Environmental Management: Principles and practice

BOX 7.1  Ecological concepts and parameters which are useful for
environmental management
* Maximum sustainable yield.
The fraction of primary production (as organic matter) in excess of what is
used for metabolism (net primary production) that it is feasible to remove on
an ongoing basis without destroying the primary productivity—i.e. ‘safe
harvest’. Under US law, maximum sustainable yield would be defined as:
maintenance in perpetuity of a high level of annual or regular periodic output
of renewable resources.
* Carrying capacity.
Definitions vary and can be imprecise. Examples include: the maximum number
of individuals that can be supported in a given environment (often expressed in kg
live weight per km
2
); the amount of biological matter a system can yield, for
consumption by organisms, over a given period of time without impairing its ability
to continue producing; the maximum population of a given species that can be
supported indefinitely in a particular region by a system, allowing for seasonal and
random changes, without any degradation of the natural resource base.
* Assimilative capacity.
The limiting resource may not be an input like food or water, it may be inability
to deal with outputs (waste products). A given environment has some capacity
to purify pollutants up to a point where the pollutant(s) hinder or wholly destroy
that capacity—this is termed the assimilative capacity.
Carrying capacity can be stretched by means of trade, technology and military
power (the latter ensures tribute from elsewhere—assuming it is available to be taken).
Often net primary productivity increases at the cost of species diversity. The timing
of resource use may be crucial: for example, rangeland might feed a certain population
of livestock, provided grazing is restricted for a few critical weeks (at times when
plants are setting seed, becoming established or are otherwise temporarily vulnerable).
If this is not done, or a disaster like a bushfire strikes, land degradation occurs and
far fewer livestock can be supported in the future.
Within even the simplest ecosystems there are complex relationships among
organisms and between organisms and environment: intertwined chains forming a
food web; complex pathways along which energy (food) and perhaps pollutants are
passed; subtle interdependencies for pollination, seed dispersal, etc. Pesticides,
radioactive isotopes, heavy metals and other pollutants can become concentrated in
organisms feeding at higher trophic levels: apparently harmless background
contamination could, through such biological magnification (bio-accumulation),
prove harmful to man and other organisms without assimilative capacity having

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obviously broken down (Carson, 1962). Today it is known that pollutants like PCBs
and DDT, which are present globally at low levels get concentrated by the food web
to such a degree that birds of prey and other predators suffer serious poisoning, and
there may be other unpleasant discoveries to be made.