The total coverage of existing wetlands is estimated to range from at least 7 million km2 to about 10 million km2, or 5-8 percent of the land surface of the world, depending upon the definition for what is included (Mitsch and Gosselink, 2007). Agricultural influences, urbanization, subsidence, and climate are all important allogenic factors in wetland study and restoration. Many other factors also affect wetlands, but they are beyond the scope of this webpage.
Wetland preservation and restoration in developed countries is relatively achievable for the funds, and institutional and legal capacity exist while it is difficult in developing countries where there is pressing needs for increased food production with the limited funds available. The impact of irrigation development is severe in ecological effects in developing countries for there are little or no available funds and there is need for increased food production in the long term (Galbraith et al., 2005).
According to the Organization for Economic Co-operation and Development (OECD 1996), it was stated that the world may have lost 50 percent of the wetlands that existed since 1900. Much of this occurred in the northern countries during the first 50 years of the century, increasing pressure for conversion to alternative land use has been put on tropical and sub-tropical wetlands since the 1950s (Wetland loss and degradation and planning for sustainable development, sylvanus_mohamad_wetlands). Drainage for agricultural production is thought to be the principal cause for wetland loss worldwide, though no quantitative data exists to back it up (Ramsar Convention, 1991). In Europe and North America, it was estimated that 56-65 percent of the available wetland had been drained for intensive agriculture by 1985. For the rest of the world the figures were 26% divided in the following manner: 27% for tropical and subtropical regions of Asia, 6% for South America and 2% for Africa. It is predicted that the pressure to drain land for agriculture will intensify in these regions. As of December 2002, 1230 wetlands of international importance were recognized based on the wetland international Ramsar sites database of 2002 in the six regions of the world, Europe, Asia, Africa, Neotropics, Oceania and North America (Galbraith et al., 2005).
The fact that scarce water resources characterize developing countries with high demand for agricultural demands shows the extensive impacts on wetlands through greater water extractions from wetlands (Galbraith et al., 2005). The causes of temporary or permanent wetland loss due to agricultural activities in these places may include draining and conversion to agricultural land, water withdrawal from rivers and streams for irrigation, damming for water storage, changed hydrologic cycle from water storage, salinization, sediment deposition, erosion, and eutrophication, pollution from use of pesticides and other chemicals and creation of wetlands (Galbraith et al., 2005).
Irrigation or water diversion could cause the submersion of wetlands or their replacement by upland vegetation communities, with consequent effects on the biota that depend on these wetlands and the services that humans hitherto derived from the systems.These result in major impacts to downstream wetland ecosystems, including dewatering and drying and submersion and replacement by aquatic communities. The ecological impacts of irrigation or water diversions for irrigation can occur a short distance from the dam or irrigation site, or many miles downstream (Galbraith, H. et al, 2005).
|Drainage tiles are installed to convert wetlands to useable land for agriculture, a major cause of wetland loss. Photo courtesy of Wikimedia Commons|
Among the few examples that led to wetland degradation and loss include (Galbraith, H. et al, 2005):
Damming, irrigation and diversion of flow of water for the purpose of agriculture have been found to be among the major causes of ecological degradation and wetland loss. In case of diversion of water streams, the scale of diversion determines the severity of the impact on wetlands. Changing traditional management practices at long-established projects such as switching from ditch-fed irrigation to concrete/metal pipes can also result in ecological impacts(Galbraith et al., 2005).
Among the other stressors to wetlands is pollution of lakes that feed wetlands, wildlife responses to irrigation, urbanization, industrial development, timber harvest and mining (Galbraith et al., 2005).
Nearby city growth could potentially have a catastrophic impact on wetlands. The effects of an increasing human population may put severe strain on an environment, whether intentional or not. In a delicate region which depends on specific conditions to sustain itself, such as a wetland, these effects might easily be disastrous. Notable examples include the impact of construction projects in wetlands and of urban runoff.
Construction within a wetland is likely to completely disrupt the natural processes which take place there. First of all, the building of manmade structures displaces the natural flora and fauna of the region, especially those which require a certain type of habitat to survive. In addition, the flow of water within the wetland is completely altered (Faulkner, 2004). Removal of the vegetation means that less water is held back during a flood event, thereby worsening it (Aber et al., 2012). Urban construction creates an increase of impervious surface and lessened sediment production, which leads to incisement of stream channels and lowering of the riparian zone water table. Wetlands also retain and transform pollutants in the water, such as nitrate, phosphorus, and heavy metals through biogeochemical and sedimentation processes, which cease when the wetland is removed, resulting in an increase of pollution in the region. Even no net loss policies are ineffective at correcting these problems, as the mitigation wetlands put in place to meet these requirements are often installed away from the original watershed (Faulkner, 2004).
|A construction project to build a highway through the Baker Wetlands, located south of Lawrence, KS. Photos by A. McElroy|
|Google Earth image showing what the area looked like in 2011, before construction began. Yellow star denotes where the above photos were taken.|
Because of the increase in impermeable land surface, overland runoff occurs much more readily in urban environments within wetland regions, further increasing flooding (USGS, 2014).This also has the effect of carrying any pollutants, such as automotive oil residue or salt from road treatments, directly into the drainage basin. In spite of this increase in runoff, groundwater recharge is not actually lessened, due to leaky water pipes, sewer mains, septic tanks, and other manmade structures (Lerner, 1990). However, these sources often contain human sewage or other contaminants. Together with the runoff, this severely reduces water quality and harms the plant life within the wetland, which often require specific water and soil conditions to thrive. This in turn, harms the animals which depend on the plants for food and shelter (Aber et al., 2012).
Subsidence is the land shifting downward relative to a datum. It is a global problem, and more than 17,000 square miles in 45 States have been directly affected by subsidence (USGS, 2013).
|Though not a wetland, the San Joaquin Valley is an excellent example of human-induced subsidence. Heavy groundwater pumping in the 20th century led to the ground level declining sharply. The signs on the pole show ground surface level in 1925, 1955, and 1977. Photo courtesy of United States Geological Survey.|
In wetland discussions, subsidence is the sinking of the surface and can be caused by a number of factors. For example, sub-surface mining has occurred throughout the southwest corner of Kansas. The extracted area can collapse and cause subsidence to occur. This is typically localized to the surface directly above the mined area.
|A subsidence event in the Tar Creek Superfund region. Images show a pond before (top, courtesy of Google Earth) and after (middle, photo by M. Barlow) subsidence occurred. The subsidence left a large sink hole opening into an abandoned mine shaft, into which all the water flowed (bottom, photo by M. Barlow). Star notes the same location in all three photos.|
Subsidence in coastal marshes involves two factors: the sinking of the marsh surface and a lack of sediment being added to the marsh surface. As materials in the marsh settle, decay, and compact, the marsh effectively sinks a little. If new sediments are not being added to the system, the entire system is going to decline in elevation. Water will now pool in these lower elevation spots when there is a particularly high tide (USGS, 2014).
As the temperature of the earth increases little by little (global warming) and the sea level begins to rise, these areas become permanently flooded, the marsh is lost, and more open water areas result. To some degree leveeing and channelization have impacted this sinking and building up of the marshes because they reduce the amount of sediment available for vertical marsh buildup. A healthy marsh system with adequate amounts of vegetation will tend to trap whatever sediment is available and promote vertical buildup (USGS, 2014).
|Degraded coastal marshes (left) may be improved by constructing terraces (right), which prevents further loss of marsh from erosion and improves water conditions to promote growth of submerged aquatic plants. Photos used with permission from Texas Parks and Wildlife Department.|
Climate change directly affects wetland habitats and biodiversity within the ecosystem. As the climate changes over a long period, plants and animals have to adapt based on their ability to survive. Plant hardiness zones may be modified with long-term climate change.
|These maps show a change in the hardiness zones in just 16 years. With a close look at Nebraska, Zone 4 appears to have moved completely out of the state. For wetland plants that thrive in Zone 5, it would appear their environment could grow and thrive. However, if plants thrive in zone 7, any wetlands in southern Oklahoma may not survive as zone 8 encroaches within that environment. Maps courtesy of the National Arbor Day Foundation.|
Climate change will also have an impact of bird and butterfly migration as the habitats change based on the availability of precipitation. During drought years, wetlands may be dried up or non-existent which affect the survival of several species that depend on the availability of food sources and suitable breeding habitats.
There are many types of allogenic changes which are capable of harming wetlands. Water drainage for agriculture is considered a primary cause of wetland loss worldwide. This tends to be worse in developing countries, which have fewer resources and less regulation. Effects caused by urbanization will also put detrimental pressure on wetlands. Construction within them displaces native plants and animals, and disrupts the overall hydrology of the region. Pollution from overland runoff or from leaky sewers and septic tanks harms the ecosystem even further. Subsidence occurs when the wetland surface shifts downward, and can have a number of different causes. Sub-surface mining may cause the above region to collapse. In coastal marshes, settling of the marsh surface and a lack of sediment influx both contribute to subsidence. This effect is worsened by global warming and sea level rise. Long-term climate change has a strong effect on wetland habitats, as it may change the plant hardiness zone of a region and affect animal habitats due to changing water availability.