Grasslands - Mountains and Plains: The Ecology of Wyoming Landscapes - page 90

Fig. 6.5. the annual aboveground net
primary production (nPP) of grassland
ecosystems is correlated with evapo-
transpiration. nPP is the amount of new
plant biomass produced in a given area
over a known period of time; evapo-
transpiration is the sum of evaporation
and transpiration and integrates the
effects of temperature and moisture
availability. each point on this graph is
for a different grassland. the response
of aboveground nPP to changes in
evapotranspiration at a single site would
be somewhat different from the response
illustrated in this graph. Adapted from
Webb et al. (1978).

500

400

300

200

100

0 0

100 200 300 400 500 600 700 800

EVAPOTRANSPIRATION (mm/yr)

By what pathways does the solar energy fixed by
photosynthesis flow through the prairie ecosystem? For
green plants, 65-85 percent of this energy is moved
internally from the leaves to roots and rhizomes, and
the plants use about 30-40 percent for their own main-
tenance. A large portion of the remainder is converted
to stems and leaves that become what is known as stand-
ing dead or detrital biomass at the end of the growing
season. Belowground detritus, such as dead roots and
dead nematodes, adds organic matter to the soil. it also
provides the base for an extensive microbial food web,
enables increased infiltration rates, and provides the
nutrients required by plants as the detritis decomposes
(see fig. 1.6). of course, fires can burn the aboveground
biomass as well, converting a substantial portion of the
energy to heat and having other significant effects in
some grasslands. 21 During some years grasshoppers con-
sume a large portion of the aboveground biomass.
on average, herbivores consume only 10-30 percent
of the total annual plant growth (above- and below-
ground), with more than twice as much energy going
to nematodes and arthropods in the soil than to large
herbivores, such as cattle (table 6.2). not surprisingly,
some ecologists have concluded that light to moderate
levels of grazing have little or no effect on plant species
composition. 22 More than plants and livestock, the food
web of rangelands is composed of bacteria, fungi, proto-

zoans, mites, insects, nematodes, and small mammals
that are mostly in the soil. Daniel Milchunas, William
Lauenroth, and their colleagues estimated that, of the
plant growth that is consumed by herbivores annually,
roughly one-third is eaten by cattle, one-third by insects
(above- and belowground), and one-third by nematodes,
with jackrabbits, rodents, and birds each consuming less
than 1 percent.
it is interesting to compare the amount of energy
available for human use in a plant-cow-human food
chain to a plant-human system. in general, people
obtain more food per acre if plants are consumed instead
of animals, because much of the plant-derived energy
of the animals has been used for their maintenance or
is converted to animal tissues or waste products that
people do not eat. Further, the second law of thermo-
dynamics dictates that every conversion of energy from
one form to another—for example, from plant to animal
tissue or from animal fat to animal movement—leads
to the production of heat. this principle accounts for
the fact that energy flows rather than cycles through
ecosystems. All solar energy fixed by photosynthesis is
eventually converted to heat, though it may persist as
biomass, soil organic matter, or fossil fuels for centuries
or more.
Human food production is usually greater per unit
area when plants are eaten rather than meat, but it is also