Grasslands - Mountains and Plains: The Ecology of Wyoming Landscapes - page 89

season species, probably because of less competition
from the plants that grew earlier and were eaten. 13 With
regard to geographic distribution, research has shown
that c 3 species are more common at higher elevations,
where one would expect cooler temperatures and less
water stress, and c 4 species tend to be more common
at lower elevations, where it is warmer and more often
drier. 14
coexisting animals also illustrate resource partition-
ing. For example, bison, jackrabbits, and some grass-
hoppers tend to eat more grasses, whereas pronghorn
and some insects (including other grasshopper species)
eat more forbs. 15 thus, competition between representa-
tives of these two groups is minimized. When the food
habits of bison, cattle, pronghorn, and sheep are com-
pared, bison and pronghorn are most different, cattle
are similar to bison, and sheep are similar to pronghorn.
From an evolutionary perspective, bison and pronghorn
would be expected to have the most dissimilar food
preferences because they have coexisted for the longest
time. Similarly, elk and deer have coexisted for millions
of years, and their food preferences are quite different.
elk consume more grass, and deer more twigs and leaves
of broad-leaved plants (called browse). of course, the
diets of all coexisting animals are determined to some
extent by what is available. considerable overlap may
occur in some seasons.

the solar energy used for photosynthesis in grass-
lands, and all ecosystems for that matter, is typically
less than 2 percent of the total impinging on the land-
scape. nearly all remaining solar energy is reflected,
used for the evaporation of water, or for heating the soil
and other ecosystem components. the low percentage
of the solar radiation fixed by photosynthesis leads to
an annual total net primary production (above- and
belowground) of 500-900 g/m 2 in mixed-grass prairie,
enough to maintain a diverse fauna of herbivores and
carnivores—large and small. 17
As expected in semi-arid ecosystems, plant growth
in grasslands is strongly influenced by water availability
(fig. 6.5), though not in a linear manner. As the amount
of water increases, nitrogen can become limiting. 18
notably, plant growth also is stimulated indirectly by
a heavy rainfall event that facilitates a pulse of organic
matter mineralization, thereby providing more nutri-
ents. Various disturbances also can influence growth, as
described later in this chapter. 19
two other factors that influence plant growth in
grasslands are the rate of water infiltration into the
soil and whether precipitation comes as snow or rain.
Rain that does not infiltrate the soil evaporates quickly
and is unavailable to plants. therefore, because coarse-
textured soils have higher infiltration rates, they often
have higher rates of plant growth (the inverse texture
effect; see chapter 3). if more of the precipitation comes
as snow, the meltwater becomes available more slowly,
and there is more time for infiltration before the water
evaporates or is transpired. Generally, the mixed-grass
prairie receives the majority of its annual precipitation
as rain, though snow can accumulate on the leeward
sides of ridges and some taller plants (such as shrubs).
Plant growth and other aspects of grassland activity
are often episodic, with pulses of growth, decomposi-
tion, and nutrient transfers occurring after rain wets the
soil. Moreover, because the rainfall is often patchy, the
amount of plant growth and available biomass varies
from place to place. in African grasslands and savan-
nas, patchy rainfall patterns determine the movements
of large herbivores and the nomadic people associated
with them. Patchy rainfall could also have affected the
movement of people, bison, and other animals in west-
ern north America, distributing the grazing pressure
over a large area. 20

Grasslands from an Ecosystem Perspective

Energy Flow

As in all terrestrial ecosystems, green plants make up
more of the grassland biomass than any other group of
organisms—96 percent, according to some estimates. 16
the next largest component is not the large herbivores,
as is often assumed, but rather microscopic organisms,
such as bacteria, fungi, mites, nematodes, and proto-
zoans. of these organisms, the weight of fungi has been
estimated at four times that of bacteria. earthworms are
rare in semi-arid grasslands, but there may be several
million nematodes and a hundred thousand mites in
one cubic yard of grassland soil. Surprisingly, the total
weight of all mammals, birds, and insects combined is
less than 1 percent of the ecosystem's living biomass,
even in grasslands grazed by domestic cattle.