Encyclopedia of biodiversity encyclopedia of biodiversity, (7 volume set) ( PDFDrive ) 2367

Grazing, Effects of

structure and species composition. This is especially true in
attempts to prevent exotics and woody species from altering
grasslands. Although fire may be a necessary disturbance in
some systems, such as tallgrass prairie, fire alone is often not
sufficient for restoration of biodiversity. After 9 years, in annually burned tallgrass prairie with nitrogen-addition, species
diversity was 48% lower than it was 5 years before burning
started and 66% lower than in unburned plots. In contrast,
mowed, annually burned, nitrogen-addition plots had more
than twice the species diversity than similar unmowed plots.
Therefore, mowing prevented a decrease in species diversity
that would have occurred with only fire and nutrient addition.
When bison grazing replaced mowing, plots that were grazed
and burned had the highest species diversity of all plots.
Similarly, in a Mediterranean grassland study, cattle grazing
led to an increase in species diversity, whereas burning
resulted in no significant difference.

Forests
Many tropical forest trees produce large fruits that either fall
near the parent tree or are dispersed by birds, primates, and
other large mammals to sites remote from the adult tree.
Dispersal is so critical for some of these species that their seeds
require gut passage through dispersal agents before they can
germinate. Because of the reliance on dispersers and the
heavy predation on seeds near the parent canopy, it has been
suggested that seed predators and dispersers are critical for
maintaining tree species diversity in tropical forests. By selectively preying on seeds that fall near parent trees, specialized seed predators may decrease the survivorship of seeds
near parents, preventing the occurrence of monospecific patches of forest trees and facilitating a more even and
diverse assemblage of species. Because larger animals disperse

the seeds of more than half of woody plant species, they play a
critical role in removing seeds from sites of high predation
and in introducing new species into different patches within a
forest. The hunting, poaching, and habitat modifications
that have left many forests depauperate in these animal dispersers have been suggested to be a cause of lowered plant
diversity in tropical forests. If this is correct, then effective
management of tropical forests may require the replenishment
of large-bodied seed dispersers. In addition to grazing by
larger animals, plants are also attacked by a diverse set of
microbial pathogens and parasites. Recent investigations have
shown that soil microbes may also play important roles in
preventing monospecific patches. If microbes that attack a
common plant build up in soils near that plant, they can
lower the probability of that species’ seedlings being successful
in that site and thus promote diversity by favoring unrelated
plant species in the vicinity of a long-established adult of a
different species.
Livestock introduced to serve as seed dispersers can partially replace native dispersers that disappeared during the
Pleistocene megafaunal extinction. The feeding activities of
these introduced species can increase the range of some plants
that produce large, fleshy fruits. In a lowland deciduous forest
in Costa Rica, introduced horses and cattle feed on the fruits
of jicaro (Crescentia alata) and disperse their seeds. In areas

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with livestock, jicaro are common. In areas where livestock are
absent, jicaro is relatively rare and occurs primarily in small,
spatially restricted patches.
Many seed dispersers subsist exclusively on fruit for at least

part of each year. This suggests that removal of seed dispersers
by anthropogenic activities could have several repercussions.
First, decreased seed dispersal could eventually lead to a decrease in the number of mature seed-producing parent plants.
This would in turn lead to a decrease of important wildlife
food resources, which could then lead to even lower numbers
of seed dispersers. This situation can be especially critical if a
particular tree species is pivotal in maintaining the health of
dispersers during food-limiting seasons. For example, Casearia
in tropical rain forests produce rich fruit during seasons of
relatively low fruit abundance and likely supply much of the
diet of local fruit-eating dispersers. In addition, this species
supports frugivorous birds that are important dispersers of
many other tree species during seasons when fruit is more
abundant. The disappearance of animals that feed on Casearia
fruits, leading to decreased recruitment of Casearia, could have
far-reaching effects on the forest community. This scenario is
thought to have occurred with jicaro. Frugivorous bats also
feed on jicaro, and it is thought that a decrease in jicaro following the loss of Pleistocene seed dispersers led to a decrease
in bats and then to a corresponding decrease in other fruitproducing plant species on which bats feed.

Lakes
Attempts to regulate the community composition of lakes
have produced a mature science in which the separate and
interactive effects of both physiochemical and biotic interactions are reasonably well understood and used in novel
ways to manage lake ecosystems. Success in these systems has
been achieved by fusing an understanding of biotic processes
such as competition and predation (traditionally studied by
population and community ecologists) with an understanding
of the role of physiochemical processes (traditionally studied
by limnologists and ecosystem ecologists). Investigations by

Steven Carpenter and colleagues have been especially important in stimulating this field.
Problems of harmful algal blooms, fish kills, and general
eutrophication of lakes have become increasingly apparent.
Initial attempts to explain these problems, and lake productivity in general, as a function of nutrient levels revealed
that nutrients could vary considerably in lakes with similar
biotic communities and that nutrient levels (also known as
bottom-up effects) explained only approximately 50% of the
variability in lake productivity. This finding prompted lake
ecologists to investigate the effects of trophic cascades (or
top-down effects) in structuring these ecosystems. By understanding and manipulating both the bottom-up effects of
nutrients and the top-down effects of consumers, lake
ecologists have been remarkably successful at altering biotic
communities and fundamental processes (e.g., productivity
and nutrient cycling) occurring in lakes.
Because lake communities are structured by interactions
between physiochemical conditions and biotic processes, initial attempts to manage lakes by managing nutrient input met