Botany · Ch 8 — Environmental Issues
Alien invasive species
Alien invasive species
An invasive alien species is one that is not native to the ecosystem or country it is found in, spreads on its own once introduced, interferes with native species' biology and survival, threatens the wider ecosystem, and causes measurable economic loss; the World Conservation Union ranks invasive alien species as the second most significant threat to biodiversity worldwide after habitat loss outright. Introductions happen through several channels — accidental transport via air or sea ports, deliberate import of wild germplasm by research organisations, or natural dispersal by birds where the species produces edible fruit. Once established, invasive species tend to be fast-growing and highly adaptable, altering soil chemistry through changed litter quality, disrupting soil fauna and decomposition processes, and outcompeting or chemically suppressing neighbouring native vegetation. Four species illustrate this in Tamil Nadu specifically: Eichhornia crassipes (water hyacinth), a free-floating South American aquatic ornamental that grows explosively, depletes dissolved oxygen (driving eutrophication), blocks sunlight and creates breeding habitat for disease-carrying mosquitoes (notably Anopheles) and snails while hampering fisheries, agriculture, recreation and hydropower; Lantana camara, a South American ornamental shrub ranked among the world's worst invasive species, which chemically (allelopathically) suppresses germination and root growth of surrounding plants, though its stems are now put to use as a fibre for baskets, furniture and household items after control; Parthenium hysterophorus, introduced accidentally via imported food grain, which suppresses native vegetation and pasture fodder availability, causes crop yield loss, and produces allergenic pollen linked to rhinitis, asthma and dermatitis in humans; and Prosopis juliflora …
What this figure shows. A photograph of Eichhornia crassipes (water hyacinth), a free-floating aquatic weed native to South America, shown as a dense mat of glossy rounded leaves with inflated bulbous leaf-stalks (petioles) that keep the plant buoyant, and a spike of pale lavender-blue flowers. The plant is depicted covering the surface of standing water, illustrating how its rapid, dense growth blocks sunlight penetration, depletes dissolved oxygen and chokes waterways, which is the visual basis for the chapter's discussion …
What this figure shows. A photograph of Lantana camara, an invasive flowering shrub native to South America, showing its characteristic small tubular flowers clustered into flat-topped, multicoloured heads (commonly orange, yellow, pink and red within the same cluster) on a many-branched, prickly-stemmed shrub with rough, ovate, toothed leaves. The image illustrates the ornamental appearance that led to its original introduction, in contrast to its documented status as one of the world's worst invasive weeds d …
What this figure shows. A photograph of Parthenium hysterophorus (commonly called carrot grass or congress grass), an invasive weed native to South America, showing its finely divided, fern-like pale-green leaves and numerous small white flower heads borne on branched, erect stems. The image depicts the plant as it typically appears colonising waste ground, pastures and farmland edges, illustrating the weed described in the text as suppressing native and c …
What this figure shows. A photograph of Prosopis juliflora, a thorny invasive tree/shrub native to Mexico and South America, showing its characteristic feathery bipinnate compound leaves, long paired thorns along the branches, and a dense, many-branched, drought-hardy woody habit typical of arid and semi-arid colonising stands. The image illustrates the species as it appears forming dense thickets on coastal and desert wasteland in states such as Gujarat, Andhra Pradesh and Tamil Nadu, where it was introduced to contr …