Human Impacts

Humans have had, and continue to have, serious impacts upon wetlands in general, and marshes in particular. Some human impacts include draining, damming, eutrophication, and alteration of food webs. Let us consider these in turn.

One of the most obvious ways in which humans affect wetlands is by draining them. When the wetlands are drained, the soil becomes oxidized, and terrestrial plants and animals replace the wetland plants and animals. Often, drainage is followed by conversion to agriculture or human settlement, entirely removing the marshes that once existed. Vast areas of farmland in Europe, Asia, and North America were once marshes and have now been converted to crops for human consumption. Many countries now have laws to protect wetlands from further development, although the degree of protection provided, and the degree of enforcement, varies from one region of the world to another. Wetlands are also often included in protected areas such as national parks and ecological reserves.

Construction of dams can also have severe negative effects upon wetlands. The dams may be built for flood control, irrigation, or generating electricity. The wetland behind the dam may be destroyed by the prolonged flooding, whereas the wetlands downstream are disrupted by the lack of normal flood pulses. A single dam can therefore affect a vast area of wetlands. The degree of damage depends upon the pattern of water level fluctuations in the reservoir behind the dam, but in general large areas of marsh are lost both upstream and downstream from the dam. Sediment that would have expanded and fertilized wetlands during periodic floods becomes trapped behind the dam. Most of the world's large rivers have now been significantly affected by dams. To protect wetlands, it is necessary to identify rivers that are still relatively natural and to prevent further dams from being constructed. In other cases, it is possible to remove dams and allow natural processes to resume. An artificial levee can be considered a special type of dam that is built parallel to a river to prevent it from flooding into adjoining lands. Levees harm marshes by preventing the annual flooding, and by allowing cropland and cities to move into floodplains.

Humans can also affect wetlands by changing the nutrients in the water. Sewage from cities provides a specific 'point source' of nutrients, particularly nitrogen and phosphorus, that enter water courses then spread into wetlands. Activities such as agriculture and forestry provide 'diffuse sources' of nutrients, where runoff from large areas carries dissolved nutrients, and nutrients attached to clay particles, into the water and into adjoining marshes. The added nutrients can stimulate plant growth, which may seem to be beneficial - but it often leads to significant changes in the biota. Rarer plants and animals that are adapted to low fertility are replaced by more common plants and animals that exploit fertile conditions. Rapid growth of algae, followed by decay, can eliminate oxygen from lakes, causing fish kills. Protecting the quality of marshes therefore requires two sets of actions. First, it is necessary to control the obvious point sources of pollution by building sewage treatment plants. Second, it is necessary to use entire landscapes with care, with the broad objective of reducing nutrients in runoff. This can involve carefully timing the fertilization of crops, maintaining areas of natural vegetation along watercourses, fencing cattle away from stream valleys, minimizing construction of new logging roads, and avoiding construction on steep hill sides.

Herbivores are common in wetlands, and a natural part of energy flow from plants to carnivores. Common examples of large herbivores include moose, geese, muskrats, and hippopotamuses. Humans can disrupt wetlands by disrupting the natural balance between herbivores and plants. Herbivores can increase to destructive levels in several ways. When humans introduce new species of herbivores, rates of damage to plants may increase greatly - for example, nutria introduced from South America are causing significant damage to coastal wetlands in Louisiana. When humans reduce predation on herbivores, they may also increase to higher than natural levels. Killing alligators may damage wetlands by allowing herbivores such as nutria to reach high population densities; similarly, the loss of natural predators may be one of the reasons that Canada geese have multiplied to levels where they can destroy wetlands around Hudson Bay. There is also evidence that when humans harvest blue crabs, snails that the crabs normally eat begin to multiply and damage coastal marshes. These types of effects are difficult to study, since the effects may be indirect and take place over the long term.

Road networks are a final cause of damage to wetlands. The obvious effects of roads include the filling of wetlands, and the blocking of lateral flow of water into or out of wetlands. But there are many other effects. When amphibians migrate across roads to breeding sites, vast numbers can be killed by cars. In northern climates, the road salt put on roads as a de-icer can flow into adjoining wetlands. Snakes may be attracted to the warm asphalt and killed by passing cars. Invasive plant species can arrive along newly constructed ditches. Overall, roads change a landscape by accelerating logging, agriculture, hunting, and urban development. As a consequence, the quality of the marshes in a landscape is linked to two

Table 1 The world's largest wetlands (areas rounded to the nearest 1000 km2)

Rank

Continent

Wetland

Description

Area (km2)

Source

1

Eurasia

West Siberian Lowland

Bogs, mires, fens

2 745 000

Solomeshch, chapter 2

2

South America

Amazon River basin

Savanna and forested floodplain

1 738000

Junk and Piedade, chapter 3

3

North America

Hudson Bay Lowland

Bogs, fens, swamps, marshes

374000

Abraham and Keddy, chapter 4

4

Africa

Congo River basin

Swamps, riverine forest, wet prairie

189000

Campbell, chapter 5

5

North America

Mackenzie River basin

Bogs, fens, swamps, marshes

166000

Vitt et al., chapter 6

6

South America

Pantanal

Savannas, grasslands, riverine forest

138000

Alho, chapter 7

7

North America

Mississippi River basin

Bottomland hardwood forest, swamps, marshes

108000

Shaffer et al., chapter 8

8

Africa

Lake Chad basin

Grass and shrub savanna, shrub steppe, marshes

106000

Lemoalle, chapter 9

9

Africa

River Nile basin

Swamps, marshes

92 000

Springuel and Ali, chapter 10

10

North America

Prairie potholes

Marshes, meadows

63 000

van der Valk, chapter 11

11

South America

Magellanic moorland

Peatlands

44 000

Arroyo et al., chapter 12

Modified from Fraser LH and Keddy PA (eds.) (2005) The World's Largest Wetlands: Ecology and Conservation. Cambridge: Cambridge University Press.

Modified from Fraser LH and Keddy PA (eds.) (2005) The World's Largest Wetlands: Ecology and Conservation. Cambridge: Cambridge University Press.

factors: the abundance of roads (a negative effect) and the abundance of forest (a positive effect). Although it may not be obvious, halting road construction (or removing unwanted roads) and protecting forests (or replanting new areas of forest) may have important consequences for all the marshes in a landscape.

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