Alien species (IAS) have been introduced or spread outside their natural past or present distribution. They are also called introduced species.
Invasive alien species are plants, animals, pathogens causing economic or environmental harm or adversely affect human health. In particular, they adversely affect biodiversity, including decline or elimination of native species - through competition, predation, or transmission of pathogens - and the disruption of local ecosystems and ecosystem functions.
Since the 17th century, invasive alien species have contributed to nearly 40% of all animal extinctions for which the cause is known (CBD, 2006) .
Why Do Alien Species Become Invasive?
Invasive plants have a few things in common:
They lack enemies, in that not many animals will eat them and few diseases affect them in their new environment
They produce seeds or berries prolifically, which are often bird-dispersed
They are “generalists,” meaning that they can grow in many different conditions
They grow quickly
Characteristics of Invasive Species
Rapid reproduction and growth
High dispersal ability
Ability to survive on various food types and in a wide range of environmental condition
Ability to adapt physiologically to new conditions (phenotypic plasticity) .
Ability of invasive plants comes from inherent genetic variation rather than new mutations.
Adaptability of invasive plants lies in their quick genetic changes in a new environment, rather than the species inherent ability to tolerate many conditions.
Key Facts on ZSI List of 157 Alien Invasive Animal Species
Of total 157 species:
58 are found on land and in freshwater habitats- 19 species of fish, 31 species of arthropods, 3 of molluscs and birds, 1 of reptile and 2 of mammals
99 are found in marine ecosystem: Genus Ascidia accounts for maximum number of species (31) Followed by:
Arthropods (26)
Annelids (16)
Cnidarian (11) Bryzoans (6)
Molluscs (5)
Ctenophora (3)
Entoprocta (1) .
Important Alien Species
Achatina fulica (African apple snail) :
Most invasive among all alien fauna in India.
This mollusc was first reported in Andaman and Nicobar Islands.
Now found all across country threatening habitats of several native species.
Paracoccus marginatus (Papaya Mealy Bug) :
Native to Mexico and Central America.
Believed to have destroyed huge crops of papaya in Assam, West Bengal, and Tamil Nadu.
Phenacoccus solenopsis (Cotton Mealybug) :
Native of North America.
Severely affected cotton crops in the Deccan.
Pterygoplichthys pardalis (Amazon sailfin catfish) : It is responsible for destroying fish populations in the wetlands of Kolkata.