| Aliens
in Sri Lanka
What is an Invasive Alien Species?
An alien species refers to
any organism, which occurs outside its natural range or dispersal
potential. They are commonly referred to as non-native, foreign
or exotic species. An invasive alien species (IAS) is an alien organism
that becomes established in natural or semi-natural ecosystems or
habitat, is an agent of change, and threatens native biological
diversity.
Why should we worry about IAS?
In fact, IAS are nothing new. We already know them as pests and
diseases of agriculture, humans and livestock. What is new is that
we have recognized them to be a very serious threat to biodiversity
and that they pose one of the greatest biological threats to our
planet’s ecological and economic well being.
The impact
of IAS are immense, not often very obvious but highly significant
economically, socially and ecologically, and often irreversible.
It has been well documented that IAS have resulted in massive and
rapid losses of biodiversity, especially in island ecosystems.
Hundreds of
species extinctions have been caused by IAS. Therefore, the direct
ecological cost of IAS it the irretrievable loss of native species
and habitats. In addition, the direct economic costs of IAS run
into many billions of dollars annually. For instance, most alien
invasive plants function as weeds that reduce crop yields and also
increase cost of control, while some invasive plants contribute
to degradation of catchment areas and irrigation systems, resulting
in huge costs to clear them.
Furthermore,
some invasive animal species function as pests and pathogens of
crops, livestock and forest plantations, resulting in increased
costs to control/eradicate them.
Some factors, which make threats posed by IAS very important to
Sri Lanka are: status; greater diversity of habitats; its revolutionary
history, which has resulted in high levels of species endemism in
the southwestern part of the country; increased degradation and
fragmentation of natural habitats due to development activities;
and the current highly threatened status of many endemic species
and their habitats.
What
processes and activities lead to invasion by alien species?
The introduction of plant and animals species beyond their natural
range is closely linked to the history of civilization. Colonisation
by foreign powers in particular led to massive transoceanic movements
and exposed ecological systems and native species of new stresses
and threats. In the modern era of globalisation, the 4 T’s
– trade, transport, travel and tourism have sharply accelerated
the rate of species’ movements. Establishment of an alien
species that may become invasive is generated by two major categories
of activities.
1.Intentional
introductions :
a)
Deliberate introductions for use in biological production systems
such as agriculture (e.g., Ipil ipil), forestry (e.g., Mahogany)
and fisheries (e.g., Mozambique Tilapia), landscaping (e.g., Velvet
plant) and for recreational (e.g., Rainbow Trout) and ornamental
purposes (e.g., Water hyacinth, Knife fish).
b)
Introduction for use in containment or captivity (e.g., Golden
apple snail, domestic cat, Red-eared slider turtle), from which
there is a known risk of escape or release to the wild due to
negligence.
2.Unintentional
introductions:
Accidental introduction of species through pathways involving transport,
trade, travel or tourism – ‘hitchhikers’ or ‘stowaways’
(e.g., Ship rat, Slugs).
Which species invade?
IAS
are found in all taxonomic groups; they include introduced viruses,
fungi, algae, mosses, ferns, higher plants, invertebrates and vertebrates.
In general,
IAS are opportunists – those that can take advantage of disturbances
to habitats and ecosystems and colonise or expand their populations.
About 1 – 2% of all introduced species are likely to become
invasive.
What
types of ecosystems are susceptible to alien invasive species?
While all ecosystems can be invaded, some are more vulnerable than
others. Ecosystems particularly vulnerable to IAS include :
- Ecosystems
that are geographically or evolutionarily isolated (islands, lakes,
mountains etc.). Availability of vacant, under- or unutilized
niches facilitates the establishment of IAS in these ecosystems.
- Degraded
and stressed ecosystems.
- Inland water
systems.
- Enclosed
marine systems, such as bays and estuaries.
- Agricultural
systems.
What
are the conditions that favour the establishment and spread of IAS?
In general, the conditions that favour the establishment and spread
of IAS can be summarized under external factors and inherent factors.
External factors
include the availability of empty or unutilized niches that occur
naturally or created by habitat destruction/degradation, absence
of natural enemies and diseases, and synergistic interactions with
global climate change. Inherent factors of IAS include their high
reproduction and dispersal capabilities, ability of IAS to tolerate
sub-optimal levels of resources and also to exploit chemical changes
such as pollution/eutrophication.
What
are the alien invasive species in Sri lanka?
Observations made during the past five years have led to the documentation
of 20 species of invasive alien animals and 38 species of invasive
alien plants spreading in natural and semi-natural ecosystems in
the different bioclimatic zones of Sri Lanka.
The invasive
alien animals include 15 species of vertebrates (Freshwater fish
– 9, Reptiles – 1, Mammals – 5) and 5 species
of invertebrates (molluscs – 5).
The invasive alien plants include 2 pteridophytes, 2 monocots and
34 dicots.
Among the total
IAS documented in Sri Lanka, 10 species of fauna and 13 species
of flora are included in the list of 100 of the world’s worst
IAS. At present, the ornamental fish trade and the ornamental plant/horticulture
trade are two major contributory sources to the introduction of
IAS in Sri Lanka.
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