78
Basic biogeography: estimating biodiversity and mapping nature
as ‘ an assemblage of species populations which occur
together in a particular habitat (or ecosystem) and
which interact with each other ’ .
Returning to Tansley ’ s description of the ecosystem,
it can be seen from the above quote that he framed his
remarks in relation to Clements ’ s earlier term ‘ biome ’ ,
commenting in a somewhat reserved endorsement
that Clements ’ s usage of this term for ‘ the whole
complex of organisms inhabiting a given region is
unobjectionable … ’ (Tansley, 1935 , p. 299). He went on
to comment (p. 301) that ‘ the biome is determined by
climate and soil and in its turn reacts, sometimes and
to some extent on climate, always on soil. ’
Tansley was at pains to emphasize that his concept
of the ecosystem was to be applied at any and all scales
of analysis, and that while for systems analytical
purposes we have to ‘ … isolate systems mentally for
the purposes of study … ’ , such a separation is largely
artifi cial, as systems ‘ … overlap, interlock and interact
with one another … ’ (p. 300). Hence, within the conceptualization of ecosystems, it is the case that interlinkages and fl ows are fundamental, and the separation
into distinct units in space is seen as artifi cial.
So, at what scale does the biome emerge as a natural
unit? The clues here are given in the emphasis on the
regional scale, and on the signifi cance of climate and
edaphic controls on the form of the biotic community.
We may, therefore, defi ne the biome as a major type of
natural vegetation that occurs wherever a particular
mix of climatic and edaphic conditions is encountered,
or we may equate it with the notion of a major ecosystem type. The latter usage translates rather better into
the marine realm than ‘ natural vegetation type ’ .
Hence, the fundamental factors infl uencing the distribution of these major vegetation or major ecosystem
types are energy and water regimes (Figure 4.11 ).
Edaphic conditions also play a major role and, especially in sub - tropical regions, interact with fi re regimes
to generate shifting mosaics of woodland and savanna
ecosystems. Biomes are not recognized by the particular species that are dominant or characteristic, but by
their physiognomic features. So, we may delimit temperate deciduous woodland, thorn scrub, tundra, etc.
(note that different versions of biome schemes use
slightly varying subdivisions (e.g. Figures 4.11 and
4.12 )). We can therefore recognize the same biome in
different parts of the world, even though the biotas
involved have little or no directly shared biotic history.
Plants and animals have evolved in parallel (convergent evolution) in these different regions to present
constant interchange of the most various kinds
within each system, not only between the organisms
but between the organic and the inorganic. ’
(Tansley, 1935 , p. 299)
In essence, then, Tansley ’ s concept is an expression of
the interrelationships between organisms and their
environment, fundamental to which is the continual
transfer of energy and chemicals between the organic
and inorganic component parts. This functionalist
approach to ecosystems was developed and championed in particular by the American ecologists Eugene
and Howard Odum, who focused much of their
research in the 1960s on the fl ow of energy and nutrients through ecosystems.
However, Tansley ’ s article was written as a response
to a body of theory concerned with the composition of
communities, especially of vegetation communities, in
which the ecologist F.E. Clements (e.g. 1916 ) was the
prime mover. Along with many early 20th century
ecologists in America and Europe, Clements promoted
the idea of ecological communities as natural units,
fairly tightly organized assemblages that reoccur in
time and space as a function predominantly of climatic
controls. This is encapsulated in Clements ’ s idea of the
monoclimax, a single climatically determined solution
in the form of a mature vegetation type for each climate
type.
This dominant phytosociological paradigm was
challenged by Gleason (e.g. 1926 ), who promoted the
view of communities being impermanent outcomes of
inherently individualistic responses of the constituent
species. Tansley was similarly unconvinced by the
notion of monoclimax, preferring the notion that there
are alternative mature vegetation communities (the
idea of the polyclimax) to be found naturally within a
single regional climate zone.
Today, as outlined in Chapter 3 , the Gleasonian view
of communities has largely triumphed (Matthews,
1996 ). Communities are recognized as essentially temporary phenomena, without the tight organizational
structure implied by the Clementsian view of communities as quasi - organisms (having properties like
organisms) or super - organisms (a stronger form of the
same idea, implying tighter integration still). On the
other hand, modern statistical analyses also demonstrate plenty of non - randomness to the structure of
assemblages, meaning that classifi cation of communities has considerable utility (see below). For practical
purposes, therefore, we may recognize the community
Basic biogeography: estimating biodiversity and mapping nature
as ‘ an assemblage of species populations which occur
together in a particular habitat (or ecosystem) and
which interact with each other ’ .
Returning to Tansley ’ s description of the ecosystem,
it can be seen from the above quote that he framed his
remarks in relation to Clements ’ s earlier term ‘ biome ’ ,
commenting in a somewhat reserved endorsement
that Clements ’ s usage of this term for ‘ the whole
complex of organisms inhabiting a given region is
unobjectionable … ’ (Tansley, 1935 , p. 299). He went on
to comment (p. 301) that ‘ the biome is determined by
climate and soil and in its turn reacts, sometimes and
to some extent on climate, always on soil. ’
Tansley was at pains to emphasize that his concept
of the ecosystem was to be applied at any and all scales
of analysis, and that while for systems analytical
purposes we have to ‘ … isolate systems mentally for
the purposes of study … ’ , such a separation is largely
artifi cial, as systems ‘ … overlap, interlock and interact
with one another … ’ (p. 300). Hence, within the conceptualization of ecosystems, it is the case that interlinkages and fl ows are fundamental, and the separation
into distinct units in space is seen as artifi cial.
So, at what scale does the biome emerge as a natural
unit? The clues here are given in the emphasis on the
regional scale, and on the signifi cance of climate and
edaphic controls on the form of the biotic community.
We may, therefore, defi ne the biome as a major type of
natural vegetation that occurs wherever a particular
mix of climatic and edaphic conditions is encountered,
or we may equate it with the notion of a major ecosystem type. The latter usage translates rather better into
the marine realm than ‘ natural vegetation type ’ .
Hence, the fundamental factors infl uencing the distribution of these major vegetation or major ecosystem
types are energy and water regimes (Figure 4.11 ).
Edaphic conditions also play a major role and, especially in sub - tropical regions, interact with fi re regimes
to generate shifting mosaics of woodland and savanna
ecosystems. Biomes are not recognized by the particular species that are dominant or characteristic, but by
their physiognomic features. So, we may delimit temperate deciduous woodland, thorn scrub, tundra, etc.
(note that different versions of biome schemes use
slightly varying subdivisions (e.g. Figures 4.11 and
4.12 )). We can therefore recognize the same biome in
different parts of the world, even though the biotas
involved have little or no directly shared biotic history.
Plants and animals have evolved in parallel (convergent evolution) in these different regions to present
constant interchange of the most various kinds
within each system, not only between the organisms
but between the organic and the inorganic. ’
(Tansley, 1935 , p. 299)
In essence, then, Tansley ’ s concept is an expression of
the interrelationships between organisms and their
environment, fundamental to which is the continual
transfer of energy and chemicals between the organic
and inorganic component parts. This functionalist
approach to ecosystems was developed and championed in particular by the American ecologists Eugene
and Howard Odum, who focused much of their
research in the 1960s on the fl ow of energy and nutrients through ecosystems.
However, Tansley ’ s article was written as a response
to a body of theory concerned with the composition of
communities, especially of vegetation communities, in
which the ecologist F.E. Clements (e.g. 1916 ) was the
prime mover. Along with many early 20th century
ecologists in America and Europe, Clements promoted
the idea of ecological communities as natural units,
fairly tightly organized assemblages that reoccur in
time and space as a function predominantly of climatic
controls. This is encapsulated in Clements ’ s idea of the
monoclimax, a single climatically determined solution
in the form of a mature vegetation type for each climate
type.
This dominant phytosociological paradigm was
challenged by Gleason (e.g. 1926 ), who promoted the
view of communities being impermanent outcomes of
inherently individualistic responses of the constituent
species. Tansley was similarly unconvinced by the
notion of monoclimax, preferring the notion that there
are alternative mature vegetation communities (the
idea of the polyclimax) to be found naturally within a
single regional climate zone.
Today, as outlined in Chapter 3 , the Gleasonian view
of communities has largely triumphed (Matthews,
1996 ). Communities are recognized as essentially temporary phenomena, without the tight organizational
structure implied by the Clementsian view of communities as quasi - organisms (having properties like
organisms) or super - organisms (a stronger form of the
same idea, implying tighter integration still). On the
other hand, modern statistical analyses also demonstrate plenty of non - randomness to the structure of
assemblages, meaning that classifi cation of communities has considerable utility (see below). For practical
purposes, therefore, we may recognize the community