Environmental Management: Principles and practice

The ecosystem
The biosphere is composed of many interacting ecosystems (ecological systems),
the boundaries between which are often indistinct, taking the form of transition zones
(ecotones) where organisms from adjoining zones may be present (it is possible for
organisms to be restricted to an ecotone only). Large land ecosystems or biomes
(synonymous with biotic areas) can be recognized. These are areas with a prevailing
regional climax vegetation and its associated animal life, in effect regional-scale
ecosystems. Biomes usually reflect climate but are also likely to be shaped by the
incidence of fire, drainage, soil characteristics, grazing, trampling, etc. (e.g. desert
biomes or grassland biomes). The biome concept seeks to extend the ideas of
community among vegetation and animal populations to cover the patterns of life
within both (Watts, 1971:186). The term ‘ecosystem’ was coined by Tansley in 1935,
and has become the basic functional unit of ecology (Tansley, 1935; Park, 1990:107).
It is an assemblage of organisms living and interacting in association under certain
environmental conditions, with, according to Miller (1991:112), six major features:
interdependence, diversity, resilience, adaptability, unpredictability and limits. An
ecosystem boundary can be defined at organism, population, or community level,
the crucial thing being that biotic processes are sustainable within that boundary. It
is possible to have different physical and functional boundaries to an ecosystem. No
two ecosystems are exactly the same, but one may recognize general rules and
similarities. There are two ways of viewing ecosystems: (1) as populations—the
community (biotic) approach, in which research can be conducted by individuals;
(2) as processes—the functional approach (energy flow studies), best investigated
by a multidisciplinary team.

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Ecosystems can be subdivided, according to local physical conditions,
into habitats (places where an organism or group of organisms live) populated
by characteristic assemblages of organisms (e.g. a lake ecosystem may be
composed of gravel bottom habitats rock bottom habitats, and mud bottom
habitats). Biomes and habitats may be subdivided into communities, which
may consist of several populations of different species that live and interact
together in a particular place.
In a stable ecosystem each species is assumed to have found a position, primarily
in relation to its functional needs: food, shelter, etc. This position, or niche, is where
a given organism can operate most effectively. Some organisms have very specialized
demands and so occupy very restricted niches (e.g. the water-filled hollow of a
particular bromeliad plant, itself with a restricted niche), others can exist in a wide
range of niches. A species may be using only a portion of its potential niche; or
alteration of a single parameter affecting competition with other organisms may
suddenly open, restrict or deny a niche for an organism.