Principles of biogeography - Fundamentals of the Physical Environment - page 486

evolved these obligate ties by living together in an
organically interdependent community. Thus an eco-
system such as oak woodland can be understood and
explained only by referring to the functions and processes
involved in the interaction of its component parts,
including climate in the air layer, the soil layer and the
living organisms of both.
The second property of plant and animal communities
is that they are collections of organisms whose common
denominator is tolerance of the particular environment
that they share. Some ecologists believe that this is the
most important feature of communities, and any other
relations are optional rather than obligatory facultative
relationships . This is the individualistic community
concept of the American ecologist Gleason, put forward
in 1939. It is in contrast to the organismic community
concept of Frank Clements of 1916 in which the plant
community is likened to a 'super-organism' which func-
tions by means of the connections between all organisms
in the community. Most ecologists would probably lie
closer to Clements than to Gleason in their views, and
would recognize that there are plant communities that
repeat themselves over geographic space. A plant com-
munity can therefore be defined as 'the collection of plant
species growing together in a particular location that
show a definite association'.
Vegetation can be studied at a range of different scales,
starting at an individual plant at the lowest level up to the
vegetation of the entire globe, i.e. the biosphere or
ecosphere . The Canadian ecologist Stan Rowe proposed a
system of nested levels for both vegetation and ecosystems.
Each level occupies a smaller and smaller area. Table 20.1
shows a modified version of Rowe's system. Biogeogra-
phers have historically studied vegetation at all scales; the
larger-scale biomes and formations were popular in the
nineteenth century when the world's surface was first
being explored and mapped. This generalized scale of
working has enjoyed revived popularity in recent years
as our techniques for studying global systems have
improved. For much of the twentieth century, however,
biogeographers focused on the plant community level, as
it is a very convenient scale for fieldwork. Biologists also
work at various levels of study, and there is an overlap
between the subject matter of the biologist and that of the
geographer. Table 20.2 shows the views of the American
ecologist Odum on similar levels of study in biology.

Table 20.1 Scales of vegetation and ecosystem study

Scale Vegetation Ecosystem

Large All vegetation Biosphere or ecosphere

Vegetation formations Biomes

Vegetation types Regional ecosystems

Plant communities Local ecosystems

Small Species populations Single organism-habitat

and individuals system

Table 20.2 Scales of organization in biology

Large-scale Biosphere

Ecosystem
Community
Population
Organism
Organ system
Organ
Tissue
Cell

Small-scale Protoplasm

by Sir Arthur Tansley in 1935, when writing about British
vegetation. For him 'an ecosystem can be defined as a
spatially explicit unit of the Earth that includes all of the
organisms along with all components of the abiotic (non-
living) environment within its boundaries'. By itself the
term 'ecosystem' does not connote any specific dimen-
sions. Tracing the frequently gradual boundaries can be
more difficult for some terrestrial than for aquatic systems,
where the presence of water helps to identify lateral
boundaries of rivers and lakes. The terms landscape ,
environment , terrain and ecosystem are often used inter-
changeably by ecologists to mean a specific landsystem or
land area whose interrelated parts are rocks, landforms,
soils, topoclimate and organisms. What emerges is that
it is impracticable to understand the dynamics of any
vegetation community without a parallel examination of
geology, topography, soils, hydrology and microclimate
which together make up the habitat of plant and animal
life. The value of the ecosystem concept is that, by focusing
on living organisms and physical environment together,
it brings understanding of what is functionally and
structurally important in the landscape. An alternative
definition of an ecosystem would be: 'the biological and
non-biological components of the landscape which exist
as an adjusted system whose parts are interrelated'.

THE ECOSYSTEM CONCEPT

Table 20.1 introduces a real distinction between 'vegeta-
tion' and 'ecosystem'. The term 'ecosystem' was first used