Environmental Engineering Reference
In-Depth Information
metabolic rates increase with a 10 C rise in temperature, although Gillooly et al. argued
that the Boltzmann factor is mechanistically more reasonable and provides a better fit to
data than Q 10 .Q 10 's are often
2, so metabolic rates often approximately double with a
10 C rise in temperature. Mass-specific metabolic rates are higher in small organisms than
in large organisms, typically scaling as M 2 0.25 , where M is individual body mass ( Peters
1983 ). Respiration (and of necessity, A , I , and E ) is much higher in homeotherms (“warm-
blooded” species) than in poikilotherms (“cold-blooded” species; Peters 1983 ). Finally,
metabolic rates are under evolutionary control, so they can respond to environmental and
ecological pressures. For example, animals that live in ground waters, which are very poor
in food, have lower metabolic rates than would be predicted from their body masses and
temperatures.
The partitioning of energy among the parts of the energy budget can be described by a
series of efficiencies. Many of these have been used by ecologists ( Kozlovsky 1968 ), but
three are of primary importance:
B
A
=
I
5
assimilation efficiency
;
ð
3
:
2
Þ
which describes how good a consumer is at extracting energy from its food
P
=
A
5
net growth efficiency
;
ð
3
:
3
Þ
which describes the partitioning between growth and respiration
P
=
I
5
gross growth efficiency
;
ð
3
:
4
Þ
a combination of the previous two efficiencies, which describes the overall efficiency with
which food is converted to consumer tissue
Again, vague terms such as ecological efficiency , although widely used in the literature,
should be avoided because they could refer to any of several efficiencies and lead
to confusion. Typical values for these three efficiencies are given in Table 3.1 . All three
are highly variable, commonly depending on at least four variables: diet, temperature,
metabolic type (homeothermy vs. poikilothermy), and physiological status ( Figure 3.3 ).
Among animals, assimilation efficiency depends strongly on the quality of the diet, so that
TABLE 3.1 Typical values (%) for the three most widely used ecological efficiencies.
Assimilation Efficiency (
A/I
) Net Growth Efficiency (
P/A
) Gross Growth Efficiency (
P/I
)
Plants
1
2
30
75
0.5
1
Bacteria
5
60
Poikilotherms
10
90
10
60
5
30
Homeotherms
40
90
1
5
1
4
From Kozlovsky (1968) , Wiegert (1976) , Humphreys (1979) , May (1979) , Schroeder (1981) , Brafield and Llewellyn (1982) , Valiela (1984) ,
and del Giorgio and Cole (1998) .
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