February 27, 2007

Know Your Biomes IV: Tropical Savanna

A cheetah crouches, shoulders hunched, barely visible through golden stems. The antelope on the edge of the herd has stopped chewing, and scans the horizon with a nervous eye. As it takes a step forward to rejoin the safety of the group, the cheetah makes her move, bounding with impossibly huge leaps towards her prey. The entire herd is on the move with her first step, but the stray is dangerously lagging behind. It flies only for a few seconds before the cheetah leaps one final time, clinging to the young antelope's rump with all her strength, pulling the animal to the ground for the coup de grace.

This should be a familiar scene for anyone with a passing interest in wildlife films; the great African savannas are often filmed to illustrate with detail the great theater of competition and predation in the wild. Visibility plays a large factor in this choice; the savannas have very few trees, and are home to some of the largest groups of the largest mammals in the world, not only in Africa, but also in South America, India and Australia. The same drama could not be effectively captured in a rain forest; animals are much less visible generally.

But that does not mean that the creatures of the savannas are immediately apparent. Most are well camouflaged, especially the predators, matching the golds, browns and sage greens of the flora, or appearing as a distant rock, as in the case of rhinos. Huge herds of ungulates pound the hard earth of the savanna, following the summer rains as they sweep across the enormous stretches of land that savannas occupy. As in all biomes, the climate sets the standard of living.

Like the tropical dry forests, the savanna is doused with rain in the summer seasons (often in just a few weeks depending on the year), and suffers from extreme drought for the rest of the year. What keeps dense plant growth at bay? Well, the answer to that question is threefold:
  • Fire: After the five month long dry season, lightning storms swirl violently over the plains, igniting the parched vegetation. These fires can be prolific, clearing huge tracks of grasses and saplings. The grasses grow back quickly, but millennia of fires like these have selected a small number of fire resistant trees and shrubs that are constantly kept in check by the annual fires.
  • Soil: The soils of the tropical grasslands are relatively impermeable, causing the heavy rains to skim along the surface instead of soaking deeply into the ground.
  • Herbivory: Animals also keep the density of non-poacean plants at a relatively low level. The herbivores of the African savanna, for example, have evolved to fill specific niches* (the specific factors that influence a species existence, in this case feeding) in which they consume plant material at different levels of the ecosystem. Giraffes and elephants browse the upper and middle levels of leafy trees, respectively, while wildebeests and zebras graze on different levels of grasses. The large herds of ungulates roaming the plains are also restrictive to growth.
The number and diversity of trees differ from savanna to savanna around the globe, however. In the Miombo Woodlands, trees of the genus Brachystegia can reach heights of 80 feet (25 meters) or more, assisted by a nutrient fixing fungus in its roots (mycorhizae). There are a few dozen species of this tree in a relatively small area.

Insects, as usual, play a large part in recycling nutrients from dead and dying tissue. Termites are perhaps the most visible of these detritivores (detritus eating) in just about every savanna across the world, living in mounds in excess of two meters high, each home to millions of foraging termites. In the Cerrado Savanna of South America, the giant anteater plays Godzilla, wandering through entire cities of termite mounds and dismantling them with its powerful claws to expose tunnels full of workers and lapping them up with its sticky tongue (links to video of anteater feeding).

Modern humans are believed to have originated in the African savanna before trekking out to inhabit every other biome in one way or another. The savanna is used for supporting non-native livestock in most cases, but there is a movement for domesticating antelope and other ungulates due to the naturally higher feed to meat ratio observed in these animals.

Next we'll move out of the tropics and into the desert.

*We will return to niche theory in a later post; Cheetah image courtesy of schani

This post is part of a series of Basic Concepts: Ecology (Intro, Biomes I, II, III). For the entire list of Basic Concepts in Science, visit Evolving Thoughts.

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