Funday Times

Impacts of Invasive Alien Species

The major impacts of IAS can be summarized under the following categories: 1) Direct exploitation or destruction of native species; 2) superior competitors for resources; 3) hybridization with native species; 4) agricultural pests and other impacts.

Water hycinth

Direct exploitation or destruction of native species:
This is the biggest threat posed by IAS. Most IAS destroys or depletes native species by being active predators or herbivores. In addition, alien invasive fauna can also function as vectors, which transmit diseases to vulnerable native species. Among the invasive alien fish in Sri Lanka, five species are active predators of native aquatic fauna.

The Rainbow Trout occurs in cold streams of the montane zone, mainly in the Horton Plains National Park, which is known to be the sole locality of the endemic shrimp Caridina singhalensis. As the trout is known to feed on aquatic organisms such as crabs, shrimps, insects and other fish, it is possible that it has affected the populations of the above endemic shrimp as well as other endemic crabs in the montane zone streams.

The Clown knife fish is a large voracious carnivore, which feeds on slow moving native fish. Spreading rapidly in the streams, rivers, ponds and marshes of the wet zone, it may have already affected the populations of the endemic fish, which are mainly distributed in this zone.

The Guppy, which is a prolific breeder distributed in aquatic habitats throughout Sri Lanka, has been observed to feed on the eggs of native amphibians.

The recently introduced Red-eared slider is a carnivorous turtle that can pose a serious threat to native aquatic vertebrate fauna. The domestic/feral cat, being a stealthy predator, is a serious threat to native reptiles, birds and small mammals,.

The domestic/feral dogs also pose a similar threat to native vertebrates. It has been documented that domestic/feral dogs avidly search and feed on the eggs of marine turtles, in coastal areas. It is quite possible that some of the alien invasive fish and molluscs also function as vectors of various diseases that affect native fauna.

Giant Sensitive plant (Flower)

Superior competitors for resources:
All invasive alien fauna act as superior competitors of resources and thereby displace native plants and animals. For instance, among the alien freshwater fish species, the Mozambique Tilapia is non-selective in its diet and breeds prolifically, enabling it to colonise tanks, reservoirs and slow-flowing rivers, while displacing some native inhabitants, which are considered highly threatened endemic species in Sri Lanka.

The feral buffalo and cattle pose a threat to native large herbivores such as deer, sambhur and elephant by competing for limited food resources, especially in the dry zone habitats. The wallowing habit of buffaloes in water makes the aquatic habitats muddy, thereby deterring other animals such as elephants, that visit water holes to drink water.

The invasive alien plants form thickets, and shades out and gradually displaces native vegetation. At the extreme level, the invasive alien plants may entirely modify the structure and function of an ecosystem, which could occur in many ways. For instance, the invasive plant species can produce substances that are toxic to other native plant species and thereby make the soil unsuitable for the original native plant communities. The aquatic invasive alien plants that form dense mats (i.e., Water hyacinth and Salvinia) tend to accumulate greater amount of sediment, and this, coupled with high loss of water through increased transpiration ultimately converts wetland into terrestrial habitats. The final outcome is the narrowing down of native biological diversity in a particular locality.

Strawberry guava

Hybridization with native species
The two species of exotic mammals Ship rat and domestic/feral buffalo have become agents of hybridization, where they have interbred with indigenous sub-species. Three sub-species of the ship rat, have been accidentally introduced to Sri Lanka by ships. These have now interbred with the two local sub-species to form mixed populations. Similarly, the domestic buffalo has interbred with the native wild water buffalo, to form feral populations.

Agricultural pests:
Several species of invasive alien fauna function as agricultural pests throughout the island. The Ship rat and the house mouse are major grain feeding pests of rice. The giant African snail and slugs, are pests of agricultural/horticultural crops. The two recently introduced garden slugs are posing a serious threat to vegetable cultivations in the Nuwara Eliya district. It is also reported that farmers are using substantial doses of pesticides to control these garden slug species, without much success. The feral buffalo also cause damage to agricultural crops, mainly in the dry zone of Sri Lanka. The introduced aquatic snail belonging to the Genus Pomacea, commonly called the Golden apple snail, was imported to Sri Lanka during the early 1980's by the ornamental fish traders. Today, it is rapidly spreading in aquatic ecosystems throughout Sri Lanka, as it is a prolific breeder. Recently, the Department of Agriculture was able to identify one specimen of Apple snail as Pomacea bridgsi. However, there seem to be two species of Pomacea that are currently spreading in Sri Lanka, and taxonomic work is in progress now. It would be interesting to know whether Pomacea cannaliculata is present in Sri Lanka, as this species is considered to be a serious pest of rice, causing severe economic damage especially in the Southeast Asian region.

Some species of invasive flora, function as weeds in agricultural ecosystems, and thereby result in economic losses to farmers. These include Parthenium, Water hyacinth, Salvinia, Siam weed, Lantana, Mile-a-minute, Cogon grass, Guinea grass, Koster's curse, T. diversifolia and M.invisa.

Other impacts of IAS:
Some IAS facilitates the establishment and spread of another alien invader or contributes to other environmental disasters. For instance, the feral buffalo and cattle feed on the pods of Mesquite and hence facilitate the spread of the latter alien invasive plant in the arid zone. Furthermore, the disturbances to natural habitats caused by feral buffalo, cattle and unmanaged domestic pigs facilitate the establishment of alien invasive plants. Some invasive alien plants such as Lanatana camara and Chromolaena facilitate the spread of forest fires as well, causing severe damage to native ecosystems and species. Certain invasive alien animals function as vectors of diseases that affect man and livestock. For instance, the ship rat functions as a vector of Leptospirosis virus, while the feral cats and dogs are vectors of the rabies virus.

Are there any beneficial attributes of IAS to native biodiversity?
Some invasive alien plants provide food and cover for native fauna. For instance, the thick Gorse stands in the Horton Plains area serve as a good habitat of the endemic Black-lipped lizard and several amphibians, providing them with food (i.e., insects attracted to flowers) and cover. It is also a nesting habitat of birds.

The invasive alien shrub Eupatorium riparium in the montane region serves as a browsing plant of the sambhur. The berries of Lantana camara are fed upon by several species of birds. The fruits of Strawberry Gauva and Cactus are eaten by langurs and monkeys.

Information from Dr. Channa Bambaradeniya, IUCN - Sri Lanka Country Office
Coordinated by Duminda Sanjeewa Balasooriya

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