Salvinia molesta
Salvinia molesta, commonly known as giant salvinia, or as kariba weed after it infested a large portion of Lake Kariba between Zimbabwe and Zambia, is an aquatic fern, native to south-eastern Brazil. It is a free-floating plant, with fronds that are long and broad, with a bristly, waterproof surface caused by the hair-like strands that join at the end to form eggbeater shapes. The fronds are produced in pairs with a third modified root-like frond that hangs in the water. It has been accidentally introduced or escaped to countless lakes throughout the United States, including Caddo Lake in Texas, where the invasive species has caused eutrophication to occur.
In Europe, Salvinia molesta has been included since 2019 in the European Union's List of invasive alien species of Union concern. Its inclusion means that the species cannot be imported, cultivated, transported, commercialized, planted, or intentionally released into the environment in the European Union.
Description
Salvinia molesta is a complex of closely related floating ferns and can be difficult to distinguish from each other. This water fern is often grown as an ornamental plant but has escaped and become a noxious pest in many regions worldwide. There are a few different growth forms for S. molesta; the primary growth form is an invading form with small flat leaves, while the tertiary or mat form has large, crowded, folded leaves. Under the best conditions plants can form a two-foot-thick mat. These mats can put a halt to recreational activities on lakes and waterways. S. molesta has been used to extract nutrients and pollutants from the water. When this plant is dried out, it can be used as mulch.The phenolic compounds 6'-O--beta-D-glucopyranosyl ester, 4-O-beta-d-glucopyranoside-3-hydroxy methyl benzoate, methyl benzoate, hypogallic acid, caffeic acid, paeoniflorin and pikuroside can be isolated from Salvinia molesta.
Growing conditions
Salvinia molesta reproduces only by asexual reproduction, but it is capable of growing extremely quickly, starting from small fragments and doubling in dry weight every 2.2–2.5 days. It grows from fragments that have broken off or dormant buds that have been detached from the main plant. Each node has five buds, so the potential for rapid spread is high. It also produces spores that are genetically defective and do not produce viable offspring.Salvinia molesta prefers to grow in slow-moving waters such as those found in lakes, ponds, oxbow lakes, streams, ditches, marshes, and rivers. It prefers nutrient-rich waters such as those found in eutrophic water or those polluted by wastewater. It does not usually grow in brackish or salty waters, but has been reported in streams with a tidal flow in southeast Texas. It handles well with dewatering, and while it prefers to grow in moderate temperatures, it will tolerate low or very high temperatures. The United States Geological Service believes that it could grow in zones 7a, 8, 9, and 10 of the USDA Plant Hardiness Map.
Environmental conditions can have a strong effect on this plant. S. molesta can survive on a mud bank for a short period of time, but because of the dry conditions, it cannot survive there permanently. It grows best at a pH of 6–7.7 and at a water temperature of. Growth can be increased in high light intensities. S. molesta cannot grow in high salt concentrations due to the increase in salt causing a decrease in chlorophyll.
Distribution
Salvinia molesta is native to Brazil, but is now widely distributed in tropical and subtropical areas. This floating fern is known for its capability to take over large bodies of slow-moving fresh water. S. molesta has been naturalized in Texas and Louisiana, but has now also been found and reported in Alabama, Mississippi, Florida, and Georgia. It can also be found where the lower Colorado River borders Arizona and California. "While S. molesta rapidly colonizes new states, current populations are too small to assess, but have been targeted for eradication". The naturalized regions of Texas have 14 drainage basins that contain infested water bodies; these are used as impoundments on tributaries that flow near federally protected wetlands. In October 2020 Texas Parks and Wildlife detected S. molesta on Lone Star Lake while surveying fish populations.Invasion
The plant originated in southeast Brazil and was exported as part of the pet industry to be used in aquaria and garden ponds. From there, it escaped or was deliberately released into the wild. It may also have been brought in with fresh, iced fish. Once in a waterway, it can be spread by infested boats, which not only spread it to new areas, but also break up the plant, allowing it to propagate. It also is spread by waterfowl. S. molesta has been spread by contaminated aquatic plant stocks, boats, and other watercraft. The movement of water spreads S. molesta, and the sale and exchange of S. molesta materials increases chances of release to the environment.The rapid growth rate of Salvinia molesta has resulted in its classification as an invasive weed in some parts of the world such as Australia, United Kingdom, New Zealand, and parts of the United States. Surfaces of ponds, reservoirs, and lakes are covered by a floating mat thick. The plant's growth clogs waterways and blocks sunlight needed by other aquatic plants and especially algae to carry out photosynthesis, thereby deoxygenating the water. As it dies and decays, decomposers use up the oxygen in the water. It also prevents the natural exchange of gases between the air and the body of water the plant has invaded, causing the waterway to stagnate. This can kill any plants, insects, or fish trapped underneath its growth.
Its ability to grow and cover a vast area makes it a threat to biodiversity. Large infestations covering a wide area may also pose a problem to migratory birds, as they may not be able to recognise an infested waterway when flying overhead, and not stop at it. S. molesta provides ideal conditions for the breeding of mosquitoes that carry disease. The growth habit of Salvinia is problematic to human activities including flood mitigation, conservation of endangered species and threatened environments, boating, and irrigation.