Environmental Engineering Reference
In-Depth Information
secondary production is sometimes confused with yield or biomass accumulation. To see
the difference between production and biomass accumulation, consider an example.
Suppose there are 10 rabbits in a field. Over the course of a year, 40 rabbits are born and
grow to adult size, but 20 rabbits are run over by cars, 10 are eaten by foxes and owls, and
10 die of broken hearts (rabbits are very sensitive). After a year, there will again be 10 rab-
bits in the field, so there has been no accumulation of rabbit biomass. But secondary pro-
duction (summed growth) of rabbit tissue was 40 rabbits. This tissue was grown by the
rabbits and became available to consumers of rabbits. We might be tempted to argue that
gross production was 40 rabbits, but net production (which might be what counts) was
zero. There is no such thing as gross and net secondary production. The equivalent of
gross secondary production (i.e., net secondary production plus consumer respiration) is
assimilation by consumers. Further, note that accumulation of consumer biomass is incon-
veniently scale-dependent and tends to approach zero over large scales of space and time
(otherwise we'd be up to our necks in rabbits), so is not often an interesting thing to study.
As a result, ecologists rarely think about or measure biomass accumulation of consumers.
Less commonly, secondary production is defined as the production of primary consu-
mers, with the production of their predators being referred to as tertiary production. This
definition is based on an outdated view of food webs, in which microbes and omnivory
were regarded as insignificant. If it is desirable to distinguish among the levels of the tra-
ditionally defined (nonmicrobial) food web, it may be helpful to refer to them as the sec-
ond, third, and fourth trophic levels in the sense of
Burns (1989)
.
METHODS TO ESTIMATE SECONDARY
PRODUCTION
Methods to estimate primary production (chiefly CO
2
uptake, O
2
production, and bio-
mass accumulation) are well known, conceptually simple, and often easy to apply (see
Chapter 2). In contrast, the highly varied methods used to estimate secondary production
are relatively obscure, conceptually complicated, usually difficult to apply, and often
encumbered with large uncertainties. Further, primary production is most often measured
on the entire community of producers, whereas secondary production usually is estimated
for individual populations or guilds of consumers, but almost never on the entire con-
sumer community. Several approaches have been taken to estimate secondary production
(
Table 3.2
): (1) relating the measurable uptake of a labeled substance to production;
(2) combining estimates of the turnover rate of consumer tissue with estimates of con-
sumer biomass; (3) combining demographic information with individual growth to calcu-
late biomass accrued by a population; and (4) using empirical models.
Radioactive tracers are used to estimate bacterial or fungal production. Bacteria are
incubated with trace amounts of tritiated thymidine or tritiated leucine. At these low con-
centrations, bacteria (and no other organisms) are assumed to take up and incorporate
these labeled molecules into DNA or protein, respectively. After incubation, the amount
of radioactive label taken up by bacteria is measured, and converted to units of carbon
growth using a series of conversion factors. Fungi are similarly incubated with trace
