Herbivory
Historically most savanna ecosystems have supported
significant levels of herbivory by large animals. This was
apparently true of South America until the Pleistocene
extinctions. Today only Africa still retains its
megafauna. The collective influence of over 40 species
of hoofed mammals, some grazers (feeding on grass),
some browsers (feeding on leaves from woody plants),
and some generalists that both browse and graze, has
had a significant influence on African savanna ecology.
Presumably that was also the case when a diversity of
large animals occupied Neotropical savannas.Human Influence
On certain sites savanna formation correlates with
frequent cutting and burning of moist forests by humans.
Increase in pastureland and subsequent overgrazing
(mostly by cattle) has resulted in an expansion of savanna
(plate 14- 6). Cutting and burning, if too frequent,
destroys the thin, upper layer of humus necessary for
rapid decomposition of leaves by bacteria and fungi and
recycling by surface roots. Once the humus layer is lost,
nutrients cannot be efficiently recycled and more rapidly
leach from the soil, converting soil from fertile to infertile
and making it more suitable for savanna vegetation.
In some areas in South America deep- rooted grasses
imported from Africa to furnish fodder for cattle have
come to dominate savannas, replacing native species.
These grasses, in particular Andropogon gayanus and
Brachiaria humidicola (often called African elephant
grasses), now are estimated to cover about 35 million
ha (86.5 million ac) of savanna.Are All Savannas “Natural”?
Some ecologists have suggested that Neotropical savannas
have essentially resulted from human activity rather than
environmental causes. This claim is unsubstantiated
by historical evidence, however. The fossil record of
South American megafauna shows that savanna and
open woodland were present in many areas throughout
much of the Cenozoic era. Evidence exists that savanna
vegetation grew in parts of the Amazon Basin as recently
as 13,000 to 30,000 years ago. What remains controversial
is just how much savanna was present.
Neotropical savannas demonstrate the highest plant
species richness of any savanna ecosystems, includingthose of Africa. In numbers of both herbaceous and
woody species, Neotropical savannas rank first. This
high species richness suggests that evolution of savanna
species has been occurring throughout much of the
Cenozoic era, particularly in the late Miocene. Savanna
is as distinctive and intrinsic to the Neotropics as it is to
Africa and other regions.
There is a dynamic, temporal interface among
grasslands, savannas, and dry forests. One expands as
the others contract in a climatically driven, edaphically
influenced, long- term process that has produced
and continues to produce far- reaching effects on
evolutionary patterns of both plants and animals.Neotropical Savannas and Dry
Forests: Some ExamplesDry Pine Savanna of BelizeThe savannas of Nicaragua, Honduras, and Belize
are populated abundantly by Caribbean Pine. Riding
through miles of savanna along the Southern Highway
in Belize, one notices that many of the pines have
dark fire scars on their trunks. Lightning strikes cause
fires during dry season, and the effects of dryness and
periodic fires combine to preserve savanna. Caribbean
Pines tolerate occasional mild fires better than
other tree species in Belize. Grasses also thrive in an
environment with periodic fire.
Throughout much of southern Belize east of the Maya
Mountains, the dominant ecosystem type is savanna,
abounding in Caribbean Pine but also supporting manyPlate 14- 6. Savannas have expanded in some areas because of
intensive grazing by introduced cattle. The flying birds here are
Blue- and- yellow Macaws (Ara ararauna). Photo by John Kricher.chapter 14 don’t miss the savannas and dry forests 253