Frontiers in Ecology and the Environment, 6(9), 485–492. http://sci-hub.tw/10.1890/070064Didham, R. K., Tylianakis, J. M., Hutchison, M. A., Ewers, R. M., & Gemmell, N. J. (2005). Are invasive species the drivers of ecological change? Trends in Ecology & Evolution, 20(9), 470–474. http://sci-hub.tw/10.1016/j.tree.2005.07.006GUREVITCH, J., & PADILLA, D. (2004). Are invasive species a major cause of extinctions? Trends in Ecology & Evolution, 19(9), 470–474. http://sci-hub.tw/10.1016/j.tree.2004.07.005Bauer, J. T. (2012). Invasive species: “back-seat drivers” of ecosystem change? Biological Invasions, 14(7), 1295–1304. http://sci-hub.tw/10.1007/s10530-011-0165-xBrian Mealor ‘Invasive Species – Passengers or Drivers of Ecosystem Change?’  – http://www.wylr.net/columns/guest-editorials/245-uw-staff/5242-invasive-species-passengers-or-drivers-of-ecosystem-change

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