Principles of terrestrial ecosystem ecology.pdf

256 11. Trophic Dynamics
eating rate may be constrained by processing
time. Animals that are more selective and have
a lower feeding rate generally eat food that is
more digestible, contributing to their shorter
passage time.
Assimilation Efficiency
Assimilation efficiency depends on both the
quality of the food and the physiology of the
consumer. Assimilation efficiency is the proportion
of ingested energy that is digested
and assimilated (An) into the bloodstream
(Fig. 11.7).
E
assim =
A
I
(11.4)
Unassimilated material returns to the soil as
feces, a component of the detrital input to
ecosystems.
Assimilation efficiencies are often higher (5
to 80%) than consumption efficiencies (0.1 to
50%). Carnivores feeding on vertebrates tend
to have higher assimilation efficiencies (about
80%) than do terrestrial herbivores (5 to 20%),
because carnivores eat food that has less structural
material than is present in terrestrial
plants. Carnivores that kill large prey can avoid
eating indigestible parts such as bones, whereas
most terrestrial herbivores consume the indigestible
cell wall structure in combination with
cell contents. Among herbivores, species that
feed on seeds, which have high concentrations
of digestible, energy-rich storage reserves, have
a higher assimilation efficiency than those
feeding on leaves. Leaf-feeding herbivores,
in turn, have higher assimilation efficiencies
than those feeding on wood, which has higher
concentrations of cellulose and lignin. Many
aquatic herbivores have a particularly high
assimilation efficiency (up to 80%) because of
the low allocation to structure in many algae
and other aquatic plants. Even in aquatic
ecosystems, however, herbivores that feed on
well-defended species have low assimilation
efficiencies. Assimilation efficiencies of herbivores
feeding on cyanobacteria, for example,
can be as low as 20%.
The physiological properties of a consumer
strongly influence assimilation efficiency. Rumi-
n
n
nants, which carry a vat of cellulose-digesting
microbes (the rumen) have a higher assimilation
efficiency (about 50%) than do most
nonruminant herbivores. One reason for the
high assimilation efficiency of ruminants is the
greater processing time than in nonruminants
of similar size, giving more time for microbial
breakdown of food. Homeotherms typically
have higher assimilation efficiencies than do
poikilotherms due to the warmer, more constant
gut temperature, which promotes digestion
and assimilation. Homeotherms therefore
have an advantage over poikilotherms in both
consumption and assimilation efficiency.
Production Efficiency
Production efficiency is determined primarily
by animal metabolism. Production efficiency
is the proportion of assimilated energy that
is converted to animal production (Fig. 11.7).
Production efficiency includes both growth of
individuals and reproduction to produce new
individuals.
od
Eprod
=
A
Pr
(11.5)
Assimilated energy that is not incorporated
into production is lost to the environment as
respiratory heat. Production efficiencies for
individual animals vary 50-fold from less than
1 to greater than 50% (Table 11.2) and differ
most dramatically between homeotherms (Eprod
1 to 3%) and poikilotherms (Eprod 10 to 50%).
Homeotherms expend most of their assimilated
energy maintaining a relatively constant body
temperature. This high constant body temperature
makes their activity less dependent on
environmental temperature and increases their
capacity to catch prey and avoid predation but
makes homeotherms inefficient in producing
new animal biomass. Among homeotherms,
production efficiency decreases with decreasing
body size because a small size results in a high
surface to volume ratio and therefore a high
rate of heat loss from the warm animal to the
cold environment. In contrast, the production
efficiency of poikilotherms is relatively high
(about 25%) and tends to decrease with
increasing body size. Some large-bodied
n
n