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INVASIVE ALIEN SPECIES


FLORA

For a small island, Sri Lanka is rich in biological diversity. Yet, this natural wealth is under threat from various sources, including invasive alien species. IAS grow rapidly, compete vigorously, push out native species and alter ecosystems. Their impacts are enormous and they have the potential to cause damage to the environment, human health, livelihoods and the economy.
 
 
 
 

Water hyacinth

Eichhornia crassipes (Mart.) Solms

Family: Pontederiaceae

Common name: Water hyacinth (English); Japan jabara (Sinhala)Water hyacinth

Synonyms: Pontederia crassipes

Character identification:

This species is a free-floating aquatic plant. The body of the plant comprises of indeterminate vegetative stems that are very short, producing a cluster of many fibrous roots, and stolons which give rise to new plants at their apices. Leaves are in rosettes bearing large swollen petioles for buoyancy, a terminal inflorescence and numerous roots hanging in the water (Heywood et al., 2007). The deep purple roots have a fine, feathery appearance. When the water is nutrient rich, roots will be shorter and petioles longer, whereas in nutrient poor waters petioles are shorter and roots are longer (Duke, 1983). Flowers are pale lilac coloured, borne on a long spike with two bracts at the base.

Total length: Plant is 30-50 cm high.

Morphologically similar species:

It can be mistaken for the native Diya habarala (Monochoria vaginalis) in Sri Lanka. However, E. crassipes can be easily distinuguished by the presence of swollen petioles.

History and introduction:

Native to Brazil and other South American countries. It was introduced and naturalized in tropical African and Asian countries, including Sri Lanka. It was introduced to Sri Lanka in 1905 as an ornamental plant from Hong Kong. This species spread due to negligence or deliberate introduction to different parts of the country (Bambaradeniya, 2002).

Present distribution:

E. crassipes shows an island-wide distribution in Sri Lanka inhabiting fresh water bodies such as tanks, canals, marshes, ponds etc. (Bambaradeniya, 2002).

Dispersal and reproduction (pollination and seed dispersal):

E. crassipes reproduces both vegetatively and sexually, but vegetative reproduction is responsible for the plant’s rapid spread and colonisation of water bodies. It occurs through the formation of stolons, which produce daughter plants. Propagated mainly by runners (stolons) which produce new plants.

Impact on native species and habitats:

E. crassipes can affect water resources by obstructing navigation routes, and water-flow, which can cause flooding; degrading water quality for domestic and recreational use and causing problems for hydroelectric power generation (Gopal, 1987). Due to its rapid growth rate, the plant can easily overtake the habitat of native species such as M. vaginalis.

Direct exploitation / destruction of native species:

Freshwater environments are affected as the weed grows throughout the year and develops into dense large, free-floating, monospecific islands or mats which compete with other aquatic species for light, nutrients and oxygen (Gopal, 1987). The plants also reduce the temperature, pH and dissolved oxygen content, and increase carbon dioxide content in the water. This oxygen depletion often kills fish, lowers the natural ability of the water body to absorb pollution, and creates septic and odorous conditions. The thick cover blocks sunlight from reaching native aquatic plants, some of which support waterfowl (Duke, 1983). They compete with paddy for light and other nutrients reducing the paddy yield.

The plants provide a habitat and food for several vectors of diseases, like mosquitoes and a species of snail known to host a parasitic flatworm which causes schistosomiasis (snail fever) (Duke, 1983).

Current uses:

Eichhornia crassipes is ideal for compost and is used as fertilizer. In Sri Lanka water hyacinth is mixed with organic municipal waste, ash and soil, and is composted and sold to local farmers and market gardeners (Calvert, 1998). It also can be used as animal fodder (National Academy of Science, 1976), and can also indirectly serve as fish feed (Gopal, 1987). Additionally, it is used for waste water treatment in textile processing (Gamage & Yapa, 2001). There are records of Eichhornia being used as offerings in Buddhist temples and recent observation reported the utilization of fibrous parts of the plant for handicrafts (e.g. in Wahakotte).

Natural threats (pest / predators):

Two species of weevil that are known to feed on water hyacinth were introduced in the 1970s, firstly to the USA (Calvert, 1998).

Prevention and control:

Weevil species Neochetina eichhorniae was introduced to Sri Lanka in 1988 by the Agriculture Department to control this species. However, this species has not been successful in controlling water hyacinth.

Herbicides have been used in other countries to control the plants for many years. This is particularly effective when infestations are small. Mechanical removal can be used as the best short-term solution (Gopal, 1987). Ctenopharyngodon idella (Chinese grass carp) is a fast growing fish which eats aquatic plants. It has also been used in other countries to control the plant. The fish will eat submerged or floating plants and bank grasses. It will eat up to 18-40% of its own body weight in a single day and can be used for weed control (Gopal, 1987; Schoonbee et al, 1984).)

Special notes:

The use of herbicides potentially poses a health risk if people collect water for drinking and washing from the affected area.

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IAS Documentary
Information for the public knowledge about Invasive Alien Species in Sri Lanka...

AS policy Intro
Information about the policies of Invasive Alien Species in Sri Lanka...



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