Environmental Engineering Reference
In-Depth Information
Figure 4.2
A conceptual framework illustrating how landscape systems function in terms of a sequence of events and
processes affecting the economy (gains and losses) of vital resources. In healthy systems, these gains and losses fl uctuate, but
in the long term they are in a dynamic equilibrium.
water is lost and soil erodes (#2b) from the landscape.
Broadly, the reserve represents a pool or bank of eco-
system resources such as soil moisture, soil nutrients,
seeds and the biomass of plant, animal and decom-
poser organisms. When water reserves exceed a
threshold, organisms experience a pulse of growth.
For example, seeds germinate and establish new
plants. Such pulses of plant growth provide the
resources (food, fi ber, fi rewood and forage) that people
the world over utilize. This use of resources is what we
call offtake processes (#3) such as the consumption of
forage by livestock, which are subsequently locally
consumed or taken to markets. Part of the growth
pulse also delivers
feedbacks
(#4); these gains can be
viewed as being largely biological feedbacks (#4a) or
physical feedbacks (#4b). Biological feedbacks include,
for example, the rebuilding of soil nutrient reserves by
the decomposition of plant biomass produced during
growth pulses. Physical feedbacks include, for example,
increases in the size and/or density of vegetation
patches due to pulses of plant growth, which then
enhances the capacity of the landscape to obstruct
overland fl ow and hence infi ltrate more water during
the next rainfall event.
We all know that ecosystems and landscape systems
fl uctuate in the short term in their resource gains and
losses, which we metaphorically depict as weights that
are balanced (Figure 4.2) but fl uctuating due to sea-
sonal effects and minor disturbances. However, in
'healthy' landscapes, in the long term, there is an
overall balance between gains from feedbacks and
losses due to offtakes. This dynamic balance between
gains and losses can be greatly affected by distur-
bances, which in turn may cause excessive offtakes
(e.g. fi re and grazing). Disturbances reduce biological
and physical feedbacks from growth pulses (e.g. the
amount of biomass reserves and the size of run-on
patches of vegetation).
In 1997, we published the fi rst version of our con-
ceptual framework consisting of a sequence of dynamic
landscape processes, and which included the notion of
' out - of - balance ' systems because of excessive losses
relative to gains (Tongway & Ludwig 1997). Since then,
we have updated and refi ned the original concepts a
