Invasive Alien Species (IAS) include any plant or animal that is introduced into an environment where it does not naturally belong. The plant or animal survives and gradually establishes itself in the new environment and multiplies rapidly, replacing the original/ native species leading to biotic homogenisation, replacement of native species and reduction in biodiversity of the region. This leads to gradual deterioration of ecosystems affecting food and water security, health and livelihoods [1].

Invasive species are characterised by their fast growth rate; ability to reproduce in adverse environmental conditions such as extreme heat, pollution; fast reproduction rate and dispersal rates; ability to survive on wide range of food items and their ability to associate with humans. Invasive alien species can be introduced accidentally or intentionally, and can cause considerable harm to natural ecosystems, biodiversity, livelihoods, water and food security and human health and economy [1].

For example, invasive mosquito species like Aedes albopictus that originated in southeast Asia and has now spread to all continents except Antarctica have led to the spread of infectious diseases such as dengue fever, chikungunya, zika and yellow fever [2]. Invasive alien plants such as common ragweed is known to affect human health through pollen causing hay fever, asthma, and allergic dermatitis. Plants like water hyacinth are extremely aggressive and are able to double their growth in 1 to 2·5 weeks, and can affect freshwater biodiversity by decreasing oxygen levels and thus harming native fish populations and lead to deterioration in the water quality of aquatic ecosystems [2].

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How do invasive species enter natural environments

Human migration, demand for food resources often lead to introduction of commercially important exotic agriculture and aquaculture species into the environment. Some of these species can accidently escape into the wild and then evolve as an invasive [3].

Ornamental trade, unscientific aquaculture practices, sport fishing, mosquito control, natural disasters and ballast water can also lead to the introduction of invasive alien species into water bodies. A number of exotic ornamental fish, aquatic plants and other small pet animals are sold in states such as Andhra Pradesh, Chandigarh, Gujarat, Karnataka, Kerala, Maharashtra, New Delhi, Tamil Nadu and West Bengal and these have been found to evolve as invasive [3]. For example, mosquito fish, tilapia, giant gaurami, guppies, platyfish are now being found in the Chalakudy river in the Western Ghats, a biodiversity hotspot [3].

Studies show that construction of dams and river linking and beautification projects are also leading to the introduction of invasive species in freshwater bodies in India [3].

Other examples of IAS include carrot grass that gained entry into India through imported wheat from the USA in the 1950s, the entry of invasive river tamarind that got into India through the social afforestation programmes undertaken by the Government of India, and the proliferation of Mozambique tilapia, a fast-growing fish introduced as a source of income for disadvantaged fishing communities, but which is now replacing the native fish communities in Indian water bodies threatening livelihoods [4].

Coastal ecosystems are also under threat due to introduction of invasive alien species. Coastal ecosystems support unique habitats such as estuaries, lagoons, mangroves, backwaters, salt marshes, mud flats, rocky shores and sandy stretches that house a number of endemic and IUCN threat category species [5]. They also support three Gulfs (Gulf of Mannar (East coast), Gulf of Kachchh and Gulf of Khambat (West coast), and three major reefs atoll, fringing and barrier which support rich coral and fish diversity and several endemic and endangered species [5].

The entry of invasive alien species through ballast water discharge can have the most deleterious impacts on marine diversity. Shipping transfers over 80 percent of the world’s cargo and about 12 billion tonnes of ballast water throughout the global oceans each year. Ballast is defined as any solid or liquid that is brought on board ship to increase the draft and regulate the stability or to maintain stress loads [5].

The marine/ocean water loaded for ballast purposes contains a variety of organisms starting from microorganisms to smaller chordate and aquatic plants. Many of the aquatic species die before they reach their destination because of the stressful ballasting and de-ballasting processes and the environmental conditions prevailing inside ballast tanks that are unfriendly for the survival of the species [5].

However, some of the individuals/ species are able to survive and slowly establish themselves in these new habitats and evolve as an invasive species in due course of time and often outcompete native species and disturb the ecological balance of the native habitats thus threatening the livelihoods of the coastal communities. Natural disasters such as floods, tsunamis and cyclones also play an important role in introducing invasive species in marine habitats [5]. However, there is very little information available on modes of entry of invasive species into marine waters besides aquaculture.

Tragic tale of Erai