Plant (kingdom Plantae)
Plants are the primary food source of most of all other organisms on earth, and many biomes and ecosystems function with the vegetation type as a basic criterion in determining the community structure and size. Plants are introduced for a variety of reasons including ornamental uses, food, improvement of the environment, alleviation of soil erosion and many other purposes. Despite their immobility in general, they are able to disperse owing to physical factors such as wind and water, and biotic factors such as mammals and birds that feed on the fruits of some.However, without the population control of the consumers some hardy plants can easily grow out of control and crowd out native communities, changing the entire structure of the habitat. We will look at a few prominent ones.
eg 1 Common Water Hyacinth
Phylum: Angiospermophyta (Flowering Plants)
Class: Liliopsida (Monocotyledons)
Order: Liliades
Family: Pontederiaceae (Water Hyacinth)
Scientific name: Eichhorinia crassipes
This weed has been maintained as one of the worst invasive species in the world. In thousands of places, such as Florida, stretches covered by this plant can exceed 50000 hectares in size, and no less than 50 tonnes of Water Hyacinth has been recovered per hectare where the weed has found conditions to be particularly advantageous. Where this applies, Water Hyacinth multiplies like a Mouse of the Plant Kingdom, being capable of doubling its population in a mere fortnight, having the highest growth rate of any known plant. Water Hyacinths reproduce both by asexual and sexual methods, that is, both by seed and from stems radiating from the underwater base of the plant, which develop into individual plants. Proliferation of the Hyacinth is also due to their extreme physical tolerance in freshwater habitats, as they survive in exorbitantly low nutrient concentrations and wide ranges of water pH and temperatures; they are even immune to some toxins. The seeds, mainly dispersed by animals, can maintain their viability for up to 20 years.
The Water Hyacinth is identified by its round, waxy, thick, glossy leaves, which are 10 to 20cm in diameter, densely an profusely veined, and mildly undulate at the sides. The flowers are purplish to pink in colour, with the 6 upper petals bearing blue-fringed yellow splotches in the centre. It can grow to a metre tall, rising right out of the water and covering the swamp in dense mats. Aquatic and free-floating, it is not hard to conceive its natural range in the Amazonian basin, where it is a major food provider for many animals. It has been introduced as a well-chosen ornamental plant to Florida, Egypt, Asia and Australia in the 1880s, and by 1950 it has managed to spread throughout the Nile Valley into Lake Victoria. It is also a noxious weed in China where it has infested 17 provinces in the last century.
The weed obscures the water surface of reservoirs, stagnant ponds and slow-flowing rivers where suitable habitats occur. This prevents light and deters oxygen from entering the water, undermining the source of energy of submerged wetland plants and kills other aquatic life, especially fish dependent on higher oxygen quantities. Water uptake by the Hyacinth also contributes to a high evaporation rate and the water quality is also degraded. Dense interlaced mats of Water Hyacinth dies and decays, causing eutrophication as a large amount of nutrients is released, which leads to algal bloom. They have proved to be a nuisance in reservoirs as the pollution of water can cause human health hazards, while transport and irrigation activities are also hindered. The economic and ecological effects are pronounced.
The possible benefits of the plant, though, inspire control methods that may be effective on this weed. Water Hyacinth is useful in making various products from high quality paper and possible water detoxification service still under research, to fuels for cooking stoves and fodder for livestock. It is hoped that in some tropical countries that this could reduce the harm done by this non-native plant. Other biological controls of the plant include Manatees, certain Weevils, Moths and Fungi, but most are still being accessed for practicability. In China, two weevils, Neochetina bruchi and N. eichhorniae have been introduced and have achieved some success in keeping the numbers of Water Hyacinth down.
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eg 2 Giant Reed
Phylum: Angiospermophyta (Flowering Plants)
Class: Liliopsida (Monocotyledons)
Order: Cyperales
Family: Poaceae
Scientific name: Arundo donax
The Giant Reed is an aggressive invader of disturbed sites where enough moisture is present in the soil. A perennial grass growing to a staggering 10m in height, it forms dense stands of vertical stalks that are uniform and branchless, preventing other animals from moving through it except small rodents. Few animals utilize it as habitats and no animals have been recorded to take it as a preferred food item; its monotonously branchless stems does not serve to allow birds to construct nests amongst its foliage, and the Giant Reeds lack a canopy in which native birds could forage. The bluish-green leaves of these plants are packed with silicates, which are essentially the main component in glass, and there is also a range of toxic chemicals present in the stems, which effectively render the plant unpalatable for many insects --- no animals have ever been observed to consume the leaves of this weed in the new range. The plumy flower heads are large and contain closely packed flowers. Fulfilling far less food or habitat requirements of the native fauna in introduced areas than the original riparian vegetation, for example willows and cottonwood forests that supports larger populations of insects and insectivorous birds, the Reeds take up a lot of space and crowd native plants out of existence, reducing the range of many native animals. They provide little shielding to the water in smaller streams and water temperatures increase, which leads to more alteration in the habitat of the aquatic life, affecting endangered amphibians such as the Arroyo Toad of California. It also restricts water flow and encourages sedimentation, the silt accumulating in the canals and estuaries destroying the habitats of streams and wetlands. Invasive Reeds in North America seem to rely totally on rhizomes as the propagation method and no seedlings have been observed, so dispersal is generally within the infested neighbourhood; growth rate of the Reed is amongst the highest of terrestrial plants, at 0.7 metres per week.
While the Reed is simply out of synch with the environment around in places as South-east Asia, South Africa, Australia, Pacific Islands and the Caribbean Islands, it is a constituent of some important riparian habitats in South Asia to the Mediterranean basin, where Reed-dependent animals thrive in. It also enabled the development of some wind instruments to take place as the mouthpiece (reed) is largely made of the stems of the Giant Reed.
No biological control seems to be effective on the Giant Reed, and manual removal is time-consuming and difficult to carry out, as the rhizomes have to be completely removed for the method to be effective. Chemical control, therefore, is the last resort. Again this is not desperately helpful as there is hitherto no specific herbicide for the Giant Reed.
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eg 3 Prickly Pear Cacti
Phylum: Angiospermophyta (Flowering Plants)
Class: Magnoliopsida (Dicotyledons)
Order: Caryophyllales
Family: Cactaceae (Cacti)
Scientific name: Opuntia sp.
There are many species of Cacti that are native to the Mexican deserts and as they constitute a significant part of the native flora, local people exploit them in many ways. Its fleshy stem can be used as food, made into jam, syrup, and marmalade; the chemicals extracted from it are widely made into medicines, notably that Opuntia extracts can stabilize blood sugar level and combat diabetes. Exported cosmetics also originate largely from Cacti, and various products include lotions, hand gels, shampoo and so on. It is also a cultural icon of Mexico and exists in many legends, local folklore and music. The very existence of these Cacti is under risk from another invasive species from South America, the Cactus Moth (Cactoblastis), ironically, which has in fact been used as the biological control that eventually decimated Cacti populations in the notorious outbreak in 1920s in Australia --- 24 million hectares, or 75% the area of Japan, of Queensland was brimming with 2-metre high Cacti.
Prickly Pear Cacti are tall, with some species growing to 2m in height. It has succulent stems enlarged for water storage, and spines from leaves with minimized surface area to prevent water loss --- the immature young shoots undertake photosynthesis largely, but not the mature Cacti. The flowers are bisexual and the fruit is an edible berry.
These xerophytes has been introduced into Australia, New Caledonia, Solomon Islands, Australia, South Africa, Yemen, Eritrea, Ethiopia and Somalia mainly as ornamental plants alternate food source for livestock, and cattle hedges, but have proved to be highly competitive and displaced many native plants in arid conditions similar to that of its native range. They are dispersed through floods, which allow infestations around the banks of the rivers, and segments can regenerate if left unattended on the ground.
Despite the biofouling capacity of the Cacti they provided shelter for many native animals such as the Death Adder, various Lizards and others, as the Cactoblastis Moths were introduced to control the Prickly Pear and their grubs were a food source for reptiles. The control schemes are hugely successful and since the Cactus Moth were released in 1926, the Cacti infested areas have virtually been all cleared, and as the Moth is a highly specific parasite of the Cacti, native wildlife have scarcely been harmed. This is one of the few examples of entirely successful biological control. Where the Moth fails to thrive due to the climate being too cold, however, herbicides still have to be implemented; but as the Cacti is also a plant thriving under high temperatures, this does not present much of a problem.
eg 4 Buckthorns
Phylum: Angiospermophyta (Flowering Plants)
Class: Magnoliopsida (Dicotyledons)
Order: Rhamnales
Family: Rhamnaceae (Buckthorns)
Scientific name: \
The Buckthorn is a tall shrub or rather a small tree in terms of size, growing to 7m in height and 25cm in trunk diameter. The growth form is dense and typical of shrubs, with stems radiating from the base of the plant. The crowns are by contrast sparser with fewer stems. Leaves are ovate in shape, smooth on the sides, and serrated on the edges in European Buckthorn, rather smooth on Glossy Buckthorn --- which also has glossy leaves, hence the name. In winter, having shed them, Buckthorns are quite distinct in appearance from many other plants. They have hairy terminal buds on curvy twigs, lumpy from leaf scars, ending in thorns. When viewed in cross-section, the heartwood is pinkish-orange while the vascular tissues are yellow in colour. A dioecious plant --- meaning there are male plants and female plants, but no hermaphrodites, the females are in berry in autumn and can be easily told apart from many native shrubs as they retain foliage till late in the season, and correspondingly produce leaves again earlier in spring.
These shrubs are originated from Eurasia, and were introduced widely into Eastern North America as an ornamental shrub. As early as 1849 the Buckthorns were planted in hedgerows in Wisconsin, and since then they have spread remarkably quickly throughout the East, north to New England and South to Missouri. The Common Buckthorn is causing problems in the understorey of oak and maple woods, other riparian woodlands and the prairies, where they crowd out native plants and prevent them from regenerating by competing with them for space. In hedgerows, pastures, abandoned fields and even rocky areas, they limit the amount of light available to native plants. They prefer drier, well-drained soils. The Glossy Buckthorn prevails in floodplain forests, reducing the biodiversity of herbs and shrubs. They can be found in acidic bogs, calcareous fens and sedge meadows, where the water content of the soil is higher. Both of them can grow well under intense sunlight and lack of nutrients, and have wide ranges of physical tolerance. They reproduce profusely and efficiently by seed and these are capable of long-distance dispersal through animal vectors, mainly berry-feeding birds. These hardy plants, when cut, can sprout again quickly; their long leaf retention periods has the effect of shading out many wild herbs and deterring their growth early in the season. When high light levels are available to the pullulating seedlings, they could form interlocking thickets rapidly, blocking out so much sunlight that native trees and herbs do not germinate; Buckthorns also release allelopathic chemicals that inhibit the growth of neighbouring plants, and the native flora is at a disadvantage, and consequently restricted in range.
To control this weed, the isolated pioneers of the species must be eradicated before seeds could be produced and further transported to other places again. Due to their dioecious nature, female plants could be picked out and selectively cut down, and treated with stump herbicides. Burning is rather ineffective under dense stands of thorny shrubs, and has to be applied over the course of several years, as the seeds are long-lived. Chemicals are applied generally in the autumn and winter months, when the Buckthorn plants are easily recognized. Triclopyr and fosamine are examples of non-specific herbicides that has been applied to the weed, and these requires a lot of manpower to be sufficiently effective as the damage of native plants cannot be avoided unless care is taken to a large extent. In wetlands, the restoration of the original water level can help eradicate the Glossy Buckthorn, but this method is generally difficult to carry out.
eg 5 American Rope (Bitter)
Phylum: Angiospermophyta (Flowering Plants)
Class: Magnoliopsida (Dicotyledons)
Order: Asterales
Family: Asterceae
Scientific name: Mikania micrantha
Commonly called mile-a-minute, this invasive vine, or climber plant, has its origins in the South American rainforests, where it grows near rivers and in disturbed areas near forests. Seemingly farcical, the vine has been introduced to South Asia originally to camouflage airfields during the World War II. Presumably its seeds have been carried accidentally to many other islands in the region including the Philippines, Hong Kong and Indonesian Islands, west to Mauritius and east to Papua New Guinea. As recent as 1998, the weed has been discovered in Queensland, Australia, and must have persisted in the country for a decade before. It is widely found now in the shrublands, riparian zones, disturbed regions, natural forests, plantations and wetlands in these tropical or subtropical countries.
A slender perennial vine, Mikania micrantha has heart-shaped leaves growing to 13cm long, small dense clusters of greenish-white flowers, and minute ( 2mm ) oblong seeds that are black in colour. The seeds can be dispersed by animals or by wind, and once germinated, the shoots can shoot up by 27mm a day, which is alarmingly quick, hence its colloquial name. A single plant is capable of covering 25 square metres in a few months, and 40000 seeds can be produced annually. They also reproduce vegetatively, and the offspring produced are no less vigorous in growth. Optimum conditions for their growth are high humidity, sunlight, organic matter, nutrient level, and soil moisture. They entwine any vertical support from trees to fences and walls, and kill other plants conceivably in this way. By developing layers of leaves around the branches of shrubs and trees they compete with their host for light, and at the base they deprive the host of water and nutrients. They also release growth inhibitors that affect other shrubs and trees, reducing competition in this way. As a consequence the vine has become the worst weed in India for tea, damaged rubber plantations in Malaysia, disturbed coconut plantations in Samoa. They often cause problems in banana, cacao and oil palm plantations as well. After human disturbance, very often, ecological voids are created and with reduced competition from other plants, Mikania could proliferate very rapidly, jeopardizing the environment and any remaining plant species.
The vine is very hard to control as their stems are capable of vegetative propagation. The complete removal of the vines is necessary, which is not practical. Herbicides remain the most reliable method of eradication, but a number of biological controls that seems to be specific to the vine have been identified, and experiments on these are being carried out with the hope of holding the vine under control. These insects include a Thrip (Liothrips mikaniae), a Bug of the genus Teleonemia and several other Beetles. Fungal pathogens are also being researched upon in the hope of exploiting them as biological control agents.
eg 6 Wattles
Phylum: Angiospermophyta (Flowering Plants)
Class: Magnoliopsida (Dicotyledons)
Order: Fabales (Beans and Acacia)
Family: Mimosaceae (Wattle)
Scientific name: \
Reaching 20m in height, these evergreen trees are leguminous plants highly prized in its native range of Australia and Tasmania for its pods, and are important in native ecosystems as their roots harbour mycorrhizal fungi, which native marsupials like to feed on. The Black Wattle has dark, hairy, bipinnate leaves that are dark olive in colour, and pale yellow flowers in globular flower heads, while Earpod Wattle have sickle-shaped leaves that are irregularly twisted. Fruit pods are large, flat and oblong, and when split, reveals black seeds linked to the pod by arils. The Wattles are found in riparian zones in general, and have been introduced mainly as potential sources of food, wood, bark products such as a chemical called tannin, and as an ornamental species to South Africa and various Pacific Islands, some of which, like Hawaii, has their own Acacia species. In South Africa it has been termed a number one threat of native plant life by severe competition, as the Legumes are better adapted to nutrient-poor soil, with the symbiotic nitrogen-fixing bacteria rhizobium in their root nodules. The Trees' impact on the local ecosystems and livelihood of the people, however, can be attributed to their thirst - they consume a lot more water than native plants do, and the Black Wattles are sucking the source of life of many endemic Karoo plants in South Africa dry. In South Florida, the Earpod Wattle has been found to adapt well to both dry and wet conditions, and is severely affecting native plants through direct competition. The Acacia species are mostly found on grasslands and disturbed habitats, where they outcompete many native species.
On the other hand many introduced Acacia species are quite useful in plantations as tannin, extracted from the bark of Acacia species, is used in tanning and dying leather. The wood is also useful and other chemicals in the Tree are used as astringents and styptics. The Trees are also used to deter soil erosion when grown on mountain slopes. Therefore there are ambivalent feelings in some locations about whether to remove the species or not. Where the Acacia species are causing dire concern people have used glyphosate, which is an herbicide, to remove them.