Exotic species are those plant species, which are not native to a particular area. These species have been introduced by human activities to a location where they do not naturally occur and so they are termed "exotic", "non-indigenous", "non-native" or simply "alien". The exotics threaten to alter the natural composition of forest; they restrict, prey on or compete with native populations and disrupt the integrity of site. They are major ecological threat. Some research suggests that the plant inhibits the growth of other plants in its vicinity.
When these non-native species cause ecological problems, they are termed as "invasive" or "harmful exotic species". Many times they turn out into a weed. Such species primarily invades disturbed habitats, degraded forests / thickets and left out or fallow fields crowding out native vegetation of upland forests, forming single species stands. For Example: In India, once introduced, the species like Lantana camara (Tantani) becomes invasive and slowly suppresses the growth of original vegetation. Once it starts flowering and fruiting, the seeds spread over a large area and germinate very fast. Then it becomes very difficult to eradicate them. Some of them are prolific seed producers. One can observe many such examples like Australian Acacia, Gliricidia spp. and Leucaena leucophyla (Subabhul). They start growing easily from the cut stump.
Monoculture of such trees spoils the natural look of the area whereas indigenous trees easily suit in the frame and start healthy processes. Native plants provide shelter and food for wildlife. They attract a variety of birds, butterflies, and other wildlife by providing diverse habitats and food sources.
Again the composition of Indigenous plants varies region wise, for example, plants occurring naturally in the high rainfall area can not sustain in dry region. So "Any indigenous" is not a sufficient criterion for plantations. Species chosen must be site specific and ecologically appropriate to that particular area.
Still there are some common trees all over India which are easy to plant and readily available in the market: