Kerala's plant biodiversity faces a severe threat from 89 alien invasive species, which were recorded in a survey commissioned by the Kerala State Biodiversity Board.

Of these, 19 present a high risk; many were found displacing and destroying a large number of native species, causing environmental and economic loss.

Around 40 per cent of the varieties belonging to Brazil, Trinidad, Costa Rica, Chile, and Mexico were believed to have reached the State mostly through timber and food grain imports, said K. V. Sankaran, director of the Kerala Forest Research Institute, Peechi, Thrissur, which conducted the survey and risk assessment.

The list comprises 11 trees, 39 herbs, 24 shrubs, and 15 climbers. The high risk species include Acacia mearnsii (Black wattle), Antigonon leptopus (Mountain rose), Arundo donax (Giant reed), Chromolaena odorata (Siam weed), Ipomoea cairica (Kolambipoo), Mikania micrantha (American vally, Kaipu vally, Dhritharashtra pacha), Mimosa diplotricha var. diplotricha (Anathottavadi), Prosopis juliflora (Sali) and Sphagneticola trilobata (Singapore daisy).

While some species were brought for agriculture and forestry, some others inadvertently reached the State. With the years, they have established and spread, displacing the natural vegetation, including medicinal plants, and reduced the availability of fodder, researchers say.

As part of the survey, around 4,000 points were identified for generation of data, and each point was selected on the basis of the presence of plants with visually aggressive growth. The species thus listed were checked against the catalogue of the native flora. The invasive plants were then subjected to the Invasive Species Risk Assessment, as per the Invasive Species Assessment Protocol developed by NatureServe, Virginia, U.S., said T. V. Sajeev, who led the field studies.

The research team also included T. A. Suresh, R. R. Ragesh and K. K. Subin.

The listed plants were at various stages of invasion and colonisation, and different strategies were required for the management of each. Sesbania bispinosa and Senna siamea have started spreading, and they were noticed only in a few localities. However, Hypoestes sanguinolenta and Heliconia psittacorum have started reproducing. Mimosa diplotricha var. diploticha and Spahgneticola trilobata have established satellite populations. Chromolaena odorata and Lantana camera have started naturalisation, Dr. Sajeev said.

Pollinating insects usually preferred these species as they produced more pollen grains and nectar than the native ones. The resultant fall in the pollination rate of the native plants would affect the local biodiversity and its regeneration. Dr. Sajeev pointed to earlier reports of pollen grains of Acacia and Parthennium having caused allergy among humans.

Dr. Sankaran reckons that the quarantine measures at sea and airports should be made stringent to control the arrival of invasive varieties. Imported timber should be treated with pesticides as the wood would carry seeds and eggs of plants and insects. A large number of countries resort to such measures for protecting their biodiversity.