What is invasive? With ref to native/endemic/alien and invasive
Alien, or exotic or introduced, species are non native species to a region that have been introduced by people, either deliberately or accidentally. If this alien plant is able to reproduce in the introduced region, it is considered a naturalized species. If any of these naturalized aliens are able to spread into and change the systems by displacing the native biota or become a threat to valued environmental resources, they are considered invasive.
Where do they come from
Humans have been deliberately transporting plants and animals from one part of the world to another, whether for food, fibre, fuel, or other purposes. Good examples are crop plants, livestock, plantation trees, and aquarium fish.
Sometimes, species get introduced accidentally, or inadvertently. These could be as hitch-hikers on species that are deliberately introduced (e.g., diseases of introduced plants, weed seeds mixed with imported grain) or as stowaways (e.g. marine organisms in ballast water). Also deliberate introduction of a species for any other purpose without knowing its invasive potential. In most cases, such introductions are unsuccessful. But when they do become established as invasive species, the consequences are catastrophic.
For example Acacia mearnsii (Black Wattle) was introduced to Nilgiris in 1861 as a plantation tree to supply fuelwood and charcoal to European settlements in Ooty and surrounding areas. Black Wattle is now one of the most widely spread invasive plants across the Western Ghats.
Characteristics of invasive species
Invasive species possess similar characteristics to ‘Pioneer Species’ (the species that first colonize new habitats created by disturbance). They can reproduce rapidly and have high dispersal ability.They can adapt physiologically and survive on a wide range of environmental conditions. They can outcompete native species for resources like soil nutrients and sunlight. Invasive species may not have natural predators or competitors present in the invaded ecosystem which will help them to grow uncontrollably.
Why do they pose a threat?
They can threaten native biological diversity. They can have economic impacts. They can also harm human health and wellbeing. Once established, some invasive alien species have the ability to displace the native biota, sever nutrient cycling, disrupt fire regimes, and also modify the pattern of plant succession.
For example Lantana camara, a South American flowering shrub, which was brought to India, via Europe, as a garden ornamental and hedge plant about 200 years ago (in 1809) is known to suppress native plant species, including NTFP that people collect. It has modified native habitats, making them unsuitable for a number of wildlife species. It has increased the risk of severe forest fires. It formed dense thickets, decreased visibility and made forests more dangerous for people by increasing chance encounters with large wildlife.
How bad is it? What does future look like
Invasive alien species are the second most cause for the loss of biodiversity across the world. It is a greater threat than pollution, harvest and diseases combined. In India, it is estimated that we have about 1600 introduced plant species. Of these 225 are invasive plants; 134 more are likely to become invasive in the near future. The damage caused by invasive plants in agriculture and forestry of India was estimated in 2001 to be 91 billion dollar per year, even by leaving aside the management costs and the ecological effects like local species extinction, alteration in ecological functioning and reduction in ecosystem services.
What is the citizen's role?
Before we can manage invasive species, we need good information that would allow us to prioritise species (and habitats) to manage. This information includes answers to questions such as: Which are the invasive alien species of most concern? Where do they occur? How widespread are they? What are the habitats that are most susceptible to invasion?
Such information could come from an atlas of invasive species. This can only be created through participatory mapping, with the involvement of Citizens, Scientists and Forest Department.