Updates
November 2001:- First text-based beta
outline. Not all topics have been described.
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Introduction
'Wetlands' is an education/training resource
to support the study and applications
of conservation management to maintain or create wetland habitats. It arose from
discussions with industrialists, teachers and community leaders associated with the
BAIS project who felt that a holistic knowledge systems embracing ecology and
culture were needed to promotion conservation management at the community
level. The ecology and mangement of wetlands was chosen as the first topic
because of its worldwide importance to stem the decline of biodiversity. Also, it its
diversity, wetland conservation has to take into account the widest possible range
of management factors from regulating water levels to re-establishing rare species
in conjunction with traditional systems of wetland husbandry. The information and
examples are developed from the Nature Conservation Review. The review was
published by the Nature Conservancy Council in 1977. Essentially it is an account
of Britain's heritage of wildlife and habitats, and sets out criteria by which the
importance of sites for nature conservation can be judged. The system of grading,
which was largely the creation of Dr Derek Ratcliffe(subsequently Chief Scientist of
the NCC), was useful to begin to safeguard the more important sites. but the
present consensus is that every semi-natural habitat, no matter how small, is worthy
of protection and enhancement.
'Wetlands' is an interactive development
of Chapter 9 (Peatlands) of the review with
an emphasis on the limiting factors that have to be addressed in managing
wetlands.
At the moment there is a bias towards
British East Anglian sites because of the
involvement of people and organisations from this region in providing examples of
projects and plans.
Definition of wetlands
Wetlands are characterised by soils poor
in oxygen, which often contain toxic
chemicals and support bacterial activity which removes nitrate nutrients.
Wetlands are areas of land defined by
a water table which is permanently or
frequently high, leading to waterlogged or flooded soils. Aquatic-marginal wetlands
are created by rivers, lakes and the sea. Mires are fed by groundwater, overland
runoff or precipitation. Groundwater fed mires normally develop from infilling a lake
or an arm of the sea. There is a clear successional sequence, the end point of
which is often Sphagnum mire.
Aquatic marginal wetlands have nutrient
supplies replenished from the parent water
body and, if seasonally dry, through decomposition, whereas permanently flooded
mires may be nutrient poor.
Biodiversity and productivity
Wetland plants generally have very specific
tolerances to environmental conditions,
leading to marked vertical zonation over very small elevational ranges. Their
precise requirements ensure that they are very sensitive to fluctuations in water
level. Sphagnum moss can, however, control the water table to its own advantage,
creating mires which often cover large areas. Few vascular plants possess the
adaptations required to persist in Sphagnummires.
Seasonally flooded wetlands ('flood wetlands')
and those on the edge of open water
bodies ('fringe wetlands') are among the most productive environments on Earth,
whereas rain-fed mires have very low productivity, but even slow decomposition in
the waterlogged conditions leads to a buildup of partially decomposed plant
remains -peat.
Wetland diversity is enhanced by seasonal
changes, particularly in those which are
seasonally flooded and therefore support terrestrial and aquatic organisms at
different times of the year. Grazing is relatively uncommon, although many natural
wetlands are grazed by large aquatic or semi-aquatic mammals. Most primary
production is consumed as detritus. Human exploitation of wetlands is often
seasonal, including grazing livestock on rich pasture during the dry season and
fishing during the wet season.
Global extent
Calculation of the worldwide extent of
wetlands is difficult because they are very
scattered and definitions are often hard to apply. Wetlands do noi form a discrete
biome, so their extent cannot be estimated by delineating an appropriate climatic
type. A reasonable estimate would be that wet lands cover about 6% of the Earth's
land surface or around 8.5 million km2, of which coastal wet lands account for about
a quarter of the total. Freshwater wetlands are, however, very unevenly distributed.
Their greatest extent is in two climatic zones - the boreal anc tundra of the Northern
Hemisphere, and more fragmented, but still sizeable, patches in equator ial
regions. It has been estimated that approximately half of al natural freshwater
wetlands are in an almost con tinuous expanse of mire across Canada, Alaskj and
Russia, and a further quarter is aquatic marginal vegetation associated with rivers
anc floodplains in the Amazon region of South America. Outside these regions,
wetlands are generally small and scat tered. There are exceptions, such as the
Florida Everglades in the USA and the Okavango ir Botswana, but more typical are
those on narrow river floodplains, or small, discrete wetlands ir glacial hollows, such
as the prairie pothole region of the north-central USA or the border region o
Northern Ireland.
To the area covered by natural wetlands
mus be added cultivated rice paddies,
which cover i further 1.3 million km2, almost 90% of which ii in southeast Asia. The
distribution of natural wet lands shows a marked trough in northern subtropical
latitudes; to a large extent this is expected, because this marks an aric climatic
zone, but it is also the latitudinal regior with the greatest concentration of rice
paddie.
Coastal wetlands, in contrast to freshwater
wetlands, show a relatively even
distribution around the world's coasts, although saltmarshes are confined to
temperate regions and mangals to the tropics, with very few areas of overlap. This
distribution is related to climate: mangroves, some species of which grow in
excess of 10m high, easily outcompete saltmarsh plants by overshading, but are
very sensitive to frost. In Florida, one of the few places where both wetland types
coexist, black mangrove (Avicennia nitida) is normally dominant, but is occasionally
killed by frost, allowing saltmarsh species to flourish until it recovers.
Benfits
Wetlands absorb excess nutrients and flood
water, as well as being a valuable
source of natural resources. Losses, particularly to agriculture, have been
extensive. There are now some examples of creative conservation aimed at
returning drained agricultural land to its former ecological state.
