Seasonality is a feature of many wetlands,
particularly freshwater flood wetlands,
whose flooding will be related to seasonal patterns of rainfall. This, in turn, results in
seasonal patterns of abundance among wetland organisms. Both terrestrial and
aquatic assemblages may occupy the same site, the community changing rapidly in
response to water levels.
The vertebrate cycle
Spatial variations in topography enhance
diversity, as do temporal changes in
water level, particularly in seasonal flood plains where different stages in the flood-
drainage cycle favour different groups of species. There is seasonal movement in
and out of floodplain wetlands by river fish and similar migrations occur among
terrestrial and semi-aquatic groups.
The vertebrate cycle of an idealised north
temperate wetland illustrates this well.
During high water levels in the winter, the wetland is dominated by aquatic birds,
most of which are wintering species which migrate to breeding grounds further
north in the spring. As the water level recedes, aquatic species are confined to ever
smaller areas but terrestrial grazers and, later, fruit and seed-eating species move
in, exploiting the high productivity When water levels rise once more, aquatic
specie; return. In contrast, terrestrial birds may enter reedbeds in large numbers
during migration or ir search of food during winter.
Floodplain wetlands are not simply the
haunt, however, of river or terrestrial species
which move in when conditions are appropriate. There are also true wetland
species, adapted to cope with the seasonally changing conditions. Examples are
amphibians, such as frogs, which require pools of water during the spring for larval
development but become more terrestrial as adults, seeking out open water once
more in the autumn as a site for hibernation. An alternative strategy, exemplified by
the lungfish of tropical Africa and South America, is to spend the dry season in a
dormant state, known as aestivation, buried in the ground awaiting the return of the
floods. Lungfish are also able to exploit the permanently anoxic waters of the
interior of large fringing wetland.
Wetlands of the southeastern USA flood
during spring and early summer, leading to
a contrasting cycle in which detritus-based food chains predominate during the
growing season, when primary production is high, while herbivore pathways
dominate over winter, when primary production is low. This is because herbivorous
mammals such as deer and rodents are driven away by rising water levels, leaving
only specialist semi-aquatic species, such as muskrat (Ondatra zibethica);
herbivorous birds either migrate to northern breeding grounds or, if resident, shift
their diet to take advantage of large numbers of emerging insects. During the
summer, therefore, the flooded wetlands are dominated by fish and amphibians,
along with aquatic invertebrates, most of which are detritivores or predators, and it
is only during the autumn, when water levels recede and, coincidentally, primary
production declines, that herbivorous mammals can invade.
The human exploitation cycle
The vertebrate cycle has its parallels
in human exploitation of wetlands. The Inner
Niger Delta in Mali supports 2-3 million head of livestock, which move, along with
their human guardians, in an annual cycle dictated by flood water. During the rainy
season from August to October, the delta is flooded and livestock are grazed on
the surrounding higher ground. In October and November they gather around the
edge of the delta, ready to move into the newly exposed pasture as the floods
recede. At first, they range widely but, as the dry season progresses, they
aggregate around permanent water holes and channels, before moving out to
higher ground once more as floods return in July. In a further analogy to the
vertebrate cycle, as the livestock and human pas-toralists move out, the fish - and
the people who fish for them - return.
Sedentary agriculturalists, too, can exploit
the predictability of the seasonal flood.
Seasonal floodplains receive an annual input of generally nutrient-rich sediment
and, when the soil aerates, can be very fertile. This fertility can be exploited by
growing crops during the dry season. Managing the flood water using drainage
channels and sluices in levees, to hasten its removal, is a long-established and
effective system, and has been practised along the Lower River Nile in Egypt, for
example, for thousands of years. Water meadows are a consequence of this
technique. The flooding ensures that they are naturally fertilised every year, so they
can be very productive and heavily grazed over summer. The result is fertile,
botanically rich meadows, with waterlogged drainage ditches supporting a great
variety of wildlife, and the value of water meadows to conservation in areas heavily
modified by human activity is well understood.
Calculation of the extent of wetlands
is further compounded by the scale of their
loss, particularly in Europe, North America and southeast Asia. One of the most
infamous of the world's lost wetlands is the East Anglian Fenland of eastern
England, reduced from 3380km2 during the 17th century to around 10 km2 today.
The USA has had its area of wetland cover reduced from maybe 900000km2 in the
16th century to around 400000 km2 today, but losses have been unevenly
distributed, California and Ohio having lost 91 and 90%, respectively, of their
original wetland cover (Anderson, 1996). Such losses occur through active
destruction, particularly to create agricultural land, but also indirectly by lowering the
water table.
The Goto Dofiana in southern Spain is
one of Europe's largest wetlands, and is
protected as a National Park and a series of Natural Areas acting as buffers to
development, but extraction of groundwater in the surrounding region has reduced
its supply to the extent that its persistence as a wetland is under severe threat.
Natural successional processes lead to
the loss of many wetlands to terrestrial
environments, but these are countered by creation of new wetlands, particularly
along coasts and river floodplains. The process of destruction has, however, been
accelerated throughout the world by drainage and reclamation, while, at the same
time, natural wetland creation processes are being impeded. As more rivers are
impounded and coasts walled, so fewer new wetlands will be created to replace the
losses.
