Geoscience Reference
In-Depth Information
8
Wetland change
8.1 Introduction
framework remains relatively stable for extended
periods, then internal (autogenic) developments
dominate wetland evolution. Somewhat stronger
and more variable external factors could either
accelerate or inhibit internal developments, still
with relatively continuous, but changing wetland
conditions. In extreme cases, major changes
by external factors may bring such instability
that the wetland functions break down and the
wetland ceases to exist. In regard to peatlands,
Charman (2002) identii ed i ve primary factors
for their development and change, namely
climate, geomorphology, geology and soils, bio-
geography, and human activities. These i ve
factors apply to all other kinds of wetlands as
well.
All wetlands are subject to both autogenic
and allogenic processes which are operating
constantly at different rates and changing
through time. These various factors often inter-
act with each other in feedback relationships.
As a simple example, consider the role of cloud
cover for the Earth's climatic system. During
an episode of global warming, more water
would evaporate from the oceans, lakes and
wetlands, thus leading to increased atmos-
pheric moisture and greater cloud cover. As
cloud cover increases, more incoming solar
radiation is rel ected into space resulting even-
tually in a cooler climate and a return to pre-
warming conditions. This scenario illustrates a
negative feedback relationship, which leads to
long-term stability.
All aspects of the Earth's environmental system
change. Some features change quickly, others
imperceptibly slowly. Some changes take place
in a constant or cyclic fashion; other changes
are irregular, episodic, or even chaotic in nature.
Wetlands are no exception to this dynamic situ-
ation. As parts of the greater environmental
scheme, wetlands respond to outside inl uences
and may in turn affect those outside conditions.
Wetland changes may be considered under two
broad headings (Charman 2002):
• Autogenic - changes that take place within
a wetland habitat caused by natural growth
and evolution due to internal forcing factors.
These changes may be brought about by
vegetation succession, soil development,
sediment burial, and other processes that
take place within the wetland.
• Allogenic - changes induced by external
forcing factors, which are of sufi cient mag-
nitude to inl uence or alter internal pro-
cesses. External factors include changes in
climate, rising sea level, tectonic uplift or
subsidence, volcanism, i re, modii cations of
drainage, and so on.
The balance between autogenic and allogenic
effects depends on magnitudes and rates of
various processes working within and upon wet-
lands. If the external (allogenic) environmental
