Plants and animals adapt themselves to habitats and surroundings but when they are accidentally or deliberately introduced into a new habitat, these non-native species can adversely affect habitats and bioregions ecologically, environmentally and economically and are hence known as alien invasive species.

Characterised by fast growth, rapid reproduction and the ability to tolerate a variety of environmental conditions, these species outgrow local flora and compete for resources such as food, water and sunlight.

When introduced outside their natural range, alien species achieve even higher growth rates in the absence of natural pests and predators and eventually destroy local vegetation. They upset the local ecology either by aggressive competition, by secreting poisonous chemicals into the soil, depleting water supplies or by hybridising with local species. Alien plant species can modify the entire structure and functioning of the ecosystem.

Interestingly, it is the same with animal species. The introduction of rabbits in Australia led to serious ecological ramifications. As the story goes, a dozen rabbits were introduced in 1859 by Thomas Austin on his land to develop a local population of rabbits that would help him continue his hunting hobby that he so enjoyed in England.

Little did he know that rabbits would become one of the most serious mammalian pests in Australia. In 1950, there were estimated to be 600 million rabbits in Australia and vast amounts of money had to be spent to keep their population in check. Conducive weather, abundant food supply and the absence of natural pests, predators and diseases attributed to this dramatic multiplication of rabbits.

Alien species often bring with them disease-causing organisms such as bacteria, fungi and viruses that can impact local species. This is what happened to the American chestnut tree when Japanese chestnut trees were introduced in the late 1800s in the US. The latter carried the parasitic fungus C. parasitica which spread like wild fire destroying billions of American chestnut trees.

In Pakistan, the Prosopis juliflora (mesquite) was deliberately introduced for afforesting deserts in Sindh. Originally found in West Indies and Mexico, the Prosopis has now become highly invasive in Pakistan and is widespread almost all over the country.

Eucalyptus was introduced in Pakistan to deal with salinity and flooding problems and was planted extensively in Punjab and Khyber Pakhtunkhwa. It is now considered a threat to both plant and animal biodiversity since it consumes excessive underground water depriving neighbouring plants of their due water supply. It prevents the growth of other plants not just by the unusually high water intake but also by secreting poisonous chemicals in the soil.

The Punjab forest department has labelled this tree ‘environmentally unfriendly’ and thousands of eucalyptus trees have been cut in Punjab alone. Adequate research and better management could have prevented this human-induced ecological disaster. The fast-growing paper mulberry was imported from China about fifty years ago in a drive to ‘green’ the environs of Islamabad in a short time. It has now become highly invasive and is replacing native flora at an alarming rate. Paper mulberry has a very high consumption of water, which leaves little for other plant species. Not only is it a threat to local vegetation, it is the foremost cause of allergies for residents of Islamabad.

The alien invasive species of Margalla hills include the lantana, common cocklebur, carrot grass, castor oil plant, marijuana and paper mulberry. Most of these species are poisonous or unpalatable, allowing them to proliferate quickly. Carrot grass may cause skin allergies and respiratory problems in humans and livestock.

Concerned with their rapid spread, some concerned NGOs have initiated a programme of eradicating alien invasive plants from the Margallas. This involves cutting the stem, removing the root and then disposing of or burning all parts of the plant. In most cases the soil is treated with a herbicide to destroy any seeds and ensure that the alien invasive does not regrow. In Pakistan, biodiversity protection laws exist but there is a gap between theory and practice, law and its implementation. Sadly, even the CDA is ignorant about the dangers of planting exotic species. Just a few years ago, CDA officials decided to plant exotic palm trees along the federal capital’s main commercial road at a phenomenal cost. These desert growing trees were stuck in the mud in the hope that they would take root and proliferate. However, they died very soon and more time, effort and money had to be spent on removing and replacing them.

Invasive species are the second greatest threat to native species, behind habitat destruction. In the US, alien species have contributed to the decline of over 40 per cent of endangered and threatened species and their estimated damage and control amounts to $138 billion annually.  Similar studies have not been done in Pakistan, but economic costs of the damage caused by alien species would surely run in to millions of rupees.

The government needs to step up its efforts to protect the spread of alien invasive species by introducing tighter controls at ports of entry. Training of appropriate staff is also imperative to ensure that this menace is kept under control in national parks and wildlife sanctuaries.