Icecaps may dominate the globe in 5 million AD but, between the Equator and the poles, there is still a diverse range of habitats. There are equatorial rainforests, tropical grasslands, tropical deserts, temperate woodlands, coniferous forests and tundra. Over time, these habitats have been compressed into narrower and narrower bands by the cooler climate.
During the Holocene, the actions of humans threatened to destroy the equatorial rainforests. 5 million AD (much time after humankind), it has been further depleted by natural changes brought about by the new ice age. In the Amazon Basin, the forest is reduced to scattered pockets, surrounded by broad stretches of savannah which reach out to the horizon. Rainfall is lower now than it has been for millions of years and the mighty Amazon River has dwindled. The vast network of tributaries that once made it the most voluminous river in the world have all but dried up.
The dry conditions mean that the savannah is frequently swept by bushfires. These can be triggered by lightning strikes from an approaching summer tropical thunderstorm or a glint of sunlight. Once ignited, the periodic fires can sweep across hundreds of square miles at great speeds, destroying everything in their path. In the Amazon Grassland, life has adapted to drought and fire in surprising ways. The grasses rely on the fires to clear the ground of other competing plant species, such as trees. If left to grow unchecked, trees can dominate the land, cutting out the sun with their thick canopies and monopolizing the soil with their roots. Grasses, on the other hand, are fast-growing plants and easily replace their burnt leaves with new growth from underground stems. They disperse their seeds when the fire has passed, and thrive in the dry habitat. In the wet season, though, fires are less frequent.
The Amazon Rainforest that once occupied this region was far more productive than the Amazon Grassland. But when the forests in this part of the world declined, so too did the number and diversity of animal species. Because South America is connected to the rest of the world only by the narrow strip of the Panama Isthmus, a connection which is periodically broken by changing sea levels and volcanic activity, animal life has developed in the savannah in virtual isolation. Cut off from the rest of the world and faced with drastic environmental changes, those species which survived extinction have evolved new adaptations for life in this ecosystem. The animals here have lost the shelter of most trees and are now primarily exposed to wind and fire. Where once food was plentiful, animals must now cover great distances to find it.