Spelling Doom for Ecology

ENVIRONMENT

To cite an example, the notorious lantana (Lantana camara) – the tropical American shrub – was introduced in India in the early 19th century as an ornamental plant. But it has now come to invade diverse landscape including scrublands and forests. It is estimated that lantana – now counted among the top 10 worst weeds of the world – has come to invade more than three lakh square kilometres of land in India.

What do you mean by invasive plant species?

By invasive plant species, we denote those species of plants which thrive in a new environment, and by doing so, adversely affect the host environment in multiple ways. What is really alarming is that these plant species can withstand the impact of global climate change and substantially alter the ecology of a place with disastrous results. Through invasion of their new habitats, these plant species bring negative effects to the native biodiversity, leading to economic losses and health hazards.

However, all alien plants are not invasive. Some just cannot adjust to their new localities and perish, while some others do not have any negative impact on the environment.

Origin of invasive plants

In most of the cases, alien or exotic plants are first brought to a new habitat for their ornamental, floricultural or agricultural value. But over a period of time, they turn invasive and wreak havoc on the host environment. To cite an example, the notorious lantana (Lantana camara) – the tropical American shrub – was introduced in India in the early 19th century as an ornamental plant. But it has now come to invade diverse landscape including scrublands and forests. It is estimated that lantana – now counted among the top 10 worst weeds of the world – has come to invade more than three lakh square kilometres of land in India. This aggressive invader now poses a serious threat to the native biodiversity in India's protected areas. In fact, lantana has infested half of the tiger reserves in the country. The extent of the invasion can be gauged from the fact that it will take a whopping Rs 40,000 crore to weed out all the lantana in India! Across the world lantana has infiltrated into pasturelands, tea and coffee plantations, and maize and sugarcane fields.

Other factors that contribute to infestation of new territories by invasive plants are travel and trade. Seeds or plant fragments can get attached to clothing or imported goods, and hitch rides to foreign shores. Parthenium (Parthenium hysterophorus), another native of Central and South America, is believed to have been carried to India through wheat imports from the United States in 1956.

Apart from lantana and parthenium, the other species that have made India their home are Siam weed, Mexican devil and mesquite. The water hyacinth is also ubiquitous in the inland water bodies, while the alligator weed has infested both aquatic and territorial habitats in India.

All continents of the world today face the issue of exotic plant invasion. Even the remote and unfavourable ecosystems of Antarctica are not free from this menace. The invasive plant species have several traits which make them 'prosperous outsiders'. They carry higher leaf nutrients and specific leaf areas. Due to short life cycles, they reproduce in double quick time. The invasive plants produce more seeds, and have better dispersal and germination rates.

Disastrous effects of invasive plants

The uncontrolled growth of invasive plant species in forest areas creates favourable conditions for the spread of wildfires. The presence of these plants stops the regeneration and expansion of native species, thereby affecting their natural habitats. This has a negative bearing on the livelihood of people living in the fringe areas of forests and dependent on forest produce.

In India the invasive plant species, which stand at more than 200, are widely distributed in Goa, Jharkhand and the South Indian states. These species thrive in those places which have a combination of sunshine and moisture. They spread to roadsides, agricultural fields and natural landscapes with help of wind, bird poop and wildfires.

Experts point out that the invasive plants outgrow the native flora by occupying the latter's habitats and suppressing the growth of native seedlings. Some invasives also affect the avian species and livestock. For instance, thick lantana growths have been found to cause a decline in diversity and abundance of bird species in the reserve forests of Karnataka. Parthenium is also known to be toxic to livestock. This weed is harmful to man as well as it can cause breathing problems and skin diseases. The invasive plants also lead to economic losses as they run over agricultural and grazing lands.

Controlling invasive plants

Invasive plants are basically unwanted plants. Less euphemistically, they are weeds that need to be removed. Several methods are being used to control the proliferation of these exotic weeds. These methods include cutting, burning and uprooting. Mechanical removal is also resorted to, as in the case of Kerala where the method has been used to clear water hyacinth from water bodies.

Biological control has been experimented in states like Karnataka where the Mexican beetle (Zygogrammabicolorata) was introduced to get rid of parthenium. However, introduction of insects to feed on invasives can prove to be counter-productive. These insects sometimes cause damage to other plant species and can become invasive species themselves. Use of chemicals is also fraught with the risk that it may harm non-invasive plants and animals.

Lately, the concept of 'green bonds' has been mooted to tackle the issue of plant invasion. Green bonds are specifically floated for protection and restoration of environment. Corporates and firms can buy these bonds and earn interest on them. The purpose is to raise money for climate and environmental projects. The World Bank is a major issuer of green bonds. Though the green bonds have not been used for control of invasive plant species so far, experts are of the opinion that they can be a viable option to manage the plant invasion menace.

It is not an easy task to eradicate the invasive plants completely. Apart from mammoth funds, greater awareness among agriculturists, horticulturists and plant nursery owners is required to control them effectively. Managers can go for area-wise interventions to restrict the spread of invasive plants in eco-sensitive and other prioritised areas. Native species-based flora can be promoted as virgin forests or forests having dense canopy are seen not to allow easy penetration of invasive plant species.

At the individual level, we can learn to identify invasive plant species and avoid growing them in our gardens no matter how greatly ornamental they look. If you have pets, it is a good idea to clean them after they go for outdoor walks to prevent seed dispersal through their hair or fur. Cleaning clothes, shoes and travel gear can also help in prevention of spread of invasive weeds.

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