Environmental Engineering Reference
In-Depth Information
vore). The fact that energy flow occupies such a central position in modern ecology owes
much to the classic work of the US ecologist Raymond Lindeman in the 1940s. His
'trophic-dynamic' concept of ecology brought a focus at the level of entire ecosystems. In
the 1920s the English ecologist Charles Elton had written about 'the pyramid of numbers'
in terms of how food webs are organized in ecosystems. He noticed that a large number
of green plants support a smaller number of herbivores which support a smaller number
of top carnivores (Figure 22.2a). Such a pyramid is common for both populations and for
species. Other ecologists noticed that a similar trend was evident in the total weight of
living organisms (biomass) at the different trophic levels (Figure 22.2b). Whilst such
pyramids of numbers of individuals, numbers of species and biomass are common, they
are not universal. There are exceptions both on land and in aquatic ecosystems. One
complication may be that one component of the ecosystem may have a rapid turnover
rate, so that biomass figures would underestimate its importance. Lindeman realized that
if organic material is looked on as a fuel or food energy (calories or joules), then an
energy pyramid
will always be found in nature (Figure 22.2c). Whenever an ecosystem
is described in terms of a rate of energy flow (calories per square metre per day, cal m
−2
d
−1
, or thousands of joules per square metre per year, kJ m
−2
yr
−1
) through the different
trophic levels, a pyramid shape will always result, following the second law of
thermodynamics.
BIOMASS AND PRODUCTIVITY
Biomass is the mass of living organic material in a specific area or ecosystem. The units
are weight per unit area (g m
−2
or kg ha
−1
or t ha
−1
or t km
−2
). Changes in biomass from
year to year indicate the amount of energy or carbon fixed by photosynthesis and
incorporated into an ecosystem. Biomass can be measured by harvesting the above-
ground plant parts (i.e. the shoots) in a sampling plot by clipping at ground level. Large
shrubs and trees are difficult to harvest in this way and usually some parameters of
Figure 22.2
The structure of ecosystems according to
Eltonian principles: (a) the pyramid of numbers (a grass
field), (b) the pyramid of biomass (a coral reef), (c) the
