Marsh and bog, so hostile to man, are
all the more hospitable to other members of
the animal kingdom. It is not easy to draw an absolute distinction between them,
but while 'marsh' (or 'swamp') conjures up a picture of relatively low-lying ground
bordering a sluggish river or a lake, seasonally flooded but always wet, 'bog'
denotes soft, spongy ground saturated at all times but not necessarily near open
water. Many bogs have formed in ancient lakes. As rivers continuously deposit
sediments where they enter a lake they slowly fill it, creating marshy borders with
their typical water-loving vegetation of reeds and sedges. As the area of open
water grows smaller the marsh plants retreat with it; the muddy edges are littered
with decaying vegetation, mosses such as sphagnum take over and eventually
perhaps the whole basin of the former lake is filled with waterlogged sour soil
supporting typical bog plants. Lowland bog may in the course of time—or aided by
man's drainage schemes—dry out and provide good arable land. Bog also forms
on mountain slopes where water is trapped by the configuration of the land, and
here the poor, usually acid, soil is likely to remain a 'wet desert'. In certain climatic
and geological conditions peat forms from the partially decomposed remains of
plants, accumulated either under water or on its margins, at sufficient depth to
exclude the oxygen required for complete decay.
A wetland may be defined as an area of
land whose characteristics are determined
by the presence of water, either permanent waterlogging or through regular, usually
seasonal, flooding. If permanently inundated, this is to a depth shallow enough to
allow the growth of emergent vegetation or rooted macrophytes. Within this broad
definition are many options for subdivision, reflecting the diverse nature and modes
of origin of the world's wetlands, but all share important ecological similarities as a
result of the high water table, hence their consideration together here. They do,
however, fall into two fundamental types, described as aquatic marginal and mire.
