58 Ecology and Applied Environmental Science
The approach of the energy model can be expanded to include non-natural
ecosystems. An interesting example is that of the urban ecosystem, where
the function of machines and the rest of combustions are added to human
and other organisms’ respiration, increasing by a large amount the total O 2
consumption and CO 2 production within the cities. The urban ecosystem is
continuously consuming O 2 . The deficit is covered by air inflow from areas
outside the cities, since O 2 production from urban area plants is negligible.
Besides, biomass production within urban ecosystems is much smaller than
their consumption needs, whereas their decomposers cannot consume the
large amount of the created waste within these areas. Thus, the food deficit and the surplus of waste require imports of food and exports of waste,
resulting in an urban ecosystem much dependent on the operation of other
ecosystems located outside the cities (Figure 6.1).
4.2 liMiting FactorS in variouS EnvironMEntS
An important factor affecting biomass increase may, under specific conditions, become the critical factor that inhibits the production of one or more
populations or trophic levels. Its limiting action may be connected with
energy, nutrients, abiotic conditions or the influence of the biocoenosis. The
idea that organisms’ increase is controlled by the weakest link of the chain
of their needs comes from Justus von Liebig (1840), a pioneer in the study of
inorganic fertilizers and is known as the “law of the minimum.” According
to von Liebig, the amount of living matter in a given environment is limited
by the factor which is characterised by the smallest, proportionally, availability, on the condition that no other negative factors interfere.
The limiting factor in the desert is H 2 O, whereas in other terrestrial ecosystems it may be either H 2 O or CO 2 . In most marine ecosystems, where
H 2 O and CO 2 are available, primary production is usually limited by the
elements P or N, which are needed in the form of salts. Some consumer
organisms of the marine environment are limited by O 2 . In the open ocean,
the limiting nutrient may be iron (Fe).
A generalisation of the above approach can be expressed as follows: The
development of a biocommunity (or a population) is dependent on a total of
conditions and the one which comes closer (either upwards or downwards)
to the corresponding tolerance limit of the biocommunity tends to become
the limiting factor. This is a concept that can be very useful for the study of
any ecosystem or part of it. The ultimate factor limiting biosphere’s development is the incident solar radiation.
The limiting factor is defined in relation to some time scale. Generally,
there is only one in each section and phase of the ecosystem. Yet if its
availability increases, some other factor may become limiting. When conditions change, successional limiting factors appear, as for instance in
The approach of the energy model can be expanded to include non-natural
ecosystems. An interesting example is that of the urban ecosystem, where
the function of machines and the rest of combustions are added to human
and other organisms’ respiration, increasing by a large amount the total O 2
consumption and CO 2 production within the cities. The urban ecosystem is
continuously consuming O 2 . The deficit is covered by air inflow from areas
outside the cities, since O 2 production from urban area plants is negligible.
Besides, biomass production within urban ecosystems is much smaller than
their consumption needs, whereas their decomposers cannot consume the
large amount of the created waste within these areas. Thus, the food deficit and the surplus of waste require imports of food and exports of waste,
resulting in an urban ecosystem much dependent on the operation of other
ecosystems located outside the cities (Figure 6.1).
4.2 liMiting FactorS in variouS EnvironMEntS
An important factor affecting biomass increase may, under specific conditions, become the critical factor that inhibits the production of one or more
populations or trophic levels. Its limiting action may be connected with
energy, nutrients, abiotic conditions or the influence of the biocoenosis. The
idea that organisms’ increase is controlled by the weakest link of the chain
of their needs comes from Justus von Liebig (1840), a pioneer in the study of
inorganic fertilizers and is known as the “law of the minimum.” According
to von Liebig, the amount of living matter in a given environment is limited
by the factor which is characterised by the smallest, proportionally, availability, on the condition that no other negative factors interfere.
The limiting factor in the desert is H 2 O, whereas in other terrestrial ecosystems it may be either H 2 O or CO 2 . In most marine ecosystems, where
H 2 O and CO 2 are available, primary production is usually limited by the
elements P or N, which are needed in the form of salts. Some consumer
organisms of the marine environment are limited by O 2 . In the open ocean,
the limiting nutrient may be iron (Fe).
A generalisation of the above approach can be expressed as follows: The
development of a biocommunity (or a population) is dependent on a total of
conditions and the one which comes closer (either upwards or downwards)
to the corresponding tolerance limit of the biocommunity tends to become
the limiting factor. This is a concept that can be very useful for the study of
any ecosystem or part of it. The ultimate factor limiting biosphere’s development is the incident solar radiation.
The limiting factor is defined in relation to some time scale. Generally,
there is only one in each section and phase of the ecosystem. Yet if its
availability increases, some other factor may become limiting. When conditions change, successional limiting factors appear, as for instance in