Page 116 - Environment: The Science Behind the Stories

(a) Tropical dry forest 400 (a) Savanna 300

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(b) Darwin, Australia (b) Harare, Zimbabwe
Figure 4.23 Tropical dry forests experience significant Figure 4.24 Savannas are grasslands with clusters of trees.
seasonal variation in precipitation and relatively stable, They experience slight seasonal variation in temperature but
warm temperatures. Climatograph adapted from Breckle, S.W., 2002. significant variation in rainfall. Climatograph adapted from Breckle,
S.W., 2002.
Despite this profusion of life, tropical rainforests have
poor, acidic soils that are low in organic matter. Nearly all nutri- Savanna Drier tropical regions give rise to savanna
ents present in this biome are contained in the plants, not in the (Figure 4.24), tropical grassland interspersed with clusters
soil. An unfortunate consequence is that once tropical rainfor- of acacias or other trees. The savanna biome is found across
ests are cleared, the nutrient-poor soil can support agriculture stretches of Africa, South America, Australia, India, and
for only a short time (p. 239). As a result, farmed areas are other dry tropical regions. Precipitation in savannas usually
abandoned quickly, and farmers move on and clear more forest. arrives during distinct rainy seasons, whereas in the dry sea-
son grazing animals concentrate near widely spaced water
Tropical dry forest Tropical areas that are warm year- holes. Common herbivores on the African savanna include CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology
round but where rainfall is lower overall and highly seasonal zebras, gazelles, and giraffes. Predators of these grazers
give rise to tropical dry forest, or tropical deciduous forest include lions, hyenas, and other highly mobile carnivores.
(Figure 4.23), a biome widespread in India, Africa, South
America, and northern Australia. Wet and dry seasons each Desert Where rainfall is very sparse, desert (Figure 4.25)
span about half a year in tropical dry forest. Organisms that forms. The driest biome on Earth, most deserts receive well
inhabit tropical dry forest have adapted to seasonal fluctua- under 25 cm (9.8 in.) of precipitation per year, much of it
tions in precipitation and temperature. For instance, many during isolated storms months or years apart. Some deserts,
plants are deciduous and leaf out and grow profusely with the such as Africa’s Sahara and Namib deserts, are mostly bare
rains, then drop their leaves during dry times of year. sand dunes; others, such as the Sonoran Desert of Arizona and
Rains during the wet season can be heavy and, coupled northwest Mexico, receive more rain and are more heavily
with erosion-prone soils, can lead to severe soil loss where vegetated.
people have cleared forest. Across the globe, we have con- Deserts are not always hot; the high desert of the west-
verted a great deal of tropical dry forest to agriculture. Clearing ern United States is positively cold in winter. Because deserts
for farming or ranching is straightforward because vegetation have low humidity and little vegetation to insulate them from
is lower and canopies less dense than in tropical rainforest. temperature extremes, sunlight readily heats them in the 115

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