Principles of terrestrial ecosystem ecology.pdf

Consumption Efficiency
Consumption efficiency is determined primarily
by food quality and secondarily by predation.
Consumption efficiency is the proportion
of the production at one trophic level that is
ingested by the next trophic level (I n) (Fig.
11.7).
E
I n
od
consump =
Pr n-1
(11.3)
Unconsumed material eventually enters the
detritus-based food chain as dead organic
matter. On average, the quantity of food consumed
by a given trophic level must be less than
the production of the preceding trophic level,
or the prey will be driven to extinction. In other
words, the average consumption efficiency of a
trophic link must be less than 100%. There may
be times when the consumption by one trophic
level exceeds that in the preceding level. Most
vertebrate herbivores, for example, consume
plants during winter, when there is no plant
production. This is, however, typically offset
by other times, such as summer, when plants
usually produce much more biomass than
animals can consume. Situations in which consumption
efficiency is greater than 100% for
prolonged periods lead to dramatic ecosystem
changes. If predator control, for example, leads
to a large deer population that consumes more
plant biomass than is produced, the plant
biomass will be reduced, altering plant species
composition in ways that profoundly affect all
ecosystem processes (see Chapter 12) (Pastor
et al. 1988, Kielland and Bryant 1998, Paine
2000). Sometimes this occurs naturally. Some
herbivores, such as beavers, typically overexploit
their local food supply and move to new
areas when their food is depleted.
The proportion of NPP consumed by herbivores
varies at least 100-fold among ecosystems,
from less than 1% to greater than 40%
(Table 11.1). The major factor accounting for
this wide range in herbivore consumption efficiency
is differences in plant allocation to structure.
Herbivore consumption efficiency is
generally lowest in forests (less than 1 to 5%),
where there is a large plant allocation to wood.
Herbivore consumption efficiencies are higher
Plant-Based Trophic Systems 253
Table 11.1. Consumption efficiency of the herbivore
trophic level in selected ecosystem types.
Consumption efficiency a
Ecosystem type (% of aboveground NPP)
Oceans 60–99
Managed rangelands 30–45
African grasslands 28–60
Herbaceous old fields 5–15
(1–7yr)
Herbaceous old fields (30yr) 1.1
Mature deciduous forests 1.5–2.5
a Terrestrial estimates emphasize consumption by aboveground
herbivores and may not accurately reflect the total
ecosystem-scale consumption efficiency.
Data from Wiegert and Owen (1971) and Detling (1988).
in grasslands (10 to 60%), where most aboveground
material is nonwoody, and highest
(generally greater than 40%) in pelagic aquatic
ecosystems, where most plant (i.e., algal) biomass
is cell contents rather than cell walls. In
these ecosystems, more algal biomass is often
consumed by herbivores than dies and decomposes;
this pattern contributes to inverted
biomass pyramids (Fig. 11.8). In grasslands,
consumption efficiencies are generally greater
for ecosystems dominated by large mammals
(25 to 50%) than those dominated by insects
and small mammals (5 to 15%) (Detling 1988).
The toxic nature of some plant tissues (due to
presence of plant secondary metabolites) and
inaccessibility of other tissues (e.g., roots to
aboveground herbivores) constrain the herbivore
consumption efficiency of terrestrial
ecosystems. Nematodes, one of the major
belowground herbivores, consume 5 to 15%
of belowground NPP in grasslands (Detling
1988). Consumption efficiencies for belowground
herbivores are not well documented,
so whole-ecosystem estimates of consumption
efficiencies almost always emphasize aboveground
consumption. The highest consumption
efficiencies in terrestrial ecosystems are on
grazing lawns, such as those found in some
African savannas (McNaughton 1985) and
arctic wetlands (Jefferies 1988). These highly
productive grasslands are maintained as a lawn
by repeated herbivore grazing. Nutrient inputs
in urine and feces from these herbivores