Environmental Management: Principles and practice

Assimilative capacity
Discussion of resources usually focuses on inputs. As population increases and people
over-stress the land, congregate in urban areas, demand manufactured goods, and
are fed with the produce of modern farming, these all lead to outputs—pollution.
Ecosystems can render a certain amount of a pollutant harmless—their assimilative
capacity. This varies from ecosystem to ecosystem, requires time, and is affected by
the types and quantities of pollutant received. Where biotic processes cope with
pollution this capacity can be seen as a renewable resource. However, the sudden
arrival of a very toxic compound, large quantities of the usual pollutants, or unusual
weather conditions may cause a breakdown of assimilative capacity that is difficult
to repair and means undiminished pollution until it is restored. Environmental
management must consider outputs as well as inputs.
Trophic level and organic productivity
Organisms in an ecosystem can be grouped by function according to their trophic
level (the level at which they gain nourishment). Each successive trophic level’s
organisms depend upon those of the next lowest for their energy requirements (food).
The first trophic level, primary producers or (autotrophs), in all but a few cases
convert solar radiation (sunlight) to chemical energy (the exceptions include
hydrothermal-vent communities and some micro-organisms deep below ground).

---

CHAPTER SEVEN
138
Seldom are there more than four or five trophic levels because organisms expend
energy living, moving and in some cases generating body heat—and transfer of energy
from one trophic level to the next is unlikely to be better than 10 per cent efficient.
Given these losses in energy transfer, it is possible to feed more people if they eat at
a low rather than high trophic level—put crudely, a diet of grain supports more people
than would be possible if it were used to feed animals for meat, eggs or milk (it has
been calculated that only about one part in 100,000 of solar energy makes it through
to a carnivore).
The sum total of biomass (organism mass, expressed as live weight, dry weight,
ash-free dry weight or carbon weight) produced at each trophic level at a given point
in time is termed the standing crop. This needs to be treated with caution; if taken at
the end of an optimum growing period it indicates full potential; if taken during a
drought, cool season, period of agricultural neglect or insect damage, it is an
underestimate of possible production. Primary productivity can be defined as the
rate at which organic matter is created (usually by photosynthesis, although in some
situations by other metabolic processes) at the first tropic level. It can be established
in several ways. The total energy fixed at the first trophic level is termed gross primary
production. Minus the estimated respiration losses, this gives net primary productivity
(in g m
–2
d
–1
or g m
–2
y
–1
). Net primary productivity gives a measure of the total
amount of usable organic material produced per unit time. Most cultivated ecosystems,
i.e. efforts to stretch food and commodity production, are well below the net primary
production of more productive natural ecosystems. There is thus, in theory, potential
for the improvement of existing agriculture.
Thus, ecologists have developed a number of concepts and parameters, some of
which have been adopted (sometimes modified) by those seeking to manage the
environment. The most widely used are: maximum sustainable yield, and carrying
capacity (Box 7.1). These should be treated with caution. Maximum sustainable yield
may be correctly calculated, but if environment changes a ‘reasonable’ resource
exploitation strategy leads to over-exploitation. Maximum sustainable yield calculations
can thus give a false sense of security. A given ecosystem can have more than one
carrying capacity, depending on the intensity of use, the technology, etc. Some organisms
adjust to their environment through boom and bust, feeding and multiplying during
good times, and in bad suffering population decline, migrating or hibernating;
calculating carrying capacities for such situations can be difficult. Biogeophysical
carrying capacity may differ from the behavioural carrying capacity, such that a
population could be fed and otherwise sustained but feel crowded and stressed to a
degree that limits their survival. Ultimately, the more people the Earth supports, the
lower the standard of living they are likely to enjoy, and the more conflict and
environmental damage are probable (although there may be situations where human
population increase does not exacerbate environmental degradation or result in lower
standards of living: see discussion of Boserüp in chapter 2). With foreseeable technology,
adequate standards of living and satisfactory environmental quality probably demand
that human population on Earth be less than today’s 5,000 million plus. We are told by
the media that world-wide much more is spent on golf than on family planning aid; the
golfer environmental manager should reflect on this!

---

SCIENCE
139