What Is a Savanna?
Tropical savanna is grassland that has a rich scattering
of trees. Savanna forms part of an ecological gradient
from grassland (little or no trees) to dry forest (mostly
trees and not so much grass). Savanna ecosystems are
estimated to cover approximately one- fifth of the land
surface of Earth, including extensive areas within the
tropics, particularly in eastern and southern Africa.
J. S. Beard (1953) was among the first researchers to
define tropical savanna, calling it “a natural and stable
ecosystem occurring under a tropical climate, having
a relatively continuous layer of xeromorphic grasses
and sedges, and often with a discontinuous layer of low
trees and shrubs.” In Beard’s definition, xeromorphic
refers to plants adapted to withstand periodic dryness,
and a strong dry season typifies savanna ecosystems.
These xeromorphic characteristics are structural (and
by implication, physiological) adaptations such as
tough fibrous leaves that enable a plant to endure high
temperatures and periodic drought and not lose water.
Many savanna species are uniquely adapted to thrive
under hot and sunny conditions (plate 14- 1). Though
Beard’s definition states that savannas are stable, they
do experience changes in the relative amount of grass
cover and woody vegetation, for reasons such as fire
frequency and severity of dry season. Wherever
savannas are found, there is a gradient evident: patches
of grasslands grade into savannas and savannas grade
into dry forests.
Wet tropical savannas are found in tropical and
subtropical areas that have strong rainy and strong dry
seasons. One familiar example is the Florida Everglades,
a wet seasonal subtropical savanna in extreme southern
Florida (plate 14- 2). In South America the extensive
Llanos in the northeastern part of the continent and the
vast Pantanal of southern Brazil and portions of Bolivia
and Paraguay (both ecosystems are described below)
are splendid examples of wet seasonal savannas. Each
forms an ideal place to visit to see a true abundance,
indeed a spectacle, of South American wildlife.Savanna Distribution
In South America, combinations of grassland, savanna,
and dry forest are estimated to occupy about 250 million
ha (618 million ac), principally in Brazil (cerrado,
caatinga, campo rupestre, and Pantanal), Colombia
(Llanos), and Venezuela (Llanos). Large tracts are also
found in eastern Bolivia (Pantanal), and in northern
Argentina and Paraguay (Chaco). Broad expanses of
savanna and dry forest also occur in Central America
particularly in the northern Yucatán Peninsula and in
parts of Belize, Honduras, and Nicaragua. Savanna is
common in many Caribbean areas, too, particularly in
the Bahamas and West Indies (plate 14- 3).
Numerous tree species populate savannas throughout
Central America, the Caribbean islands, and equatorial
South America, including acacias, palmettos, palms,Chapter 14. Don’t Miss the Savannas and Dry Forests
Plate 14- 1. Burrowing Owls (Athene cunicularia) are common
residents of grasslands and savannas throughout Central
and South America, and their range extends well into North
America. Photo by John Kricher.
Plate 14- 2. The Florida Everglades has been called a “river
of grass.” It is a seasonal wet savanna that is dependent on
floodwaters from Lake Okeechobee to maintain the wet
savanna ecosystem. Photo by John Kricher.250 chapter 14 don’t miss the savannas and dry forests