Environmental Engineering Reference
In-Depth Information
BOX 3.1
WE DON'T LIVE BY BREAD ALONE: CONSUMER
BUDGETS FOR ELEMENTS OTHERTHAN CARBON
Consumers process materials (i.e., nutri-
ents) as well as energy (i.e., carbon). The
conceptual framework for consumer energy
budgets is easily modified to describe mate-
rial processing by consumers, with one key
difference: There are no respiratory losses
for elements other than carbon and oxygen.
For instance, following the notation pre-
sented in
Figure 3.1
for energy, we can
write the consumer nitrogen balance as:
organic matter in nutrients as it moves up
the food chain.
A consumer may be limited by the energy
content or the nutrient content of its diet,
which can have important consequences for
ecological interactions (e.g., nutrient cycling,
foraging behavior) involving the consumer.
Sterner and Elser (2002)
explored this subject
in detail; I will make only a few points here.
First, when will a consumer be limited by
nutrients, and when will it be limited by
energy? To answer this question, we note
that the elemental ratios (e.g., C:N, C:P) of
most consumers do not vary much, and may
be treated as constant within a species and
life-stage (
Sterner and Elser 2002
). Put sim-
ply, a consumer will be nutrient-limited if its
diet is so rich in energy (carbon) and so poor
in nutrients that the carbon in its diet is more
than enough to match the nutrients used for
growth plus the demands of respiration.
Putting this statement
I
n
5
A
n
1
E
n
ð
3
:
B1
Þ
A
n
5
P
n
1
U
n
ð
3
:
B2
Þ
where
I
n
,
A
n
,
P
n
,and
U
n
are ingestion, assimi-
lation, production (
growth), and excretion
of nitrogen, respectively. Likewise, we can
define a series of ecological efficiencies for
nitrogen that are parallel to those defined
for energy:
5
A
n
=
I
n
5
assimilation efficiency of nitrogen
;
ε
an
into mathematical
ð
3
:
B3
Þ
terms:
P
n
=
A
n
5
;
ε
gn
net growth efficiency of nitrogen
I
c
e
ac
. ð
P
n
=
Q
g
Þ 1
R
ð
3
:
B6
Þ
ð
3
:
B4
Þ
P
n
=
I
n
5
gross growth efficiency of nitrogen
ð
The left side of the equation shows the
amount of carbon that is assimilated as the
product of the rate at which carbon is
ingested (
I
c
) and the assimilation efficiency
for carbon (
e
ac
). The right side of the equa-
tion shows the amount of carbon needed
for growth (which is the amount of nutrient
used for growth,
P
n
, divided by the
nutrient: carbon ratio of consumer tissue,
Q
g
5
3
:
B5
Þ
Because there are no nonrespiratory
losses of nitrogen (and most elements), net
growth efficiencies of most elements will be
much higher than net growth efficiencies
for energy, especially for homeotherms. The
mismatch between large respiratory losses
of carbon and lesser losses of other ele-
ments should tend to progressively enrich
P
n
/P
c
) plus the amount of carbon
needed to support respiration. After some
