Circling the equator in wide bands around the globe, tropical savannahs are characterized by two distinct seasons—wet and dry. During the long dry “winter,” temperatures hover around 70 degrees. Most rivers and streams dry out, grasses wither and die, animals migrate for food, and fire is an ever-present threat. When “summer” returns, temperatures reach into the upper 80s. Rain dominates and drenches the parched grasslands, plants reappear and animals return to graze.
Fire-resistant bark, tree trunks that store water, and roots so long they can tap the water table far below the sun-baked soil—these are just a few of the adaptations plants have developed to thrive in tropical savannahs.
While all savannahs are populated with hardy grasses, tropical savannahs are often noted for the diversity of their plant life and for the abundance of particular species. Africa is rich in thorny acacias, Australia has its fire-hardy eucalyptus, and South America’s vast Cerrado is flush with tropical flower species.
Native animals are critical to maintaining savannah ecosystems. The ants that live on Africa’s thorn acacias deliver painful stings to any browser that tries nibbling their tree. Elephants weed out young trees and shrubs, allowing grasses to flourish, and in savannahs throughout the world, birds and fruit-eating animals disperse seeds in their dung.
These species have also evolved in such ways as to coexist despite scare resources. Vast herds of grazing animals in Kenya have specialized eating habits to reduce competition for food. Browsers, such as giraffes and gazelles, feed on leaves at different heights in the trees. Grazers, such as wildebeests and zebras, eat different grasses at different times of the year.