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Wijesundara, S. (2010): Invasive alien plants in Sri Lanka. In: I nvasive Alien Species in Sri Lanka – Strengthening Capacity to Control Their
Introduction and S pread (Eds: Marambe, B., Silva, P., Wijesundara, S. and Atapattu, N.), pp 27-38. Biodiversity Secretariat of the Ministry
of Environment, Sri Lanka.
Invasive Alien Plants in Sri Lanka
Royal Botanic Gardens, Peradeniya, Sri Lanka
Invasiveness of a species is generally understood as the ability to spread beyond its introduction
site and become established in new locations where it may cause a deleterious effect on
organisms already exists. There are various definitions to invasive alien species (IAS), which are
described elsewhere in this publication (see Chapter 1). From among the definitions described in
Chapter 1, the information provided in this Chapter is based on the definition for IAS given by
the SCBD (2009), i.e. “species whose introduction and/or spread outside their natural habitats
threaten biological diversity”.
Invasions are a basic characteristic of nature and have occurred ever since life first appeared on
the Earth. Invasion can be considered as an integral part of evolution as any other mechanism or
process. However, the unprecedented and accelerating rate of species invasions caused by the
removal of natural impediments to dispersal, deliberate introductions and ecosystem changes
caused by man have devastated ecosystems and caused enormous ecological and economic
Deliberate plant introductions date back to the early days of human civilization, that the origin
of some of the most useful crops of the world is unknown. For example, according to historical
information, nuts of the coconut palm (Cocos nucifera) have been used even in the 2nd century.
The Department of National Botanic Gardens (DNBG) and the Department of Agriculture (DOA)
have played a major role in early plant introductions. It would be impossible to estimate what Sri
Lanka owes to plants that have been introduced from other countries and successfully
acclimatized. Our principal economic crops, our tastiest fruits, our best shade trees, our most
beautiful flowering trees, our major vegetables and our best fodders have all been introduced
from other countries. Our most venerated tree, the sacred bo tree (Sri Maha Bodhi) at
Anuradhapura, was also introduced to Sri Lanka 2,300 years ago from India (MacMillan, 1908).
However, some of the plants introduced have become noxious weeds or invasive plants. The
Royal Botanic Gardens, Peradeniya played a major role in many such introductions. One of the
best examples is Water Hyacinth (Eichhornia crassipes). That aquatic plant species was first
introduced in 1905 to the Royal Botanic gardens, Peradeniya by Lady Blake, the wife of the
Governor from Hong Kong as an ornamental plant. By June 1912 it was reported from
Wattegama in the central province, in 1914 it appeared near Tangalle in the southern province,
and two years later in Chilaw in the north western province. By 1917 it was found in
Sabaragamuwa Province. By 1930, four ordinances were in action to control this weed, namely
Water Hyacinth Ordinance No. 4 of 1909, Plant Protection Ordinance No. 10 of 1924, Village
Communities Ordinance No. 9 of 1914 and Irrigation Ordinance No. 45 of 1917 (Wijesundara,
1999). Table 1 gives some of the notable IAS introduced to Sri Lanka through the botanic
Table 1. Invasive alien flora introduced through botanic gardens.
Country of Origin
Cape of Good Hope
Types of Invasive Plants
Invasive alien plants can be herbs (e.g. Alternanthera philoxeroides), shrubs (e.g. Cestrum
aurantiacum), creepers (e.g. Wedelia trilobata) or trees (e.g. Prosopis juliflora). They may invade
terrestrial or aquatic ecosystems. Some of these species such as Cuscuta are parasitic
(Wijesundara et al., 2001). According to current knowledge almost all invasive alien plants
reported from Sri Lanka are vascular plants (Angiosperms or Pteridophytes). Although a green
alga, Caulerpa taxifolia has been reported as a sea weed, data on invasive lower plants are
scanty in Sri Lanka.
Invasive alien plants in Sri Lanka
Invasive Alien Plants in Sri Lanka and Their Extent of Spread
A few researchers have previously attempted to compile lists of invasive alien flora in Sri Lanka
(Wijesundara, 1999; Bambaradeniya, 2002; Marambe et al., 2003a). The main drawback of these
attempts is the lack of proper criteria to determine the invasive nature of the listed species.
Others have either documented the spread of several invasive alien plants in a specific locality
(Ratnayake, 2008), or the spread of a single invasive alien plant species in different localities
(Marambe et al., 2000a, 2000b; Hitinayake et al., 2000; Jayasuriya, 2001; Pushpakumara et al.,
2001; Medawatte et al., 2008). Even though there are no accurate information on the degree of
infestation, Amarasinghe and Ekneligoda (1997) reported that about 8,000 ha of rice fields were
infested with Salvinia in 1988. Area invaded by A. philoxeroides was reported to be more than
200 ha of land in the southern province of Sri Lanka (Marambe et al., 2003a). This has now been
reported in the montane areas of the island. too.
Sri Lanka is blessed with a wide array of vegetation types distributed in diverse climatic zones.
Some IAS are specific to certain ecosystems or climatic zones while others are widely distributed.
Some of the common IAS and their current distribution are given in the Table 2.
Table 2. Common invasive alien flora and their distribution in Sri Lanka
degraded forests, forest
up- / low-country
fallow fields, marshy/
lowland wet zone
coastal lagoons, marshes
Open areas in lowland
rain forest edges
island-wide except in
upper montane zone
land in low country
forest edges, open
Open areas in montane
streams, canals, marshes
Table 2. Contd…
Mode of Introduction and Spread of Invasive Plants
Introduction of invasive alien plant species may be deliberate or accidental. Deliberate
introduction of an alien species applies to a collection of plants, which are useful, interesting or
ornamental, and also imported (through institutions such as Royal Botanic Gardens). Such plants
need to be treated with special care and propagated by artificial aid. A plant is said to be
naturalized when, having become independent of artificial aid of any kind, escaped from
cultivation and thrives in a wild state. Some plants, which have been deliberately introduced for
a specific purpose, have escaped from cultivation and become invasive.
dry and intermediate
river banks, fallow fields,
wastelands, dry patina
dry and intermediate
dry and intermediate
Wastelands, dry patina
thorn scrublands, edges
of dry mixed evergreen
forest, sea shore
grasslands, riparian areas
marshes, streams, paddy
Wet, sub montane
montane forests, wet
abandoned paddy fields
Invasive alien plants in Sri Lanka
For example, Salvinia (S. molesta) was introduced to Sri Lanka in the late 1930's as educational
material, but appears to have escaped and is currently a one of the most troublesome aquatic
invasive plants, blocking irrigation canals and water bodies and also invading aquatic ecosystems
and rice fields in the dry and intermediate zones of Sri Lanka (Marambe et al., 2003a).
It is believed that giant mimosa (M. pigra) was introduced in to Sri Lanka in the early 1980s to
strengthen the river banks in the Mahaweli areas. That species has now introduced into other
parts of the country by irrigation water, machinery, river sand used for construction purposes,
and lopping branches with mature pods, as a result of the use of the stems of the plant as fuel
wood by people (Marambe, 2000b).
Many other invasive species are spread by irrigation water. In some instances even the control
measures can cause further spread of the plant meant to be controlled. For example, mechanical
removal of water hyacinth has resulted in its spread due to contamination of the machinery
used for this purpose (Marambe, 1999b). According to Jayasinghe (2004) the parasitic invasive
plant, Cuscuta campestris is distributed by irrigation water in the Mahaweli areas.
Mesquite (P. juliflora) was first introduced by the Royal Botanic Gardens, Peradeniya in 1880. It
is reported that it was introduced to the Hambantota district in southern province of Sri Lanka in
early 1950s to improve saline soils, and as a form of ground cover (Algama and Seneviratne,
2000). Prosopis juliflora has become a very serious invasive plant threatening the ecosystems in
Bundala National Park, a Ramsar wetland site in southern province. The seeds of this plant are
dispersed by cattle as well as by elephants that eat the pods.
One of the best examples for accidental or non-deliberate introductions of an invasive plant is
the Congress weed (P. hysterophorus). This plant was believed to have entered the northeast of
the country in the late 1980s, through goats imported from India by the Indian Peace Keeping
Force (IPKF). Seeds of P. hysterophorus have also entered the island along with seeds of onion
and chillies imported from India as a contaminant. Since the colonial times many plant species
introduced into the island have become invasive. Several IAS have been reported during the last
few decades. Some plants show invasive characteristics after a considerable time since the first
introduction. For example, the Guinea grass (P. maximum) was not considered a weed in 1908
(MacMillan, 1908) even though it is believed to have been introduced around 1801-1802
(Wisumperuma, 2008). Similarly, there may be some species which may become invasive in the
future. A list of such species with invasive potential is given in Table 3.
It has been reported that climate change could result in both range expansion and contraction of
invasive plants (Bethany et al., 2009; Kriticos et al., 2003; Dukes et al., 1999). Although there are
no quantitative ecological studies comparing the population sizes at different intervals, some
invasive species such as Austroeupatorum inulifolium, Clidemia hirta and Dillenia suffruticosa
have shown a marked increase in population sizes during the last three decades in Sri Lanka. It
was also observed that Pinus caribaea, a plantation tree species, which did not regenerate
naturally before, has shown invasive behavior (Medawatte et al., 2008) in the Knuckles forest
region. Whether these are due to the changes in climate needs to be investigated.
Nature of Threats
The IAS can cause significant changes to ecosystems, disturb the ecological balance, and cause
economic harm to agricultural and recreational sectors. These IAS compete with native species
for space, resources and plant-animal interactions. Few researchers have documented the
harmful impacts of invasive alien flora on native biodiversity in Sri Lanka.
Table 3. A list of potential invasive alien plant species.
Myriophyllum aquaticum (Vell.)
Montane zone. Native of the Amazon
River in South America. Only
reported in Gregory’s Lake at Nuwara
Eliya Lake (Sujith Ratnayake,
Biodiversity Secretariat, Ministry of
Environment (ME), Sri Lanka - Pers.
Com.). Invasive in many countries
including USA, Pacific Islands, Japan,
New Zealand, South Africa and
Mayaca fluviatilis Aubl.
Wet zone. Ornamental aquatic.
Escaped into some water bodies in
the western province near Gampaha.
Invasive in some countries.
Tibouchina urvilleana Cogn.
Montane zone. Native to Southern
Brazil. Small population in Horton
Plans near Anderson Bungalow.
Invasive in Hawaii and some pacific
Ludwigia sedoides (Bonpland) H.
(False Loosestrife, Mosaic Plant)
Wet zone. Native to South America.
Ornamental aquatic. Escaped into
some water bodies in the western
province near Gampaha. Invasive in
Panicum trichocladum K. Schum.
(Donkey grass, creeping guinea
Wet zone. Native to Africa. Probably
imported as a pasture grass.
Spreading in Hanguranketha area.
Observed around 2002.
Setaria barbata Kunth.
(bristly foxtail grass)
Wet zone. Native to Africa.
Spreading in the mid country.
(Philippine fireworks, Valentine
All zones. Native to Phillippines.
Ornamental plant grown in many
parts of the country. Invasive in the
Cissus rotundifolia (Forssk.) Vahl
(Arabian wax cissus)
Dry Zone. Native to Africa. Cultivated
ornamental. Invasive in some
The flowers of C. aurantiacum are pollinated by an endemic bird, the Sri Lanka White Eye and
the fruits of this species are dispersed by another endemic bird, the Yellow eared bulbul. Due to
the abundance of Cestrum plants in the forest fringe, these two bird species are now found
mostly in those areas and the pollination and dispersal of native species in the montane forest
may be affected by the altered feeding habits of them.
Invasive alien plants in Sri Lanka
The spread of P. juliflora in the Bundala area has deprived large mammals such as elephants of
important habitats. It is now spreading in the lagoon shore areas of Bundala National Park,
reducing the feeding area for wading birds (Bambaradeniya et al., 2002).
As documented by Bambaradeniya et al. (2006), a rapid increase in the spread of Thorny cactus
(O. dillennii) in the coastal scrubland and seashore habitats in Tangalle and Ambalanthota areas
subsequent to the December 2004 tsunami has resulted in the loss and/or deterioration of
nesting habitats of globally threatened marine turtles that visit these areas annually. The spread
of the invasive cactus has also hindered the regeneration of coastal vegetation destroyed by the
tsunami, such as Pandanus odoratissimus, Scaevola takkada, and Spinifex littoreus
(Bambaradeniya et al., 2006).
As stated in the 2007 National Red List of Threatened Species, the spread of invasive alien plants
such as Annona glabra, Dillenia suffruticosa, and Eichhornia crassipes has led to further
degradation of the remaining marshy habitats of threatened blind eels (Monopterus desilvai and
Ophisternon bengalense) in the western province of Sri Lanka (IUCN and MENR, 2007).
According to the research findings of Gunaratne et al. (2008), the preferred habitat types of
native aquatic birds such as the Little grebe and the Pheasant-tailed Jacana are adversely
affected by the spread of S. molesta and E. crassipes in tanks and reservoirs. Similarly,
Weerakoon and Athukorala (2008) have stated that the feeding habitats of globally threatened
spot-billed pelicans in Sri Lanka are adversely affected by these aquatic invasive plants.
A large area in the natural montane grassland in Horton Plains was cultivated with potato in
1961 to 1978 period. The cultivation was abandoned in late1970s. Those areas were later
invaded by exotic, pasture grasses such as Pennisetum clandestinum, P. thunbergii and Vulpia
bromoides escaped from the nearby cattle farm in Ambewela. The original montane grassland
vegetation in the Horton Plans comprised of tussock grasses such as Chrysopogon, Cymbopogon
and Andropogon. The change in physiognomy and composition due to potato cultivation
followed by invasive alien grass species benefited the local population of sambhurs. The
presence of nutritious pasture grasses and the increased safety due to absence of tussock grass
cover for predators (leopards) may have caused the apparent increase in sambhur populations
in Horton Plains (Wijesundara, 1997). On the other hand, lack of tussock grasses may have
negatively affected the survival of leopards that used them for cover in hunting sambhur.
As Sri Lanka is an island with sensitive ecosystems consisting of a unique biodiversity it is very
important to mitigate factors affecting regeneration of species. Impact of IAS on pollination and
dispersal of native species is worth studying. Data on the statistics of distribution or economics
of the damages due to IAS in Sri Lanka is scarce. The goat weed or white weed (Ageratum
conyzoides) is said to have cost the planters ₤ 250,000 a year to control it during the time of
coffee cultivation (MacMillan, 1908).
Control and Management Strategies Adopted
Control and management of IAS need a strategic approach that encompasses prevention,
eradication, control and containment. Several legislative provisions have been enacted in Sri
Lanka (see Chapter 6 in this book). These legislative acts and ordinances can be used to prevent
and control introduction of IAS to a considerable extent. However, there are many areas to be
improved in accordance with the current needs.
In 2007 (MENR, 2007), as a part of the Addendum to the Biodiversity Conservation in Sri Lanka -
a framework for action, the Ministry of Environment (ME) of Sri Lanka has proposed to
formulate a National Action Plan for the Control of IAS in protected areas. A National Invasive
Species Specialist Group (NISSG) has also been proposed to be appointed to deal with the issues
related to the alien invasions. The Biodiversity Secretariat of the ME has conducted many
awareness programmes to educate the general public on the adverse impacts of IAS. Research
on biology, impact and control of IAS is also receiving attention. About five national level
workshops and symposia have been conducted on IAS in Sri Lanka since 1999, where useful
information related to management and control of IAS were presented (Marambe, 1999a;
Marambe, 2000a; Kotagama et al., 2001; Ranwala, 2008; MENR, 2009). Marambe et al. (2003a)
presented a draft national list of IAS, and recently, Marambe (2008) has highlighted the research
priorities on IAS.
The ME, together with the DOA, implemented a one year project on management of aquatic
weeds in 2005/2006, with funding from the FAO. This project involved awareness raising
activities and pilot scale control programmes targeting S. molesta, E. crassipes and P. stratiotes.
The Department of Wildlife Conservation (DWLC), in collaboration with the private corporate
sector has implemented a programme to manage the spread of P. juliflora and O. dillennii in
Bundala National Park.
The DOA is engaged in a biological control programme for S. molesta using the weevil
Cyrtobagus salviniae. This has been successful and it was introduced into many parts of the
country, especially in the low-country wet zone. This species has been declared as a serious pest
under the Plant Protection Ordinance. The DOA is also rearing the bio-control agents Neochetina
eichhorniae and N. bruchi and releasing the organisms as a measure to control E. crassipes.
Recently, the DOA launched a chemical control programme for E. crassipes in the northwestern
province in collaboration with the Irrigation Department. The DOA, together with ME,
universities and other governmental, non-governmental and private organizations is actively
involved in programmes to control P. hysterophorus. An extraordinary gazette notification was
released by the government of Sri Lanka in December 2000, prohibiting the movement of
materials contaminated with any part of P. hysterophorus from the infested areas (Marambe et
al., 2003a). Marambe et al. (2000b; 2003b) also attempted the control of Mimosa pigra by using
glyphosate, which is a total killer systemic herbicide.
Many NGOs, in collaboration with the Ministry of Environment and Natural Resources, are
actively involved in campaigns to eradicate IAS. Some of the programmes conducted to
eradicate IAS have become complicated due to the interactions with wildlife. For example, the
eradication programmes on U. europaeus, were aborted due to the fact that endemic lizards and
amphibians seek protection from their natural enemies in this thorny plant (Bambaradeniya et
al., 2001). Programmes conducted by the DWLC to remove L. camara from the Uda Walawe
National Park were also not successful due to various practical difficulties.
In some instances the control measures are affected by conflicts in the policies. For example, the
Forest Department is reported to promote planting C. rosea in the Knuckles reserve as a fire
prevention measure (as a biological barrier). Similarly the DWLC has been promoting the
planting of P. maximum as a food source for elephants.
Invasive alien plants in Sri Lanka
Many alien plants that have been introduced deliberately or accidentally have become invasive
as they have escaped human management. The measure adopted to control them have mostly
being through a piece-meal approach rather than being comprehensive. Early detection of and
rapid response to new invasive alien plants (species), identifying the factors that may influence
their spread, and application of control strategies requires a well coordinated participatory
approach. Several strategies, which are discussed below, could be adopted to achieve effective
control of invasive alien plants (or IAS in general) to minimize their impacts on the ecosystems of
A central coordinating committee (CCC) including all relevant stakeholders such as the
government departments and institutes dealing with wildlife, forests, biodiversity, agriculture,
botanic gardens, customs, marine environment protection, coast conservation, and scientists
from the universities, should be established under the ME. The CCC should coordinate existing
and future efforts on prevention and control of IAS. It could also enhance information exchange
among scientists, government and non governmental agencies, and private sector. Integration of
university`-based research to optimize management and prevention activities, effective
communication techniques for wider and more effective delivery of public education about
biological invasions would be some of the important role plays of the CCC to effectively deal
with issues related to IAS.
Geographical distribution and areas of spread needs to be recorded for all known IAS in Sri
Lanka. This information could be collected at grass root level using Global Positioning System
(GPS) and Geographic Information Systems (GIS). Up-to-date information needs to be collected
using the Grama Niladharis and other officers at village level and at Divisional Secretary level. All
data should be recorded and updated in a central data base maintained at the CCC. Lists of
potential invasives and information on their invisibility, vulnerability of sensitive ecosystems and
environmental factors that affect spread of IAS need to be maintained at the CCC.
As discussed elsewhere in this book, there are many gaps in the knowledge of IAS in Sri Lanka.
Some of the problems could be solved by conducting appropriate research, where it becomes a
necessity to improve facilities in the universities and other relevant institutions involved in IAS
research. Capacity building of scientists and the necessary infrastructure facilities should be
provided to the relevant institutions.
One of the main obstacles to IAS control is the lack of access to current knowledge. In many
developed countries Invasion Ecology is given a special attention. It is essential to facilitate local,
regional and international co-operation on information exchange through workshops, symposia,
seminars, public lectures, exhibitions, poster campaigns at local level and supporting the local
scientists to participate and get involved in regional and international events related to IAS.
Transboundary collaborations with relevant international knowledge networks such as the IUCN
Invasive Species Specialist Group (ISSG) and the Global Invasive Species Programme (GISP) would
also enable to share and acquire new knowledge on IAS.
Programmes such as press conferences, TV and radio documentaries, and publishing informative
articles in the print media at the national level for building awareness among the stakeholders
should be designed and carried out by the CCC. There can also be seminars, workshops,
exhibitions, poster campaigns, and public lectures at local level. Universities, agricultural schools
and technical colleges can also be encouraged to accommodate IAS in their curricula at an
The main obstacle for capacity building and infrastructure development in any discipline is lack
of funding. There should be a mechanism to channel adequate funding for human resource
development and improving infrastructure facilities at relevant institutions. It can also be helpful
if the leading research organizations such as the National Science Foundation (NSF), Council for
Agricultural Research Policy (CARP) and National Research Council (NRC) recognize IAS as a
priority area for providing research funding. The relevant private sector organizations should
also be encouraged to initiate an endowment for IAS research.
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