Environmental Engineering Reference
In-Depth Information
system, comprising a diverse complexity of components where every niche is filled in
order to maximize efficiency (Plate 22.4), to an unstable condition where there are few
components or, in the case of the plantation, just one. The human objective is to harvest
nutrients with a minimum of inputs. Inevitably, where such land-use systems expand in
their areal coverage within the tropical rain forest zone, the entire ecosystem becomes
unstable (see Chapter 28).
CONCLUSION
The International Biological Programme, which commenced in 1964, has stimulated
much data collection and ecological stocktaking in all the world's biomes. The aim was
to solve the fundamental ecological equations dealing with productivity, biomass,
nutrient status and energy assimilation. Rates of photosynthesis in different biomes vary
with light intensity, temperature, moisture and soil nutrient content. Thus latitude is a
great determinant of productivity on land through its effects on radiation, temperature,
moisture and the length of the growing season. By contrast productivity in oceans is
much more closely linked with the availability of nutrients. The productive zones in the
oceans occur where the mixing of ocean currents brings sedimentary particles to the
surface to feed the phytoplankton. This occurs more readily at mid to high latitudes.
Soil micro-organisms play a key role in nutrient cycling. The largest store of nutrients
in many ecosystems is the organic matter, whether living in biomass or dead in litter and
humus. The nutrient elements contained in those stores are mineralized by microbial
pathways of decomposition, releasing cations and anions which can again be absorbed by
plants. These microbial processes are mostly carried out by a wide range of general-
purpose soil microorganisms, but in the case of nitrogen and sulphur many reactions are
carried out by highly specialized autotrophic bacteria.
FURTHER READING
Dickinson, G., and Murphy, K. (1998) Ecosystems , London: Routledge. An extremely useful
summary of the properties of ecosystems.
Ricklefs, R. E. (1990) Ecology , third edition, New York: Freeman. The latest edition of this classic
work on ecological systems.
Tivy, J. (1993) Biogeography , third edition, London: Longman. A standard text on ecosystems with
many world examples.
KEY POINTS
1 Gross primary productivity (GPP) is the sum total of energy fixed by autotrophic
organisms through photosynthesis. That energy which is not used by the autotrophs
themselves for respiration is termed net primary productivity (NPP). Some of the NPP
will be grazed each year by herbivores, and some organisms will die and become
decomposed. The remainder will cause an increase in biomass.
2 Production in ecosystems depends on an assured supply of nutrients including water in
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