REFERENCE Mohammad Zayauddin’s article ‘New aquatic weed surfaces in Thatta’ ( March 7, Business and Finance pages), reporting the presence of aquatic invasive species salvinia molesta (kariba weed, aquatic fern) near Gharo, Thatta district. This invasive species is not a new introduction to our area. I had first reported this species from Keenjhar Lake in 1999 in a scientific article, and later in the press in 2001.

Actually, it has been around at least from the early 1990s, as I had seen its dead specimens along the dried-up margin of Keenjhar Lake for several years before I finally found living material and identified it.

Over the past 20 years I have widely travelled in southern parts of the country, particularly in Thatta and Sujawal districts, for vegetation surveys and inventorying of plant biodiversity. I have seen Salvinia molesta quite widespread in lakes and canals of these districts, covering the entire width of quite large canals in some cases.

While its infestation is particularly heavy Sujawal district canals, the species is nevertheless present in the Chotiari reservoir (Sanghar district) as well. Another invasive species, Eichhornia crassipes (water hyacinth), is also usually present at most of the places where Salvinia molesta occurs. Fortunately, any of these species has not yet reached any water body in Balochistan.

Salvinia molesta, a native of Brazil, is undoubtedly one of the most dreaded invasive species the world over. Initially it had invaded the freshwater bodies of tropical and subtropical regions in the world; but in recent years it has spread in many temperate areas as well. It causes a number of problems, including impediment in the flow of water and loss of native flora and fauna as its aggressive growth not only outcompetes the native plant species, but its thick mats also kill aquatic fauna by asphyxiation.

It is also known to harbour certain vectors of human pathogens. Both Salvinia molesta and Eichhornia crassipes cause heavy loss of water through transpiration while both being mat forming species effectively accumulate and bind large amounts of silt.

Both these processes collectively can convert a water body into a piece of land if left uncontrolled.

Till now, there does not appear to be any comprehensive programme in place to control these invasive species — except for their occasional removal from the Keenjhar and Haleji Lakes.

Prof (Dr) Surayya Khatoon

Department of Botany

University of Karachi

Published in Dawn, March 12th, 2016