Animals in Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 271

The perennial grass prairie ecosystem of the North
American Great Plains — in its aboriginal form — was
another example of herbivore diversity coupled with floral
diversity. Bison, elk, antelope, and other grazers selectively
consumed different plant species, different plant parts, at
different times in the season, coevolving with a prairie vege-
tation comprised of shortgrass, midgrass, and tallgrass
species. The prairie ecosystem also demonstrated the direct
influence of herbivores on ecosystem structure. The pro-
portion of shortgrass, midgrass, and tallgrass species was
determined primarily by grazing behavior and fire, with
shifts in one direction or the other due to abiotic factors
such as soil type and rainfall (Briske, 1996). Following the
severe reduction in wild herds of the prairie herbivores,
species composition of the native plants changed as well.
Recent restoration programs of native prairie ecosys-
tems face the challenge of how to restore this native graz-
ing pressure, or face the alternative of having to simulate
the natural grazing with fire, mowing, or the use of domes-
tic animals. At the Tallgrass Prairie Reserve in Oklahoma,
The Nature Conservancy is using herds of 2000-plus bison
and a “patch-burn” management tool to restore the prairie
ecosystem and promote its original native plant and animal
diversity. In another case, the World Wildlife Fund has a
goal of restoring 17 to 27 million acres of the Northern
Great Plains ecoregion so that it can support two bison
herds of at least 10,000 animals each, as well as much of
the accompanying plant and animal biodiversity charac-
teristic of these systems.

of the fauna generally. Returning to the example of the
Serengeti: the diversity of herbivores is what makes possible
the relatively high diversity of predators and other secondary
consumers, including cheetah, lions, hyenas, aardwolves,
leopards, wild dogs, jackals, eagles, vultures, crocodiles,
and a variety of smaller carnivores and omnivores.

C YCLING N UTRIENTS

In all natural ecosystems, herbivorous animals play an
essential role in the dynamic process by which matter is
cycled through the system. The emergent properties of
efficiency, productivity, and stability are all related to this
fundamental ecosystem process.
Ecosystems are dependent on animals, decomposers,
and detritivores to release nutrients from their storage in
plant material. Animals are therefore an important part of
the nitrogen cycle, the carbon cycle, and the phosphorus
cycle (all discussed in Chapter 2). Whether the nutrients
are released back into geologic or atmospheric reservoirs,
the initial step is consumption of plant tissue, followed by
digestion, excretion, and decomposition (Figure 19.3).

I NFLUENCING C OMMUNITY D YNAMICS

As discussed above, herbivory has a direct effect on the
vegetation of an ecosystem. This was noted in a struc-
tural sense, but it can also be understood in a functional
sense, as a factor affecting interspecific interactions and
ecosystem complexity. Grazing or foraging by herbivores
involves selective removal of certain species or plant
parts, which affects how populations of each species in
the community interact. Grazing pressure, for example,
is often a key factor preventing a particular plant species
from dominating an ecosystem through competitive
exclusion and thereby reducing diversity and complexity.
When grazing patterns change — due, for example, to
removal of native grazers, changes in herbivore popula-
tions, or introduction of non-native herbivores — shifts
in plant species dominance inevitably occur.
An example of the important role herbivores play in
community dynamics is provided by ecosystems dominated
by introduced species. In many parts of the world, invasive
non-native plant species have established dominance in
association with introduced nonnative grazing animals,
causing changes in the native ecosystems that can persist
even after the exotic herbivores are removed (Colvin and
Gliessman, 2000). Conversely, invasion by non-native
plants can become problematic because of the absence of
herbivores able to control the aliens through consumption.
An awareness of how animals, especially larger her-
bivores, function as part of community dynamics and the
other ecosystem processes discussed above can guide us
as we consider how livestock may be integrated into crop
production. As heterotrophic consumers of plants (and in
some cases arthropod and molluscan pests), livestock

E NABLING E NERGY F LOW

When herbivores eat plants, and are in turn eaten by car-
nivorous animals, energy is flowing from one trophic level
to another. You will recall that the energy flow between
trophic levels is inefficient. A relatively small percentage
of the solar energy fixed by photosynthesis and stored in
plant biomass is preserved when that biomass is converted
into animal biomass at the next trophic level. The vast
majority of the energy (up to 90%) moving from one
trophic level to another is given off as metabolic heat by
the animals or deposited as manures back into the soil
(Odum and Barrett, 2005). The energy contained in the
manures of animals is not lost, however, because it is an
essential driver of soil organism activity.
The loss of energy at each jump in trophic level means
that the biomass at each higher level must be progressively
smaller — thus the shape of the familiar “energy pyramid”
in basic ecology texts. Since plants can occupy only the
bottom level in the energy pyramid, the energy stored in
animal biomass at any level is essential to the secondary
consumers at each higher trophic level. Thus animal diver-
sity in an ecosystem is a primary determinant of the number
of trophic levels through which energy can be transferred
— that is, the height of the energy pyramid and the diversity