Environmental Engineering Reference
In-Depth Information
might represent a fundamental ecological principle. In
reality, ecological efficiencies depart significantly from
the 10% rate.
Ten years after the publication of Lindeman's work,
Odum (1957) began his study of an aquatic ecosystem
at Silver Springs in central Florida, which resulted in
trophic efficiencies of about 16% for herbivores and, re-
spectively, 4.5% and 9% for first- and second-level carni-
vores. Teal (1962), in his study of a Georgia salt marsh,
obtained trophic efficiencies of nearly 40% for bacteria,
27% for insects, and only about 2% for crabs and nema-
todes. By that time Hairston, Smith, and Slobodkin
(1960) elevated food chain dynamics to one of the key
concerns of ecology. In contrast to the previous para-
digm stating that heterotrophic numbers are primarily
limited by available energy or habitats, they generalized
this fundamental energetic concern by concluding that
herbivores are limited most often by predators rather
than by resources and hence are not likely to compete
for common resources. In this three-link food chain only
the predators are energy-limited, and their pressure on
grazers does not allow these primary consumers to regu-
late energy intake.
Earlier studies of food webs found an average of 3
food chain lengths and maxima between 5 and 7, but
later studies showed higher values, some with average
length of 5 and maxima of 11-12 (S. J. Hall and D. Raf-
faelli 1991; Polis and Winemiller 1996). The earliest ex-
planation ascribed the brevity of most food chains to
energetic constraints, but the impacts of environmental
instability limits the length long before such constraints
become operative (fig. 4.11). There is no simple progres-
sion of animal sizes along these chains: for example, in
a Caribbean tropical rain forest the fourth level of
consumers includes not only snakes but also arboreal
arachnids (Reagan and Waide 1996). In even-linked
grasslands the dominant ungulates are larger than their
predators; in odd-linked tropical rain forest small, folivo-
rous grazers are controlled by larger predators; in many
aquatic ecosystems abundant herbivores are much larger
than autotrophs, and secondary consumers and predators
are larger still. But in grasslands the presence of large
mammals strongly influences the numbers and diversity
of small mammals (Keesing 2000). With ungulates pre-
sent, the densities of small mammals are lower and their
diversity generally higher; the reverse is true if ungulates
are absent.
Large differences in average lifespans and masses of
producers and consumers result in broad-based terrestrial
trophic pyramids. Phytomass density is commonly 20
times more abundant than the mass of primary con-
sumers, and zoomass in the highest trophic level may be
equal to a mere 0.001% of the phytomass. In lakes food
chain length increases with ecosystem size, but it is not
related to productivity (Post, Pace, and Hairston 2000).
Short lifespans of marine phytoplankton and high energy
throughputs among consumers usually reverse the layer-
ing in the ocean and produce inverted trophic pyramids:
the standing heterotrophic biomass is at least twice,
often even three or four times, as large as the oceanic
phytomass.
All we can do in the absence of any grand patterns is to
describe the substantial variability in energy transfer rates.
As already detailed (see chapter 3), the share of PAR
used in converting carbon in CO
2
to new phytomass is
at best about 4%. After that rate is reduced by carbon
losses due to respiration, the efficiency of the NPP, that
is, the phytomass actually available for heterotrophic con-
sumption, averages only about 0.33% for terrestrial eco-
systems and (because of the low absorption of PAR by
