Environmental Management: Principles and practice

Ecosystems: definitions, values and function
People have long recognized functional environmental units, e.g. maquis scrubland
of southern France; the taiga forests of Siberia; Norfolk’s brecklands. Many would
view these as landscape units. Environmental managers have an alternative—the
ecosystem (ecological system), which is a basic functional unit of ecology (Golley,
1991). The term was coined by Sir Arthur Tansley (1935), and there are various
definitions, which include: ‘an energy-driven complex of a community of organisms
and its controlling environment’ (Billings, 1978); ‘a community of organisms and
their physical environment interacting as an ecological unit’ (Dickinson and Murphy,
1998); ‘an integration of all the living and non-living factors of an environment for
a defined segment of space and time’ (Golley, 1993).
A system is a set of linked components, where the linkages may not be direct—
a network or web with organisms as nodes within it (Figure 9.1). Table 9.1 suggests
two ways of classifying environmental systems, by function or degree of disturbance.
A naturalist might map the ecosystem of an animal, say a bear, by reference to the
resources it uses (i.e. as a function of the organism), so the area might alter with
seasons, differ according to the age or sex of the animal, or from individual to
individual; such an ecosystem would incorporate a number of distinct components
(valley, mountain forest, coastlands, etc., each of which could itself be recognized as
an ecosystem) (Gonzales, 1996). Alternatively, delineation could be by ecosystem
function (i.e. as a sort of landscape unit). The latter form is often selected as a working
unit by those concerned with environmental protection, the former by, for example,
a game warden.

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FIGURE 9.1 The relations between ecosystem components
Source: Van Dyne (1969:83, Fig. 3)
TABLE 9.1 Classifications of environmental systems
Note: Biosphere 2 is an enclosed environment experiment constructed some years ago in
the Sonoran Desert, USA

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Ecosystems may be recognized across a great range of spatial scales: one may
cover 10,000 km
2
, another less than 1 km
2
(one could argue the half-litre of water
trapped in a pitcher-plant, or a clump of lichen on a tombstone are ecosystems). In a
stable ecosystem each species will have found a position, primarily in relation to its
functional needs for food, shelter, etc. This position, or niche, is where a given
organism can survive most effectively. Some organisms have very specialized
demands and so occupy very restricted niches; others can exist in a very wide niche.
Niche demands are not always simple: in some situations a species may be using
only a portion of its potential niche, and alteration of a single environmental parameter
may suddenly open, restrict or deny a niche for an organism. Competition for the
niche with other organisms is one such parameter.
Ecosystems can be subdivided, according to local physical conditions, into
habitats (places where an organism or group of organisms live), populated by
characteristic mixes of plants and animals (e.g. a pond ecosystem may have a gravel
bottom habitat and a mud bottom habitat). Within an ecosystem change in one variable
may affect one or more, perhaps all other variables.
There are few ecosystems where there are no complex energy flows and
exchanges of materials across their boundaries. Even something as well-defined as a
cave may exchange water and nutrients with regional groundwater or capture debris
blown from outside (Bailey, 1986). To simplify study, ecologists have attempted to
enclose small natural ecosystems, create artificial laboratory versions (e.g. phytotrons
and growth chambers), and study very simple types (such as those of the Antarctic
‘dry valleys’). A huge hermetically sealed greenhouse complex with a crew of eight,
designed to study the function and interaction of several ecosystems, was established
in Arizona, USA, in 1991. Named ‘Biosphere 2’ (to emphasize its separation from
the Earth’s biosphere, and to reflect one motive of the experiment, which was to test
the feasibility of such facilities for life-support on Mars), it maintained a more or
less breathable atmosphere and provided almost enough food for the crew for two
years (Allen, 1991).
Ecosystems are commonly investigated by systems analysis (Watt, 1966; Odum,
1983). In the late 1940s, systems diagrams were constructed to show energy flows
between components of ecosystems. Soon similar approaches had been adopted by
many social scientists and business managers as frameworks for study and as means
of prediction (a systems approach was used by the Club of Rome to try to model
global limits) (Smith and Reeves, 1989). Applied systems theory and systems
modelling have been steadily improved and are used by environmental managers
(Perez-Trejo et al., 1993). While the ecosystem approach may not give precise results,
it does provide a valuable framework for analysis.
Adaptive environmental management approaches often adopt an ecosystem
approach. However, use of an ecosystem approach is not without problems: it can be
difficult to recognize boundaries; measurement of what goes in and comes out can
be difficult; establishing whether an ecosystem is natural, rather than modified, can
be difficult, and it is possible for organisms to migrate in or out. Also, the assumption
that an ecosystem will behave in a linear, predictable manner may be over-optimistic;
in practice ecosystem predictions are often inaccurate. Nevertheless, it is often possible

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to get some idea of an ecosystem’s energy and material distribution. Without
understanding all of the complex interactions one could model the behaviour of an
ecosystem, although with complex ecosystems this becomes difficult (Figure 9.2).
There is also a chance that some of the processes that are operating work at random,
and therefore cannot be modelled satisfactorily.
It is important for the environmental manager to integrate ecology and policy
in order to manipulate ecosystems to meet human needs and desires (Brown and
MacLeod, 1996). Ecosystems researchers must therefore ensure that they are looking
at realistic assumptions, not over-simple abstractions or misconceptions. According
to systems theory, changes in one component of a system will promote changes in
other, possibly all, components. As subsystems may interact in different ways, the
ecosystem approach is essentially holistic. A concise critique of the ecosystems
approach may be found in Pepper (1984:107–110).
Given time, natural, undisturbed ecosystems theoretically reach a state of
dynamic equilibrium (steady state). Regulatory mechanisms of (checks and balances)
of positive and negative feedbacks counter changes within and outside the ecosystem
to maintain the steady state. However, since each ecosystem has developed under a
different set of variables, each has a different capacity to resist stresses and to recover.
FIGURE 9.2 The conventional approach to studying ecosystems (a); a second approach (b)
involves abstraction of the system into a model leading to interpretation of mathematical
conclusions