Invasive species or invasive exotics, are the non-native or non-indigenous plants and animals that affect the habitats and bioregions they invade economically, environmentally and ecologically. They disrupt by dominating a region and disrupt the natural equilibrium. Alterations in genetic pool is a major threat by the invasive species.
Common features of invasive exotics include:
- The ability to reproduce both asexually and sexually
High dispersal ability
- Tolerance of a wide range of environmental conditions
Ability to live off of a wide range of food types
Scientific Name : Eichhornia crassipes
The seven species of water hyacinth comprise the genus Eichhornia. Water hyacinth are a free-floating perennial aquatic plant, native to tropical and sub-tropical South America. With broad, thick, glossy, ovate leaves, water hyacinth may rise above the surface of the water as much as 1 meter in height. The leaves are 10–20 cm across, and float above the water surface. They have long, spongy and bulbous stalks. The feathery, freely hanging roots are purple-black. An erect stalk supports a single spike of 8-15 conspicuously attractive flowers, mostly lavender to pink in colour with six petals. When not in bloom, water hyacinth may be mistaken for frog's bit (Limnobium spongia).
One of the fastest growing plants known, water hyacinth reproduces primarily by way of runners or stolons, which eventually form daughter plants. It also produces large quantities of seeds, and these are viable up to thirty years. The common water hyacinth (Eichhornia crassipes) are vigorous growers known to double their population in two weeks. Water hyacinth has been widely introduced throughout North America, Asia, Australia and they can be found in large water areas such as Louisiana, or in the Kerala Backwater. Water hyacinth proliferation can blocks waterways, limiting boat traffic, swimming and fishing. The weed prevents sunlight and oxygen from reaching the water column and submerged plants. By crowding out native aquatic plants, it dramatically reduces biological diversity in aquatic ecosystems.
Scientific Name : Salvinia molesta
Salvinia molesta, commonly known as African Payal in Malayalam, is a floating aquatic fern that thrives in slow-moving, nutrient-rich, warm, freshwater. It was first recorded in Australia in 1952 and first noticed as a pest in Kerala in 1956. A rapidly growing competitive plant, it is dispersed long distances within a waterbody (via water currents) and between waterbodies (via animals and contaminated equipment, boats or vehicles). Salvinia molesta is cultivated by aquarium or pond owners and it is sometimes released by flooding, or by intentional dumping. Salvinia molesta may form dense vegetation mats that reduce water-flow and lower the light and oxygen levels in the water. This stagnant dark environment negatively affects the biodiversity and abundance of freshwater species, including fish and submerged aquatic plants. Salvinia molesta can alter wetland ecosystems and cause wetland loss and also poses a severe threat to socio-economic activities dependent on open, flowing and/or high quality water bodies, including hydro-electricity generation, fishing and boat transport. It prefers tropical, sub-tropical or warm temperate areas of the world and grows best in still or slow-moving water bodies including ditches, ponds, lakes, slow rivers and canals. In standing water it forms stable floating mats. It grows optimally at a water temperature of between 20°C and 30°C. Buds are killed when exposed for more than two hours to temperatures below -3°C or above 43°C. Salvinia is able to tolerate salinity levels one tenth that of seawater, allowing the weed to adapt to a wide range of benthic environments. Its growth rate decreases by 25% at a salinity level of 0.3%. Growth is greatly stimulated by an increase in nutrient levels. As a consequence the weed is particularly fast-growing in areas where the hydrological regime has been altered by humans, encouraging an increase in nutrient levels (for example by increased runoff or fertiliser leaching).
Scientific Name : Lantana camara
It is also known as Spanish Flag or West Indian Lantana, is a species of flowering plant in the verbena family, Verbenaceae, that is native to the American tropics. It has been introduced into other parts of the world as an ornamental plant and is considered an invasive species in many tropical areas. is one among the 10 most harmful weeds in the world. It spreads rapidly and occupies major forestland. It inhibits natural pollination, leaving the native trees unpollinated. It is resistant to fire, and quickly grows in and colonizes burnt areas. It also fuels forest fires. It replacing native plant communities in forests ecosystems by forming dense impenetrable thickets, harbouring vectors that carry infectious diseases15 and Since its entry in 1807 it has spread all over the Indian subcontinent, and is currently a threat to the biodiversity of forests because it encroaches upon the native vegetation. The chemical and biological methods of control are not favoured because of environmental and ecological reasons, particularly due to their potential adverse effects on other biota present in forest ecosystems, and hence not practised by the managers of protected areas in India. The most common methods used in India for the control of Lantana in forests are: (i) hand pulling, (ii) slashing/chopping of the stems, (iii) burning and (iv) manual grubbing with substantial removal of the root system. These methods have had no or little effect in controlling the spread of Lantana infestation, due to their inherent limitations and absence of an integrated control strategy.
Scientific Name : Mimosa diplotricha
Mimosa diplotricha, commonly known as Nila grass is an erect, climbing, ascending or prostrate biennial or perennial shrub that often forms a dense thicket, the root system strong, often woody at the decumbent base; stems conspicuously angular throughout the length, up to 2 m tall with many randomly scattered recurved spines or thorns 3 to 6 mm long; leaves bipinnate, 10-20 cm long, moderately sensitive to the touch; pinnae four to nine pairs; leaflets 12-30 pairs, sessile, opposite, lanceolate, acute, 6-12 mm long, 1.5 mm wide; inflorescence a head, one to three in the axils of leaves, on stalks 1 cm long, hairy, about 12 mm in diameter; corolla united at least at the base (gamopetalous), pale pink; stamens twice as many as the petals; fruit a pod, spiny, three- to four-seeded, borne in clusters, linear, flat, 10-35 mm long, splitting transversely into one-seeded sections which separate at grooves or seams (sutures); seeds flat, ovate, 2 to 2.5 mm long, light brown. It may be distinguished by its angular stems with recurved thorns or spines and by its stamens which are twice the number of the petals"
Diamond cholla or branched pencil cholla
Scientific Name: Cylindropuntia ramosissima
Cylindropuntia ramosissima is a species of cactus known by the common names diamond cholla and branched pencil cholla.It is, however, the only cactus with either of these common names growing in Anza-Borrego.Its origin is fromUSA (southwestern Colorado, western Arizona, southern Nevada, eastern California), Mexico (northern Sonora, Sierra Pala), at low elevations, 500 to 2000 feet (150-600 m). Cylindropuntia Southwestern United States, California, ramosissima is native to the and of theand Northwestern Mexico, and to Baja California and its Islas San Benito.Cylindropuntia ramosissima is a decumbent or erect and tree like cactus which can approach 2 meters-6 feet in maximum height. It has many narrow branches made up of cylindrical segments, green in color drying gray, the surface divided into squarish, flat tubercles with few or no spines, or often with a single long, straight spine. Diamond Cactus flowers are inconspicuous. The flower is small and orange, pink or brownish in color.They are about the size of a quarter. If you see a Diamond Cactus flower, consider yourself lucky. According to Jaeger, "many botanists confess they have never seen one." The best time for finding the late-blooming C. ramosissima in flower is in summer after the blooming of other cacti has ceased for the year.The fruit is a small, dry, spiny body up to 2 centimeters long. Finding Diamond Cholla in flower can be a challenge. The best strategy would seem to be to stake out an area where the plants grow. In early May, start visiting it regularly, perhaps as often as once a week, looking for signs of buds or new stem growth. Diamond Cactus is a spectacular sight even when there are no flowers, especially when its spines take on a golden color and catch the light of early morning or late afternoon.
Scientific Name: Limnocharis flava
Limnocharis flava belongs to the family Alismataceae. In India, the occurrence of this noxious aquatic weed was first reported from Kerala. The earliest record of its introduction in the east dates back to 1866, when it was recorded to have been cultivated in the Botanic Garden at Bogor. It is thought that the introduction of L. flava into India may have been due to contaminated imports of rice from rice paddies in South East Asia infested with the weed. As well as unintentional spread of the seed via agricultural imports its use and cultivation as a food source may result in intentional spread of the plant into new countries. The plant mainly propagates by its seeds; a single fruit produces 1000 seeds and single plant produces about 1,000,000 seeds per year. It flowers throughout the year. The seeds are generally dispersed by mud sticking onto the feet of birds, animals, man and agricultural implements. The weed clogs irrigation tanks, channels and drainage ditches resulting in poor drainage, making lower regions of the cultivated tracks unsuitable for farming. It also obstructs the flow of water from paddy fields during heavy rainfall, resulting in damage to crops by submergence.
Water Cabbage/ Water Lettuce
Scientific Name: Pistia stratiotes
Pistia is a genus of aquatic plant in the family Araceae, comprising a single species, Pistia stratiotes, often called water cabbage or water lettuce. Its native distribution is uncertain, but probably pantropical; it was first described from the Nile near Lake Victoria in Africa. It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways. It is a perennial monocotyledon with thick, soft leaves that form a rosette. It floats on the surface of the water its roots hanging submersed beneath floating leaves. The leaves can be up to 14 cm long and have no stem. They are light green, with parallel veins, wavy margins and are covered in short hairs which form basket-like structures which trap air bubbles, increasing the plant's buoyancy. The flowers are dioecious, and are hidden in the middle of the plant amongst the leaves. Small green berries form after successful fertilization. The plant can also undergo asexual reproduction. Mother and daughter plants are connected by a short stolon, forming dense mats. The growth habit can make it a weed in waterways. It is a common aquatic weed in the United States, particularly in Florida where it may clog waterways. It has the potential to reduce the biodiversity of a waterway. Mats of Pistia block gas exchange at the air-water interface, reducing the oxygen in the water and killing fish. They also block light, killing native submerged plants, and alter immersed plant communities by crushing them. Pistia can be controlled by mechanical harvesters that remove the water lettuce from the water and transport it to disposal on shore. Aquatic herbicides may also be used. Two insects are also being used as a biological control.
Scientific Name : Oreochromis mossambica
Domination of the exotic fish, Tilapia is another threat. Considering the damage caused to native fish species and biodiversity, Tilapia is termed a ‘biological pollutant’ by the Food and Agriculture Organisation. Tilapia inhabit a variety of fresh water habitats including shallow streams, ponds, rivers and lakes. Historically they have been of major importance in artisan fishing in Africa and the Levant and are of increasing importance in aquaculture. Tilapia can become problematic invasive species in new warm-water habitats, whether deliberately or accidentally introduced but generally not in temperate climates due to their inability to survive in cool waters, generally below 60°F (16°C). In other cases, unwanted fish have been released by aquarists or ornamental fish farmers into the wild. Because tilapiine cichlids are generally large, fast growing, breed rapidly, and tolerate a wide variety of water conditions (even marine conditions), once introduced into a habitat they generally establish themselves very quickly. In doing so they compete with native fish fauna, create turbidity in the water (by digging) thus reducing the light available for aquatic plants, and eating certain types of aquatic plants causing changes in local aquatic flora. Its dominance in 'Veli' lake is reported recently.
Scientific Name: Plecostomus multiradiatus
These can be found in a wide variety of habitats, ranging from relatively cool, fast-flowing and oxygen-rich highland streams to slow-flowing, warm lowland rivers and stagnant pools poor in oxygen. They are tropical fish and populations are typically limited only by their lower lethal temperature which has been found to be about 8.8-11°C in some species. They can thrive in a range of acidic to alkaline waters in a range of about (pH 5.5.0 to 8.0) . They are often found in soft waters, but can adapt very quickly to hard waters. These fishers are also highly tolerant to poor water quality and are commonly found in polluted waters. They are known to use outflow from sewage treatment plants as thermal refugia and can readily adapt to changing water quality. These fishes may be found in from lowlands to elevations of up to 3,000 m. Some species are salt tolerant. Potential effects of Plecostomus spp. include alteration of bank structure and erosion, disruption of aquatic food chains, competition with native species, mortality of endangered shore birds, changes in aquatic plant communities, and damage to fishing gear and industry. Environmental impacts of Plecostomus spp. are not fully understood, but in locations where they are introduced and abundant, their feeding behaviours and burrowing activities can cause considerable disturbance. Their burrows have been reported as contributing to siltation problems and bank erosion and instability. Plecostomus spp. forage along the bottoms of streams and lakes, occasionally burying their heads in the substrate and lashing their tails. These behaviours can uproot or shear aquatic plants and reduce the abundance of beds of submersed aquatic vegetation, creating floating mats that shade the benthos from sunlight. By grazing on benthic algae and detritus, they may alter or reduce food availability and the physical cover available for aquatic insects eaten by other native and non-native fishes where they are introduced..
Giant African land snail
Scientific Name: Achatina fulica
The East African land snail, or giant African land snail, a species of large, air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Achatinidae. This mollusc is now known as one of the worst invasive species in the world. In recent times, the land snails have been kept as pets; however, they are illegal to possess in some countries including the United States. The Giant East African Snail is a simultaneous hermaphrodite; each individual has both testes and ovaries and is capable of producing both sperm and ova. A single snail can lay hudereds of eggs at a time. The nocturnal snails are voracious eaters of food crops .It is known to eat at least 500 different types of plants including cassava, papaya, rubber, beans, peas, cucumber and melons. They can act as carriers of viruses. Reports showed that these snails feed on the concrete structures for calcium to make their shells strong and can go into hibernation for up to three years and emerge when the climatic condition turned conducive. Many methods, including hand collecting and use of molluscicides and flame-throwers, have been tried to eradicate the giant snail. Generally, none of them has been effective except where implemented at the first sign of infestation.
Scientific Name: Bufo marinus
The Cane Toad (Bufo marinus), also known as the Giant Neotropical Toad or Marine Toad, is a large, terrestrial true toad native to Central and South America, but has since been introduced to various islands throughout Oceania and the Caribbean. It is a member of the subgenus Rhinella of the genus Bufo, which includes many different true toad species found throughout Central and South America. The cane toad is a prolific breeder; females lay single-clump spawns with thousands of eggs. Its reproductive success is partly because of opportunistic feeding: it has a diet, unusual among Anurans, of both dead and living matter. Adults average 10–15 cm (3.9–5.9 in) in length; the largest recorded specimen weighed 2.65 kilograms (5.8 lb) with a length of 38 cm (15 in) from snout to vent. The cane toad has poison glands, and the tadpoles are highly toxic to most animals if ingested. Because of its voracious appetite, the cane toad has been introduced to many regions of the Pacific and the Caribbean islands as a method of agricultural pest control. The species derives its common name from its use against the cane beetle (Dermolepida albohirtum). The cane toad is now considered a pest and an invasive species in many of its introduced regions; of particular concern is that its toxic skin kills many animals-native predators and otherwise-when ingested
Scientific Name: Rana catesbeiana
The bullfrog, Rana catesbiana, is found throughout North America east of the Rocky Mountain. Bullfrogs are typically green or greenish brown, of light or dark shade. The back and sides may be plain in color or may be spotted with dark. When present, the spots may be distinct or connected. The arms and legs are spotted or barred with dark. Underparts are white, distinctly or obscurely spotted and mottled with dark. The throat of the male may be yellow. The iris is either golden or reddish bronze. The bullfrog is America's largest frog; however, size is not a good criteria to judge the age or gender of the amphibian. The variation in this aspect of them is amazing. A frog one year old may be no more than two inches long. Much depends on the size attained by the tadpole before the transformation. Much of a bullfrog's growth also depends on food and other environmental conditions. The bullfrog is generally a solitary animal except during breeding season which runs from late May into July.
European tree sparrow
Scientific Name: Passer montanus
The Eurasian Tree Sparrow, Passer montanus, is a passerine bird in the sparrow family with a rich chestnut crown and nape, and a black patch on each pure white cheek. The sexes are similarly plumaged, and young birds are a duller version of the adult. This sparrow breeds over most of temperate Eurasia and Southeast Asia, where it is known as the Tree Sparrow, and it has been introduced elsewhere including the United States, where it is known as the Eurasian Tree Sparrow or German Sparrow to differentiate it from the native unrelated American Tree Sparrow. Although several subspecies are recognized, the appearance of this bird varies little across its extensive range. This sparrow is distinctive even within its genus in that it has no plumage differences between the sexes; the juvenile also resembles the adult, although the colours tend to be duller. Its contrasting face pattern makes this species easily identifiable in all plumages; the smaller size and brown, not grey, crown are additional differences from the male House Sparrow. Adult and juvenile Eurasian Tree Sparrows undergo a slow complete moult in the autumn, and show an increase in body mass despite a reduction in stored fat. The change in mass is due to an increase in blood volume to support active feather growth, and agenerally higher water content in the body.
Ecological impacts of Invasive species: Biological species invasion alter ecological systems in a multitude of ways. Worldwide as estimated 80% of endangered species could suffer losses due to competition with or predation by invasive species. As highly adaptable and generalized species are introduced to environments already impacted by human activities, some native species may be put at a disadvantage to survive while other species survival is enhanced. Invasive species can change the functions of ecosystems. For example invasive plants can alter the fire regime, nutrient cycling and hydrology in native ecosystems. Invasive species that are closely related with rare native species have the potential to hybridize with native species. Harmful effects of hybridization have led to a decline and even extinction of native species.
(1) Biodiversity Impact and Assessment, P.C. Trivedi (2009), Aavishkar Publishers and Distributors, Jaipur.P. 266.