Environmental Engineering Reference
In-Depth Information
protozoans) fall steeply with rising temperatures. Homeotherms have much lower net
growth efficiencies than poikilotherms. In addition, ecological efficiencies may vary with the
physiological state of the organism—for example, assimilation efficiency may vary with age.
Lindeman (1942)
suggested that the low assimilation efficiency of herbivores (and detri-
tivores) may be compensated for in part by a relatively high net growth efficiency. His
argument was that herbivores have to spend less time than predators searching for and
capturing food, and so might have lower respiratory costs than predators. Thus, gross
growth efficiency would vary less across species than net growth efficiency. This hypothe-
sis has not been tested in detail, although
Welch (1968)
found some evidence to support it.
SECONDARY PRODUCTION
The remainder of this chapter focuses on secondary production, which has received
more attention than the other terms in the ecological energy budget. Ecologists have
focused on secondary production for two reasons. First, although we will see that second-
ary production often is difficult to estimate, it is at least much easier to estimate than other
terms in the energy budgets of free-ranging organisms. Various methods have been devel-
oped to estimate assimilation or respiration of free-ranging organisms (dimethylsulfoxide
reduction, activity of the electron transport system, tracer injections of radiocesium, or
double-labeled water), but these are more or less experimental and have not been widely
used by ecologists. Second, there is a strong and persistent idea among ecologists that the
“purpose” of a food web is to transfer matter and energy into higher trophic levels, which
may be useful to humans. (To see this bias, note that systems with high “efficiencies” are
defined as those that transfer a lot of energy to higher trophic levels, not those that dissi-
pate a lot of energy through respiration, even though the “purpose” of a food web could
just as easily be said to be the destruction of organic matter as its transfer to higher trophic
levels.) Production is the mechanism by which energy is transferred through a food web,
so it naturally attracted ecologists who viewed a food web as a way to move energy up
into useful higher trophic levels. Further, although this view is simplistic, secondary pro-
duction seemed to early ecologists to be related to the yield of those higher trophic levels.
Although I won't be discussing other energetic terms much, you will note that many of
the conclusions that I reach about secondary production apply equally well to ingestion,
assimilation, egestion, and respiration. Perhaps because humans harvest wild animals
(i.e., higher trophic levels) more often from aquatic ecosystems than from terrestrial eco-
systems, the vast majority (
90%) of work on secondary production has been done in
aquatic ecosystems. Consequently, some generalizations about secondary production may
actually be generalizations about aquatic ecosystems.
.
DEFINITION OF SECONDARY PRODUCTION
growth), regardless of its fate
.It
includes the production of heterotrophic bacteria, fungi, protozoans, and animals. This
definition of secondary production is commonly misinterpreted,
Secondary production is all heterotrophic production (
5
in two ways. First,
