Geoscience Reference
In-Depth Information
tissue, plus atmospheric additions. in 1977 clark wrote,
“once a given n atom makes its initial entry into the
blue grama plant, there is a greatly increased probabil-
ity that the atom will again enter new herbage growth
(50-80 percent) of the nitrogen in the senescing leaves
of western wheatgrass and blue grama is transferred to
belowground perennial plant parts, thereby conserving
the nutrient for use when growth resumes.
As with plant growth, the accumulation and cycling
of nutrients is not uniform across the grassland land-
scape. Many years of natural erosion from ridgetops,
with accumulation on lower slopes and in valley bot-
toms—combined with pronounced differences in the
microclimate and moisture availability—have led to
All aspects of nutrient cycling and the flow of energy
and water vary from one place to another. the effect
of this patchiness on plant and animal abundance is a
major topic in modern ecological research.
stored in roots is adequate for new growth (regrowth).
in such cases, there is hardly any disturbance at all.
31
the effect of natural grassland disturbances is to
augment an already high level of spatial variability. As
described by ecologist Samuel Mcnaughton in 1985,
while writing about African grasslands, there are con-
tinual fluctuations of rainfall, grazing, nutrient availabil-
ity, and fire. these fluctuations generate pulses of rapid
plant growth in different parts of the landscape, causing
a nomadic way of life for large herbivores and humans
alike. Moreover, because of this nomadism, plants in
any particular place are not grazed continuously. today,
in north America, comparable animal movements over
large areas are often impeded by exurban developments
and various land management practices.
The Effects of Grazing
As noted, herbivores of any kind form a small portion
of the ecosystem biomass, but they play an important
role in determining the nature of grassland ecosystems.
Studying the effects of herbivory provides insights on
how coexisting plants and animals survive.
on the surface it appears as though any grazing
would be disadvantageous to plants; aboveground, the
leaf area for photosynthesis is reduced, and energy that
might be expended for the production of additional
leaves is diverted to replacing stems and leaves that have
been eaten. However, some grassland plants seem well
adapted to tolerate considerable grazing pressure, and
ecologists have hypothesized that grasslands subjected
to light to moderate grazing have higher plant cover,
more growth, and more species than do comparable
a few exceptions, however, such studies have focused
on mammalian herbivores rather than the micro-
scopic belowground “grazers” that account for most
energy flow. Scientists can construct fences to exclude
large mammals, but excluding nematodes, mites, and
other belowground herbivores is necessary to test the
hypothesis—a formidable challenge. not surprisingly,
experiments on the effects of mammalian herbivory
alone have produced mixed results.
How might mammalian herbivory lead to increased
plant growth in grasslands? there are several possible
mechanisms:
Disturbance and Succession in Grasslands
A common theme in ecological research is
secondary suc-
cession
, that is, the changes that occur as an ecosystem
recovers after a disturbance. All ecosystems are sub-
jected to various kinds of disturbances, which—though
often viewed as unfortunate by casual observers—have
occurred for millennia and allow for the coexistence of
more plant and animal species than otherwise would
happen. Also, nutrient cycling and plant growth are
often more rapid after a disturbance. As described
later in this chapter, the suppression of disturbances to
which some native species are adapted can lead to unde-
sirable consequences.
Potential disturbances in grassland ecosystems,
aside from plowing for crop production, are drought,
fire, periodic heavy grazing by native or domestic her-
bivores, and the burrowing of small mammals and the
western harvester ant. each disturbance has the poten-
tial to modify the relative abundance of the different
species. Some disturbances, such as fire or drought, are
largely physical phenomena, whereas herbivory and
burrowing are biotic. Recovery to predisturbance condi-
tions often occurs in a few months or a year if damage is
restricted to aboveground plant parts and if the energy
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