Around 2 million years back, the early humans started migrating from African continent carrying along with them, parasites, pathogens and domesticated animals to other places around the world. Thus, modern humans colonized the entire planet and can be considered amongst the most successful invasive species of all time.
Invasive species also called as alien species are non-native species of plants and animals that invade or are deliberately introduced in an area. Such species may modify or disrupt the ecosystem that it colonizes. All non-native species are invasive for example food crops like rice, wheat and tomatoes grown in the United States are not native to the region. Human activities always form a major pathway for species of microbes, plants and animals to invade new habitats.
Every species on this planet is adapted to a particular environmental condition. A species can only thrive in a new surrounding when it possess evolutionary adaptions for that environment or it has built-in competitive advantage over the native species, i.e it will compete with the native species for the same resources and eventually win, and if the new surrounding lacks natural predator of the species then they are likely to survive easily. Invasive species have a high rate of reproduction, shorter life cycles, they produce more seeds, better dispersal techniques and germinate very easily. All of these conditions can make any species successfully invade a new area.
Some species are often introduced deliberately as a form of pest control. These introduced species multiply too quickly and become invasive. For example in 1945, five cats were introduced to Marion Island, a part of South Africa in the southern Indian Ocean. The cats were introduced as pest control for mice. By 1977, about 3,400 cats were living on the island, endangering the local bird population.
Invasive species thrive because they compete with native species for food source. Bighead and silver carp are two large species of fish that escaped from fish farms in the 1990s and are now common in the Missouri River of North America. These fish feed on plankton, tiny organisms floating in the water. Many native fish species, such as paddlefish, also feed on plankton. The feeding cycle of the paddlefish is however slower than that of the carp. This has resulted in less food for paddlefish and more for carp thereby decreasing the population of paddlefish. Invasive predator species will outnumber the prey population and thus eliminate them from the ecosystem which further disturbs the balance in nature. Many times an invasive species may also serve as vectors for various diseases spreading parasites and pathogens in new areas. They cause huge economic losses when they invade agricultural and pastoral lands.
Water Hyacinth created a havoc in West Bengal. Also known as ‘Bengal terror’, ‘blue devil’, and ‘noxious species’, it was mainly introduced in our country as an ornamental plant but soon made its way to almost every water body. It directly affects irrigation, hydroelectric generation and drastically reduces the fish population and production of aquatic crops like chestnut, lotus. It also increases the risk of disease conditions caused by mosquitoes. Known for its beautiful violet flowers, it literally chokes the life out of lakes, ponds and clogs waterways.
Another commonly seen invasive species in our country is Lantana camara. Introduced in India around 1805-1810 by the British colonizers as an ornamental plant, Lantana has successfully invaded most parts of our country today. It is proven that Lantana prevents the growth of native species of plants on which herbivores thrive. Herbivores cannot consume Lantana because of certain chemical properties, this has further led to human-wildlife conflict in some parts of our country. Lantana infested habitats have seen a specific decline in the population of birds which feed on ground like Pheasants, tree pie, doves etc. It also damages the soil organisms and repel the insect fauna that forms the diet of many bird species.
Prosopis juliflora is another top invader in our country. It was introduced in India to serve as a fuel wood requirement in the rural areas. A recent study has shown that apart from threatening the local plant species this tree is also affecting the nesting success of birds.
Parthenium hysterophorus also known as congress grass is a highly invasive herb. This alien has invaded almost all Indian states, which has resulted in yield losses of up to 40% in several crops and 90% drop on forage production. It causes health problems too. Direct/indirect contact can cause skin allergies in people. This grass is toxic for livestock too.
Though there has been a confusion regarding the exact number, a 2017 study states that more than 200 invasive species occur in India. This makes our country one of the region with the highest number of invasive flora. Siam weed, Mexican devil (Ageratina adenophora) and mesquite (Prosopis juliflora) are other notorious invasive species in India.
Can Invasive species be removed? Yes they can be eradicated by mechanical methods like cutting, burning or uprooting. Water hyacinth is removed from water bodies mechanically. Biological control is also used in several states of India eg: Mexican beetle (Zygogramma bicolorata) that feeds on Parthenium. However, these methods have met little success. To prevent the seeds from dispersing and further germinating, you can clean your clothes every time you go out and even clean your pets regularly. Awareness is needed amongst agriculturalists, horticulturalists and people who do gardening or even own nursery, about the affects of invasive species and the problems caused by them.
