Invasion Of Salvina Molesta (kariba Weed) Essay, Research Paper

Introduction

The plant Salvinia molesta , generally known as Giant Salvinia or Kariba weed, is a free floating pterophyta indigenous to south-east brazil (Julien et. al 1987). Today plants and animals are shipped, sometimes accidentally, all over the globe to zoos, botanical gardens, and for commercial and agricultural purposes. A small minority of ?alien? plants, or introduced, species become ecological as well as economic disasters. Although they may have been innocuous in their native region, these species are transformed into aggressive pests or weeds that invade and dominate their new environment (Barrett 1989) Nowhere are these biological invasions more evident than in the rivers lakes and reservoirs of the world. Giant Salvinia, one of the worst aquatic weeds in the world, exemplifies this problem. It is notorious for it?s ability to colonize large areas of water in a short space of time. The explosive growth of these plants has caused difficulties in North America, Africa, Asia, and Australia

Discussion

Salvinia is a sterile floating fern native to South America (Julien et. al 1987). The basic morphological module of Salvinia is a ramet, each of which consist of a segment of rhizome bearing three distal leaves and , initially, an apical bud and a lateral bud which both have the potential to grow new ramets (Room 1988). True roots are absent and a one of the leaves is modified and hangs down into the water functioning as a root (Julien 1988). During early stages plants are smaller and leaves lie flat on the water surface. As plants grow, leaves curl at the edges in response to self competition (Jacono 2000). Mature plants can produce large quantities of sporocarps that contain many sporangia, yet plants are functionally sterile. Spores, only rarely found, are deformed and infertile, the result of improper chromosome pairing in this pentaploid species (Mitchell and Thomas 1972). As the oldest ramet in a colony becomes senescent, its rhizome segment breaks and the colony fragments into daughter colonies of unequal sizes. It is this mode of clonal propagation that help the weeds grow rapidly over large areas. As wind or water currents disperse fragments, colonies can expand rapidly over vast open stretches on water surface due to the floating nature of Salvinia (Barrett 1987). Freed from competition with other plants and guaranteed almost unlimited space, nutrients, and sunlight, the kariba weed grow and multiply at an extraordinarily pace to achieve some of the highest biomass production in the plant world. Salvinia has been know to double in size in as little as 2.2 days (Thomas and Room 1986). It can quickly cover lakes and slow-moving rivers with mats up to 1 metre thick. Factors which promote the formation of permanent mats appear to be the same as the plant?s native environment and in situations where it is a troublesome invading species. They require good light conditions and the optimal temperature for growth is between 25-30 degrees Celsius. The size and growth of these plants ultimately depends on nutrients. If considerable quantities of nutrients, especially nitrogen, are supplied, growth can be very rapid and dense mats are soon formed (Mitchell and Thomas 1972).

It is this growth rate of Salvinia that has caused such horrendous ecological and economical difficulties around the world. Thick mats of salvinia prevent the passage of large diesel-powered boats and even a single layer of plants is a major obstacle to paddle-powered canoes. As a result, transport by water and commercial and recreational fishing can be severely impeded or halted. Mats block access to drinking water by humans, domestic stock, and wildlife, clog irrigation and drainage canals, and during floods, sweep fences and other light structures before them (Thomas and Room 1986). The plant is a major weed of rice and harbors the host of human diseases such as schistomiasis (Thomas and Room 1986). By cutting off light to submerged plants, salvinia mats depress oxygen concentrations and increase those of carbon dioxide and hydrogen sulphide in water beneath, which can lead to extinction of floral fauna (Caffrey et. al 1996). The decaying Salvinia drops to the bottom of water, greatly consuming dissolved oxygen needed by fish and other aquatic life (Thomas and Room 1996). Animal Habitat is most noticeably altered by the obliteration of open water. Migrating birds may not recognize or stop at water bodies covered with giant Salvinia. This has led to economic hardships for waterfowl guides that lease the land for hunting. Local fisherman in North America have found it impossible to cast into smothered lakes and are abandoning spots once fished for bass, crappie and sunfish. Nowhere have the adverse socioeconomic effects of salvania been more serious than in the floodplain of the Sepik River in New Guinea. A few plants introduced in the 1970?s led to an infestation in the early 1980?s , in which all of the 500 km2 of lakes were affected with Salvinia covering 250 km2 of water surface. Salvinia became a major threat to the 80,000 people living in the area, which is devoid of any transportation due to annual flooding. The staple carbohydrate is obtained from sago palms and the use of canoes is vital for harvesting the trunks and towing them to villages. The movement of canoes is also essential for fishing, access to markets, schools and medical assistance (Thomas and Room 1986). Another area that was crippled by Salvinia was in Malaysia where the rice granary was invaded by Salvinia. The irrigation canals were clogged with floating Salvinia and consequently blocked the water supply to the rice fields. This led to huge hardships for the local people. In North America they are identifying with the same problem of difficulties with agricultural irrigation. In 1956 the cost of the weed to U.S. agriculture was estimated at $43,000,000, and this has likely dramatically increased (Mitchell and Thomas 1972).

There are a few reasons why Salvinia has displayed such an aggressive behavior outside its native South American range. One of the reasons is that plants and animals usually populate their native environments at densities commensurate with their ecological role in a balanced community; but when species are introduced to another part of the world, they leave behind the co-evolved competitors and enemies that normally keep their populations in check. The absence of natural enemies in the nonnative range allows alien populations to increase rapidly and leads to ?ecological release? (Raven 1999). Another key reason may be found investigating the outbreak of Salvinia in Malaysia during the 1970?s. At the time more drainage and irrigation canals were being built, dams being constructed across the rivers and changing of rice practice from the traditional transplanted method to direct seeding method was taking place particularly at Muda rice granary area. The increased usage of fertilizers resulted in the leaching of a certain amount of nutrients into the aquatic ecosystems. A fertilizer that is generally rich in phosphate and nitrate plays an important role in triggering the tremendous growth of floating weeds (Caffrey et. al 1996). A noticeable feature of rice fields that depend heavily on the use of fertilizer is the occurrence of massive infestation of floating plants such as Salvinia (Oliver 1993). Recently outbreaks of salvinia have been observed throughout the Southern United States. (Jacono 2000). It has been found in water bodies and freshwater drainages of Texas and Louisiana, Alabama, Georgia, Florida, Mississippi, Arizona, California, and Hawaii. As a result the United States has declared the Giant Salvinia a Federal Noxious Weed, meaning that the importation of Salvinia molesta into the United States and transport across state lines is prohibited by Federal Law. However one problem is that the species must be listed by individual states as a State Noxious Weed in order to be prohibited for sale or cultivation within that state, and in a few states such as Hawaii it is not prohibited (Jacono 2000). This creates a problem the spread of Salvinia. Infestations in Texas ponds have been linked to purchases from local water garden nurseries, which can also be linked to Salvinia?s prevalence in the horticultural trade. Sites where giant Salvinia occurs in cultivation can serve as sources for introduction into new regions. Human transport, including boats, trailers, etc., will spread the plant locally.

There were many early attempts to control Salvinia. They ranged from Chemical control with the use of herbicides to mechanical control measures by manually removal from small water bodies. Both of these attempts proved to be unsuccessful. Despite the availability of herbicides to kill the weed, attempts to control salvinia chemically have failed to provide long-term solutions. The reason for this is basically economic. For example, at Lake monodrama, Australia, use of AF101 sprayed from a helicopter was abandoned in 1978 after $160,000 u.s. had been spent. At the time the plant was doubling in size in 3 days, implying the need to kill more than half the infestations every 3 days if control was to be achieved (Thomas and Room 1986).

Economic constraints were also the main reason for failure of physical or mechanical controls. Once the weed is established, biomasses of about 80 tonnes per hectare fresh weight and the potential for rapid regrowth make this impractical. The use of weed harvesting machines was considered in Australia, but was abandoned because the growth of the plants exceeded the removal capacity of the weeds (Thomas and Room 1996).

The major breakthrough in Control of Salvinia came on Lake Moondara in Australia, where biological control proved to be successful against Salvinia. A new beetle species, Cyrtobagous salvinidae, was discovered in Salvinia?s native brazil that fed exclusively on Salvinia. The beetle was brought to Australia and released where it rapidly destroyed and infestation of kariba weed covering two square kilometers. The beetle?s most spectacular success occurred in Papua, New Guinea where the beetle consumed two million metric tons of the weed in just two years (Barrett 1989). Other beetle programs are currently under way all over the world. The success of the beetle is due to the fact that it feeds selectively on growing points, which must be of considerable significance to a plant which relies on vegetative propagation. The larvae tunnel through the rhizome destroying vascular tissue (Thomas and Room 1986). Since Salvinia is asexual, the world?s entire population of the plant is oostensibly genetically identical. The lack of variation, suggests that the susceptibility of the weed to C.salvinae should not differ from place to place. However, biological control is not perfect and there are a few problems to consider with biological control. C.salviniae eggs will not hatch below 19 degrees Celsius. Since salvinia will continue to grow at 10 degrees Celsius and can survive light frosts it is clear that the plant will survive in areas which, for part of the year at least, will not support C. salviniae (Thomas and Room 1996).

Conclusion

Salvinia molesta is a rapidly growing plant that has invaded and dominated many non-native areas. It has led to huge economic and socioeconomic costs throughout the world. Control of Salvinia has been achieved in many countries such as Australia, New Guinea, and Malaysia. With the current outbreaks of Salvinia in the southern United States it is important for society in these areas to realize the potential threat to ecosystems. The importance of funding must be realized and made available for programs to successfully achieve control. The concept of biological control is simple, however the usefulness of these biological control agents will probably lie more in the realm of integrated control that includes education of the public as well. Biological control seems to be the best answer to the current problem for the results seem to be effective, environmentally sound, pollution free, and non-target plants are not affected. Presumably, biological control succeeds if genetic variation is limited. Therefore, clonal propagation, which allows the kariba weed to dominate the world, may provide a way to destroy these noxious invaders.

References Cited

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Caffrey, J.M.K., Barret, P.R.F., Murphy, K.J., Wade, P.M. (1996). Noxious Floating weeds of Malaysia. Hydrobiologia 340: 121-196

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Seagrave, C. Aquatic Weed Control. 1988. Fishing News Book, Inc. Great Britain.

Thomas, P.A.., Room, P.M. (1986). Taxonomy and control of Salvinia Molesta. Nature, 320, 581-584.