Environmental Engineering Reference
In-Depth Information
Respiration
(
R
)
Ingestion
(
I
)
Assimilation
(
A
)
Growth (
G
)
or
Production (
P
)
Egestion
(
E
)
Excretion
(
U
)
I
=
A
+
E
A
=
R
+
P
(+
U
)
FIGURE 3.1
Diagram of
the consumer energy budget.
A
5
assimilation,
E
5
egestion,
I
5
ingestion,
P
5
production (growth),
R
5
respiration,
U
5
energy contained in nitrogenous compounds in urine.
for feces). Assimilated energy may be lost as respiration (
R
), excreted as the energy con-
tained in nitrogenous wastes (usually called
U
, for urine), or used for growth (
G
). Growth
often is called production (
P
), and may be further subdivided into somatic growth versus
production of gametes and so forth, depending on the purposes of the study (i.e., produc-
tion includes both growth and reproduction of the consumer). Bacteria and fungi don't
“ingest” food, so the energy budget starts with assimilation. Terms like
consumption
or
demand
, although sometimes used in energy budgets, are undesirable because they could
refer to more than one term in the energy budget.
Although I presented the energy budget as it applies to an individual organism, it is
possible to write an analogous budget for a population of consumers, a group of popula-
tions, or even the entire community of consumers in an ecosystem. It is just more difficult
to estimate the terms in the energy budget for a population or community than for an indi-
vidual organism.
Two aspects of energy budgets are especially interesting to ecologists: the magnitude of
the flows and the way energy is partitioned among the flows. The different terms of the
energy budget each have special ecological significance. Ingestion describes the effect of
the consumer on its food resource, egestion gives the input to the detritus pool, respiration
is energy that is lost from the ecosystem, and production shows both the amount of energy
that is available to the consumer to support growth and reproduction and the energy that
is available to predators.
Several factors affect the magnitude of all of these terms in individual organisms
(
Figure 3.2
). Mass-specific metabolic rates rise with increasing temperature. Specifically,
resting respiration rates scale with the Boltzmann factor:
e
2ðE
i
=kTÞ
R
~
ð
3
:
1
Þ
