Invasive species means an alien species which becomes established outside its natural ecosystems or habitats, is an agent of change and threatens native biological diversity (IUCN). Once an invasive species becomes firmly established, its control can often be difficult and eradication is usually impossible (Primental et al., 2000).
It can have some serious consequences on ecological, economic and social systems (Primental et al., 2000). Though the answer to the question ‘what makes a species invasive?’ is still unclear (Baker and Stebbins 1965, Kornberg and Williamson 1991, Lodge 1993), short juvenile period, short interval between large seed crops and larger number of seeds produce- these three variables seem to discriminate invasive species from others (Rejmanek M., Richardson M. D., 1996).
Also, resource competition has always been considered as a major mechanism for invasive plant success (Tilman, 1997; Levine et al., 2003). Phenological differences resulting in competition avoidance, niche and fitness differences, phylogenetic relatedness, recruitment limitation, indirect competition, or allelopathy have been key factors for resource competition (Gioria M., Osbome A. B.
,2014). Many invasive species possess higher values of competitively advantageous traits than native and non-invasive species. These include a superior capacity to acquire and retain resources and/or to advantageously exploit resources better than co-occurring native species (Huenneke et al., 1990; Burke and Grime, 1996; Rejmánek, 1996; Callaway and Aschehoug, 2000; Daehler, 2003). This increased competitive ability often results from rapid hybridization or genetic drift from founder populations, in the absence of herbivores or pathogens that would control them in their native environments (Richardson M.
D. et al.,2006). A naturally aggressive plant can be especially invasive when it is introduced into a new habitat.
It can also have a huge effect on other native species and biodiversity.Though the topic how invasive species shaped biodiversity was always controversial, the biotic invasion is considered one of the top five drivers for global biodiversity loss (Millennium Ecosystem Assessment 2005). It is believed that invasive species may lead to extinction of the native species via competitive exclusion, niche displacement, or hybridisation with related native species. Several studies had shown invasive species as a direct cause of native species decline (Ricciardi A 2004; Clavero M and Garci´a-Berthou E 2005). But most of these studies had been conducted for a short period of time. Also, a direct positive correlation between native species decline and invasive species dominance doesn’t necessarily mean that invasive species are the main reason for the observed change. Most invasive species take opportunistic advantage of other forms of ecosystem change, such as habitat disturbance, rather than being the drivers of change themselves (Gurevitch J.
and Padilla D.K. 2004). Interestingly the current emerging theories are telling a different story. It seems predation and not competition is the real cause of loss of biodiversity, for example, the introduction of brown tree snake in Guam island almost wiped off the native bird species but nothing happened to the other snake species (Colvin et al.
2005). Another theory states that habitat loss and not invasive species is the cause of this loss. Surprisingly, there are also cases where invasive species have improved the ecosystems as a whole and has contributed to gain in biodiversity.
For example, invasive plants which prevent water runoff and promote sedimentation have converted grasslands to forests and invasive grasses have converted scrublands to grassland. From this discussion we can see that invasive species can be the cause, a mere spectator or anything in between, regarding the loss of biodiversity. The endemic species may face a high risk of extinction due to invasive species. But extinction is a multifaceted phenomenon and are caused due to multiple factors, can’t just because of invasive species. The reason why invading species do well is simply that they have no natural predators in their new environments.
But in many cases, it was observed that invasive species become prey in the new environments and improve the native species in that ecosystems. Below here we will discuss two examples of well distributed invasive species, one plant species and an animal species. Both of these species are of concern in India too.A good example of such an invasive species is Prosopis juliflora, a thorny, shrubby species in the family Fabaceae, a kind of mesquite, which is native to Mexico, South America and the Caribbean. It has become an invasive weed in several parts of Africa, Asia, Australia and elsewhere. Like the others part of middle-east Asia, it has also become an invasive species in the Banni Grassland Reserve, India.
Prosopis juliflora, locally known as ‘gando baval’, was planted in the area to prevent the potential spread of the Rann and to make the drylands greener. Around 1961 it was introduced in Banni by the forest department in an area of 31,550 hectares along the margins of the Rann (ICAR,1977:59). Traits such as tolerance to high salinity and to persistent drought-like conditions, provide P.
juliflora extraordinary success in Banni grasslands where the groundwater is saline and annual rainfall is only 300 mm. In addition, Prosopis seeds exhibit longevity and form seed banks through which recruitment occurs as the conditions become favourable. Prosopis is thought to have allelopathic properties too, which lead to loss of indigenous plants (Asrat G., Seid A.
2017). This too may help the invasion of Prosopis in Banni. Now, P. juliflora covers an approximate area of 1,92,736.38 ha and cover more than 50% of the total Banni area (Sastry et al. 2003). In a separate study by Jadhav and others, they have shown us how P. juliflora is spreading at a rate of 26.
73 sq. km/yr while the grassland area is reducing at a rate of 30.39 sq. km./yr (Jadhav et. al. 1992).
Despite being an invasive species P. juliflora does have some economical value. It has been used for charcoal making and fuel wood due to its high calorific value.