Climate
Savannas are tropical, with high temperatures year-round, but with rainfall highly seasonal.
Soils
Savanna soils are often reddish, acid latosols, as in the tropical rain forest, but there may be gray to reddish calcareous soils also, especially in drier areas. The parent rock of the local area is important in determining the soil chemistry, as little leaching (and thus soil evolution) takes place in this dry climate.
Vegetation
Tropical grasslands usually support scattered trees, and this mixture is called a "savanna." Savannas actually encompass a broad spectrum of vegetation types from pure grasses and forbs at one end through trees and shrubs at variable densities to thorn forest at the other end, which in turn grades into tropical dry forest in areas of higher precipitation. Tree growth is controlled not only by rainfall but also by soil type; large areas of hardpan soils (often laterites) allow no tree roots to penetrate except through cracks, and the cracks determine tree distribution. Palms and legumes are important components of woody savanna floras in most regions. Tree growth is also controlled by the nearness of the water table, with trees always along water bodies, grading into gallery forest, which in turn may be vegetatively comparable to dry forests or rain forests of the area. Seasonality is pronounced, with a flush of grass growth and the appearance of many annual forbs at the beginning of the rains.
There is much argument about the origin and maintenance of savannas; some think that all or most are fire-dependent and would grow up into woodland if fire were excluded. South American savannas are often considered relatively recently human-derived because so few plants and animals are unique to them, but at same time they have been shown to be underlain by hardpan.
Diversity
Savannas are quite low in tree species diversity because of stringent ecological requirements but fairly high in diversity of herbaceous plants; it would be of great interest to compare the diversity of herbs of tropical savanna, temperate grassland, and arctic tundra. Animal diversity is fairly high, although much lower overall than tropical forested areas because of fewer vegetation layers, which in turn provide environments for fewer adaptive types. Large mammals are at their most diverse in this open environment, in which they can move about freely and yet find shelter among woody vegetation. Large herbivores are successful because of the tremendous biomass of herbaceous vegetation produced annually, and there are many carnivores to crop them in turn. This is particularly the case in Africa, where savannas dominate, and much less so on other continents, where they are limited. The tremendous diversity of ungulates in Africa is paralleled by only few species of kangaroos in Australia and virtually no large grazing animals in South America. Many distinctive African groups are confined to savanna or are more diverse there than in the tropical rain forest--elephant shrews, springhare, hyaenas, aardvark, hyraxes, zebras, giraffe, some major antelope groups, ostrich, hammerkop, shoebill, secretarybird, mousebirds, woodhoopoes, starlings, and weavers.
Plant Adaptations
In trees, most savanna adaptations are to drought--long tap roots to reach the deep water table, thick bark for resistance to annual fires (thus palms are prominent in many areas), deciduousness to avoid moisture loss during the dry season, and use of the trunk as a water-storage organ (as in baobab). In grasses, most adaptations are against grazing--siliceous spicules to deter herbivores, growth from base of the plant rather than its tip to avoid damage to growing tissue, and vegetative reproduction in many types to overgrow competing forbs. Many plants have vegetative storage organs--bulbs and corms, for example--to make it through the dry (nongrowing) season.
Animal Adaptations
Many animals have effective locomotion for long-distance migrations to coincide with the seasonal flush of growth--primarily mammals in Africa and birds in Australia. Many forms burrow to avoid predation (in open) and desiccation (during drought), and many others use these burrows. Savannas are perfect for birds of prey, with wide open spaces for hunting with their long-range vision and trees for perches and nest sites (even the terrestrial secretarybird uses them). Termite mounds are significant features, supporting a surprising diversity of termite specialists--aardvark and aardwolf in Africa and giant anteater in South America (one of most characteristic savanna animals of that continent). Ratite birds have ecological equivalents in open country on each tropical continent--ostrich in Africa, rheas in South America (as much in grassland as savanna), and emu in Australia.
There are substantial niche separations in African ungulates, even in this fairly simple environment. The primary dichotomy is between browsers and grazers, but it is not a simple one, as many species do both in different proportions. Within grazers, some species are generalists, others specialists. The proportion of grasses and forbs in the diet varies among species, as does the parts of the plant eaten, down to distinct differences in which species eat leaves, sheaths, or stems of various grasses. Finally, some species are migratory, others resident, which is correlated with diet, social system (size of herd), and defense adaptations. The abundant but patchy food and the ease of keeping in contact have promoted a high degree of sociality in savanna mammals (ungulates, baboons, lions, and others). Birds are the same, also perhaps social because of the scarcity of arboreal nest sites (weavers).
Human Effects (primarily Africa)
As the savanna is an optimal environment for ungulates, it is much used for livestock where human populations are high, as in Africa. Thus one of most significant human effects is overgrazing, primarily by cattle but also by goats in drier areas. In the past, there was much hunting for sport but with relatively minor effects. Now the illegal hunting of large animals, both for meat and salable parts such as tusks and horns, is contributing to severe population reductions and even local extinction (e.g., rhinoceroses). Human-caused fires are thought to have contributed to the extent of savanna vegetation in South America. It is surprising that so little domestication has taken place in this habitat full of diverse large animals.
