Wetlands Ecosystems Dam construction and water diversion projects also have significant effects on downstream riparian vegetation by altering peak and minimum flows, decreasing erosion and deposition of sediment, and lowering floodplain water tables Altering the natural variability of river flow can adversely affect the species composition and structure of riparian wetland vegetation Dredging, channelization, and levee construction have greatly impacted extensive wetland areas in the Mississippi River Delta Groundwater extraction is a primary cause of wetland modification because it lowers the regional water table and alters recharge and discharge patterns within the wetland and surrounding landscape Irrigation with water supplied by aquifers that are recharged in part by wetlands has led to declines in the area of major regional wetlands in North America (e.g., the prairie pothole and Nebraska sandhills regions) Although plant species of coastal wetlands may be well adapted to flooding, increases in the level or duration of flooding above that at which species have long persisted can cause serious deterioration of these wetlands Sea-level rise and saltwater intrusion are important factors in the degradation and loss of wetland forests and coastal marshes in the southeastern United States Increased flooding and salinity have been particularly damaging to forests dominated by baldcypress Landscape Fragmentation Other human activities impoverish the wetland fauna in more subtle ways, even where wetlands receive nominal legal protection Destruction of the uplands adjacent to wetlands destroys the habitat interface critical for amphibians and reptiles that migrate between wetland and upland habitats Reductions in the connectivity among wetlands (e.g., by construction of roads and levees) blocks the migration routes of amphibians, fish, and birds, and can lead to local population extinctions In contrast, dredging of canals that link major river systems causes sudden mixing of fish and reptile faunas that may have evolved in isolation for thousands of years Other Threats Anthropogenic inputs of nutrients and toxins drastically alter the chemical environment of wetlands and render many sites unsuitable for wetland plants and animals Atmospheric deposition of nitrogen and sulfur is particularly high in parts of western Europe but is also high in the northeastern United States Atmospheric deposition may be especially damaging to peatlands dominated by bryophytes (mosses and lichens), because these species are very sensitive to changes in nutrients and acidity-alkalinity Wetlands in agricultural landscapes receive excess nitrogen via ground water draining from agricultural fields and excess phosphorus primarily via surface water runoff Residential development also contributes excess nitrogen and phosphorus to wetlands Coastal oil spills can adversely affect the vegetation of salt marshes and tidal freshwater wetlands Invasive, exotic wetland species typically cause a substantial loss of native wetland species and greatly alter wildlife habitat In otherwise arid regions, invasive species use substantial 399 amounts of water and can desiccate watercourses Invasive species generally respond favorably to altered hydrologic regimes, substrate disturbance, or changes in water quality, especially eutrophication Introduction of exotic wetland animals, for example, carp, nutria (Myocaster coypu), and bullfrogs, outside their native range have greatly altered wetland environments throughout much of North America, mostly to the detriment of the native wetland fauna Purple loosestrife (Lythrum salicaria), an emergent, herbaceous species of Eurasian origin, has become a serious invasive species of open wetlands in eastern North America A single individual of this species can produce an average of 2,700,000 seeds Another very serious invasive species in wetlands of the eastern United States is the common reed (Phragmites communis), which can produce 200 to 300 culms mÀ2 in fresh and brackish wetlands through an extensive network of rhizomes Recent biological control methods (i.e., leaf- and root-feeding beetles) appear promising for L salicaria Salt cedars (Tamarix spp.) are rapid, arborescent invaders of riparian wetlands in the southwestern United States, especially downstream from dams where flooding is minimal Another invasive tree species of riparian wetlands in the western United States is Russian-olive (Elaeagnus angustifolia) In southern Florida, the invasive tree, melaleuca (Melaleuca quinquenervia), has been aggressively colonizing shallow wetlands Two other tree species that seriously threaten southern Florida wetlands are Australian pine (Casuarina) and Brazilian pepper (Schinus terebinthifolius) Glossy buckthorn (Rhamnus frangula) is becoming a serious problem in wetlands around the eastern Great Lakes basin Some native species become invasive following some hydrologic changes or an increase in nutrients and dominate wetlands previously occupied by other, more desirable species Cattail is regarded as a threat to southern Florida marshes that have been dominated by sawgrass (Cladium jamaicense) The spread of cattail is believed due mostly to increased phosphorus input from agricultural lands and anthropogenic changes to the regional hydrology Cattail is also considered a weed in many wetlands and aquatic habitats that are used for agriculture or to provide water resources (e.g., rice fields, irrigation canals, recreational lakes, and reservoirs) Mechanical (e.g., cutting, water level modification, fire, shading), chemical (selective versus nonselective herbicides), and biological control methods vary in their effectiveness, which varies widely among species Maintaining water levels over 0.5 m is effective in controlling spread of cattail Natural Disturbances Severe flooding, hurricanes, and fire are the major natural disturbances that greatly affect wetlands Although wetland plant and animal communities are adapted to each of these disturbances, events of exceptional magnitude and duration, or that occur during unusual times of the year, can have profound effects A serious concern following any of these severe events is the spread of invasive species During the 1993 flooding of the Mississippi River, submerged aquatic, emergent wetland, and floodplain species