Savannas are the tropical version of the temperate grasslands. Most savannas are caused by climatic patterns where there is a strong dry season for a large part of the year. Few trees survive in these regions, but most savannas do have some form of trees that scatter the landscape. If you look at the map below you’ll realize that a large part of Africa, nearly half its landmass, is covered in some sort of savanna. Large tracks of savanna are also found in South America, India and Australia.
An iconic scene lies before you—the wide open expanse of tall grasses blowing in the wind seems to go on forever, where the occasional tree and cluster of shrubs dots the landscape. In the distance, a massive herd of zebras graze lazily in the sun, occasionally checking their surroundings for signs of lions or other threats. You hear an elephant trumpet in the distance, and notice a small group of giraffes off to your right eating leaves from the tops of a lonely acacia tree . In front of your feet, a dung beetle scurries along the ground, trying to avoid being seen by one of the many birds overhead. This is the African savanna.
The scene above is, for many of us, the most familiar of the savanna ecosystems, but there are other savannas around the world, all with their own set of unique plants and animals surviving in this biome controlled by extremes of wet and dry from one season to the next.
Based on the lines of latitude (the imaginary lines that run east and west on our planet), we can divide the world into three regions: polar, tropical, and temperate. Polar regions are the most northern and southern (between 66° and 90° lat), with temperate regions further towards the equator (23° and 66° lat) and tropic regions in the center (between 0° and 23°) surrounding the equator line. The tropical savanna is found on various continents in the tropical region of our planet, alongside the equator at around 10°–20° latitude both North and South. Additionally, they are generally found at less than 500 meters above sea level (1640 famsl).
Huge tropical grasslands exist in Africa, Australia, South America, and India. Some well known ones include the Serengeti Plains in Tanzania, Los Llanos in Venezuela and Colombia, the Cerrados of Brazil, and the Everglades in North America. Within and between these regions exist specific ecoregions with some unique characteristics. We are going to be talking about savannas in an overarching sense, but you can find details about ecoregions with the WWFand this paper.
When we define a biome, we are interested in 2 things (1) the plant life present and (2) the abiotic factors—such as precipitation and temperature—that occur here.
The tropical savanna is generally a large flat expanse, with very few trees and a lot of grasses—at least 50% grass coverage to be precise. Two distinct seasons, summer and winter, define this biome, with a rainy wet season in summer, and a dry season in the winter time. The other defining feature of the savanna is that the temperature stays fairly constant between seasons.
Note: Savannas in the N. Hemisphere experience their wet season from May to September (summer), while in the S. Hemisphere it is from October to March (summer).
Even though the yearly average rainfall here is high, plant and animal life is limited mainly by water. This is due to the difficult droughts they must survive in the dry season, where wetlands and rivers can dry right up, and many plants die or go dormant, waiting for better conditions. In the wet season, rains replenish water bodies and actually cause large areas to become flooded, a welcome change for both plants and animals that have undergone a lot of stress during the drought.
Other than two seasons and stable temperatures, what abiotic factors define the savanna?
As mentioned, the temperature here remains fairly stable between seasons, with a maximum of about 2.5 degrees difference between distinct months. However, the daily temperature changes quite drastically, and there can be a 13–17 degree difference between night and day.
The overall mean temperature is about 25/27°C (77°–80°F), with highs up to 30°C (86°F) and lows down to 20°C (68°F).
Since tropical grasslands border the equator, the hours of daylight and nighttime are relatively equal and constant throughout the year, with a slight loss of daylight in the winter.
The low number of trees across the landscape means that the sun reaches the ground floor almost everywhere, allowing for the takeover of grasses that flourish in high sunlight.
The yearly average rainfall is around 800–1200 mm (~30–50 inches), but can get up to 1800 mm (70 inches) in wetter regions. However, this rainfall is not evenly distributed throughout the year, and dry seasons can see as little as 50–100 mm (2–4 inches) of rain in a 3–5 month period.
The lack of rain and moisture and the intense direct light also creates high evaporation rates in the dry season, causing everything to become even drier.
This seasonal difference results in huge seasonal changes in water bodies here—rivers and wetlands are full in the wet season, flooded even, and some completely dry up in the dry season, leaving little water for plants and animals alike.
Surprisingly, the soils of tropical savannas are often poor, being low in nutrients, and either lacking water, or waterlogged, depending on the season.
In the rainy season, soils become saturated and nutrients are pulled down from the topsoil, making it harder for plants to have access to them.
In the dry season, there is very little water in topsoil, although the deeper soil does manage to retain some, and the dry dead vegetation causes many fires. These fires initially release nutrients to the soil, but it also burns away some of them in the form of smoke, and ashes are easily blown away by the wind.
The low nutrients make nitrogen-fixing organisms, such as blue-green algae and certain bacteria, really important. Nitrogen-fixing organisms are able to take nitrogen from the air—atmospheric nitrogen—that is unusable to most plants, and turn it into usable nitrogen—nitrates—in the soil. Many of these nitrogen-fixing microbes are in a mutualistic symbiotic relationship with plants, where they help the plant get more nitrogen and the plant gives them carbon—it’s a win-win.
Types of soil vary between different parts of the savanna, from high-iron ultisols, to clay heavy oxisols, as well as rocky lithosols and areas of almost pure sand!
In general, savanna soil layers will look like this:
Since grasses get so tall and plentiful in the summer, and die off in the dry season, there is a lot of fuel for fires. Fires are frequent here, and are a huge part of what shapes this ecosystem. Now, by “frequent” we mean naturally every 1–5 years in most systems, though human activity has increased this frequency.
Fires occur most often, not in peak dry season, but at the very beginning of the rainy season due to lightning strikes that occur in the first thunderstorms, and the dry and dead vegetation that have yet to recover. Perhaps opposite of what you would expect, wetter places have more fire because they are more productive and end up with more vegetation, which makes for higher fuel loads when a fire does start.
Nowadays, the majority of fires are caused by people, unintentionally and intentionally. Burning to alter the landscape, such as to create fresh grass for livestock and to more easily hunt wild herbivores, has been occurring for a really, really long time. Interestingly, we think that the burning has gone on so long that the savannas we know today may be more anthropogenic than natural, and could have, at one time, contained more shrubs and trees.
Fire is pretty complex, and does both “good” and “bad” in the ecosystem.
First off, fire can kill many living plants and even animals, or in the best case, drive them out of an area. If a lot of plants and animals are killed, it changes the community significantly as the first things to colonize an area are probably different than before, and can often be invasive species; many invasive species are great at taking over disturbed habitat. As well, though, burning results in an immediate availability of nutrients—it burns off a lot of important ones as gas and smoke and changes the chemistry of the soil. Additionally, the ash that could replenish the soil easily blows away. This means fire can be good in the short term in the form of ash, but it depletes nutrients in the long run, especially if it burns too often. Last, loss of vegetation can lead to more erosion and higher evaporation rates from already dry soils.
The good news is, fires are often not intense enough to burn all the way to the ground, they mostly just “trim off the top” of the dead grasses. This means that small animals and the important roots and meristems of plants can stay protected under the soil, and the area gets a bit of a “clean up” and fresh start. Many plants are adapted to survive fire, and some even love it and depend on it for reproduction—these are called pyrophites. Certain seeds need fire to be able to germinate, and others use cues from the smoke as a signal to germinate and grow faster since the fire has created a lot of space free of competition and possibly lower herbivore presence.
Grass and fires in the savanna video: https://www.youtube.com/watch?v=A7b1bg6RrK8
Herbivory is technically a biotic factor, or living element, to the savanna, but it is a very important defining feature.
No other ecosystem has the same quantity of ungulate herbivores, meaning hoofed plant eaters, and they have a big influence on the landscape. Through both grazing and moving through areas in big groups, they change what plants can survive here (grasses being dominant as they can handle grazing pressures), alter the soil through compaction and through depositing their waste, and even have a part in the lack of trees as they often eat up saplings before they have a chance to establish into large trees.
So what are some of the important herbivores, plants, and other animals that live in the savanna? And what adaptations do these plants and animals possess to survive in the tropical grassland?
The biodiversity in the savanna is quite fantastic, and each region has something unique about it. The plant life is dominated by a variety of grasses, but also has interesting and very tough trees, sedges, and legumes. In terms of wildlife, the Cerrados in Brazil is probably the most diverse savanna, but African savannas have the highest quantity and diversity of large herbivores. Los llanos in Colombia and Brazil are known for having many endemic species, meaning found in no other place, and while they lack iconic large herbivores, they do have the capybara (Hydrochoerus hydrochaeris), a rodent reaching more than 50 kg (110 lbs) that takes on the larger herbivore role. Australia doesn’t have big herds of animals but houses the iconic kangaroo, other marsupials, and hardy eucalyptus trees, while the Indian savanna has an interesting variety of herbivores and is home to water buffalo, rhinos and the Indian elephant.
Heads up! Since the savanna is found on multiple continents, not all plant and animal life is the same depending on the region! These are some general examples; if you are researching for a report, make sure you confirm where your plant/animal is found!
In terms of plants you find grasses, sedges, legumes, some shrubs, and sporadic trees.
Grass species: These will vary based on where you go, but there is usually a mix of different species living in an area. Due to the unending nature of this list, we will just mention bahiagrass (Paspalum notatum) and bermudagrass (Cynodon dactylon), which are common in Africa and used as lawn grass worldwide, and elephant grass or Napier grass (Pennisetum purpureum), which is famous for its ability to grow taller than 3 meters (10 ft)!
Sedges: Though they may look the same at first glance, sedges belong to the Cyperaceae family, and grasses to the Gramineae family. Rushes also seem similar but are in the Juncaceae family.
You can remember with this little rhyme:
“Sedges have edges, Rushes are round, Grasses are hollow right up from the ground.”
This refers to rush blades being triangular and often sharper, where rushes and grasses have rounder blades, and grasses are much more hollow.
Trees: The savanna is home to various species of Acacia, Baobab trees, Eucalyptus, the candelabra tree (Euphorbia ingens), and even palm trees (Arecaceae sp.) in wetter, more flooded areas.
There is also a giant list of amazing savanna animals, but we will just mention a few:
Mammals: Marsh deer (Blastocerus dichotomus), savanna rabbits (Sylvilagus floridanus), giant armadillos (Priodontes maximus), ocelots (Leopardus pardalis), as well as impalas (Aepyceros melampus), gazelles (Gazella sp.), water buffalo (Bubalus bubalis), wildebeest (Connochaetes sp.), zebra (Hippotigris sp), rhinos (Family: Rhinocerotidae), giraffes (Family: Giraffidae), elephants (Family: Elephantidae), Hippos (Family: Hippopotamidae), warthogs (Phacochoerus africanus), and predators such as lions (Panthera leo), leopards (Panthera pardus), cheetahs (Acinonyx jubatus), jackals (Canis aureus), and wild dogs (Lycaon pictus).
Reptiles and amphibians: Crocodiles, including the Orinoco crocodile (Crocodylus intermedius) from Los Llanos which is endemic and restricted to only one river! Red-footed tortoises (Geochelone carbonaria), black mamba snakes (Dendroaspis polylepis), African bullfrogs (Pyxicephalus adspersus), geckos and skinks, as well as large rock monitor lizards (Varanus albigularis).
Arthropods: One of the most abundant but least seen animals in the tropical grasslands is the termite (Isoptera). These ant-sized insects eat dead grass and wood and recycle the nutrients they contain back into the grassland ecosystem. Some species of termite in Africa, Australia, and South America build colossal mud mounds for their colonies that can reach more than five meters high! Other insects also make up a huge part of this ecosystem—everything from birds to rodents and reptiles rely on insects, and other arthropods like spiders, for food. Others like the dung beetle (Scarabaeinae) and the many ant species as well are important clean up crews of biological waste.
Birds: More than 2,900 bird species are found in savannas, many which are here all year, but also lots of migratory birds coming from northern regions to spend the winters in more favorable climates. The highest diversity is found in the Cerrado of Brazil, followed by Africa, but Australia has the most endemic families. Some examples are: greater rhea (Rhea americana), cone-billed tanager, (Conothraupis mesoleuca), the colorful roseate spoonbill (Ajaia ajaja), wood stork (Mycteria americana), monk parakeets (Myiopsitta monachus), burrowing owls (Athene cunicularia), white-tailed hawks (Buteo albicaudatus), ostriches (Struthio camelus), and thousands more.
Adaptations arise in organisms as a way to solve problems in their environment. So what plant adaptations do we find in the savanna?
The extreme differences in precipitation throughout the year is very stressful for plants, as not only do they have to be drought-tolerant, but also be able to survive the possibility of too much water once the wetter season comes around.
Dealing with these extremes isn’t easy for the plants of tropical grasslands, but every species has adaptations to help it survive.
Many of the grasses go dormant throughout the dry season. They also have the capacity to live on very few nutrients, By lowering their transpiration rates, the pace that water evaporates out of the leaves, they do not experience as much water loss. Grasses in tropical grasslands are also more efficient at water use in general and can adjust their osmotic potential, which is the term to say that they change the nutrient concentrations in their cells to better take up water when there is very little available.
To be able to withstand grazing, grasses grow quickly, grow from the bottom up so newer tissues aren’t eaten right away, and have lots of their energy stored in their roots, which are not fed on as much as the above-ground leaves.
To deal with fire, some grasses have rhizomes underground that sprout back easily after a fire.
Though there are few trees, the ones that live here have some key strategies. Some have leaves that fall off during the dry season to conserve water, known as drought-deciduous trees, like the baobab. Others are evergreen, but are good at conserving water, like the acacia.
Another limiting factor for trees here is frequent fires. Trees are often vulnerable just before the wet season because they grow new leaves and are already quite stressed, so an intense fire could be too much for them. To help protect against this, mature trees have very thick bark to protect against the fires heat.
Trees deal with low precipitation by having huge roots that can get water from deep underground, and even water storage tissues, bulbs and corms, in their trunks (again, like the baobab). Since trees can often reach the water table, they need fewer physiological adjustments in the dry season than the grasses do. However, this is likely another reason that there are so few trees here, because it is really tough for a sapling to grow to a point where the roots are sufficiently large enough to take advantage of the low water table during the dry season. Though they have less control over water efficiency than grasses, some trees close, or partially close, their stomata at midday when it is the hottest to avoid really high transpiration rates.
A final important adaptation to make it in the savanna is defensive chemicals against the most numerous danger in the land, herbivores and termites. Termites, for example, can completely consume huge trees, but defensive compounds produced by the tree in both wood and leafy tissue can ward them off. Some trees, especially acacias, have a mutualistic symbiotic relationship with ants. These ants, which live in and on the tree, also defend it from any other predators.
Did you know?! To ward off giraffes, their number one herbivore, acacias not only pump bad-tasting compounds into their leaves when they start to get eaten, but they also release ethylene gas, which causes the other trees around them to do the same!
Read the excerpt on “how trees talk to each other” by Peter Wohlleben for more details.
Animals adapt as a way to solve problems presented in their environment, either from abiotic stress or biological competition. Animals can have three types of adaptations: structural (the physical traits of their body), physiological (how their hormones and metabolic systems deal with stresses), and behavioral (actions they take to better survive in an environment). So what adaptations do animals in the tropical savanna have that enable them to live here?
The dry season is a difficult time for animals. The fact that many plants go dormant or die in the dry season also means that there is a lack of food, and of course they also have reduced access to water. They must either find ways to cope with little food and water, or migrate to new areas.
Many animals and birds migrate during the dry season to wetter areas, and they will congregate more heavily around water sources than they would in the wet season.
Giraffes, for example, do not have to migrate because they have evolved a very long neck that allows them to eat from evergreen tree tops. They also have strong lips and long tongues, that they have impressive control over, which help them eat from plants with thorns and other defenses.
On a similar note, these browsers and grazers manage to co-exist by having different preferences when it comes to food and where and when they eat. This is known as niche partitioning.
Animals also have to stay cool in the daytime heat. Elephants, for example, use physical adaptations like large ears to help heat escape their bodies, but also behavioral ones like taking dust baths to cool down. Other animals, like hippos, spend the day in the water and graze at night. In fact, many tropical grassland animals are nocturnal—herbivores and carnivores alike.
Did you know that the cheetah is the only large diurnal cat?
As well, a common behavioral strategy is to burrow underground and stay in the cool earth during the day, like meerkats and many reptiles. Smaller animals may also reside in burrows to hide from predatory birds that do very well in these large open spaces where they have wide, clear views to search for prey, hot air updrafts to help them soar, and essential resting and nesting sites in the trees that are present.
Animals that burrow, and some that don’t, also look for their food below ground when vegetation gets scarce, digging for roots and tubers and other vegetation that will also have a higher water content than the above ground plant tissue.
If you thought the king of the savanna was the lion, you are wrong. It is actually the honey badger (Mellivora capensis). These animals are the roughest, toughest, and possibly the coolest (of course this could be up for debate) of the African savanna. To start, they have small eyes and ears so they can avoid harm while fighting, as well as loose, yet tough skin that makes it very hard to pin down, as they can just wiggle out. This feature also helps them to almost completely avoid bee stings, even when it sticks its entire head into a beehive to feed on bee larvae and honey. Its neck skin is especially thick to avoid throat injury. They also support impressive teeth and claws, which help them to run, burrow, and catch prey. The honey badger has been found to feed on 60 different prey items! Last but not least, honey badgers are resistant to snake venom and when defending against, or fighting off, other large animals, they have been known to attack the scrotum with their teeth and claws—definitely not an animal you want to mess with!
We have a longer history with the African savanna than any other biome, and today the savannas around the world support people as a place to live, for agriculture, and as a key destination for the tourism industry.
Many distinct ancient tribes and more recently established communities live in all savannas around the world. The year round warm weather and flat areas make for desirable places to live and develop, especially as we get better at technology to bring water to people and agriculture during the dry seasons.
Here is some more information on peoples of the African Savanna.
So, with all that we do here, what are some direct and indirect human impacts on the savanna?
Direct impacts on the tropical savanna by humans
General development and agriculture are some of the most harmful practices to the savanna, along with the oil industry, poaching, and unsustainable tourism. For example, according to the WWF more than 70% of South American savannas have been converted to agriculture.
To make areas ready to plant on, large fires are started on purpose, which as mentioned before can result in short term nutrients, but over a long period of time, leads to less nutrients and high erosion, making this area poor for plant growth in the future. As well, because of lower nutrients in general, larger areas need to be used to get the same yield. Last, irrigation of crops can greatly deplete natural water sources.
Livestock can also cause many issues. First, we alter and take up a lot of land for things like cattle. Also, we keep livestock in small spaces where they can totally deplete an area. This is different from how large wild herds normally behave—feeding in one area and moving along—which allows the place to recover. Interestingly, this movement is also important as it allows some time for harmful urea from waste to properly degrade in the soil. Some wild herbivores excrete waste in different patches than where they feed to avoid getting sick from waste; when cattle are penned up like this, they do not have much choice, even though they do not exhibit this behavior naturally. Additionally, there are issues of human-animal conflict where farmers shoot wild predators that may attack their livestock or herbivores and birds that feed on or destroy crops.
Converting land and developing is very harmful in these open systems because animals from these areas need large undisturbed patches to thrive.
Climate change is also causing more heat, less rain, and more extreme weather events which can (1) further increase fires here, making them much too frequent (2) make it more difficult for plants and animals to survive, and (3) allow invasive species to take over areas that used to be cooler and wetter.
What we are doing to mitigate these issues and what can you do?
Sustainable practices in development, agriculture, and tourism are becoming more mainstream, and with more people advocating for change and working hard to do things better, we can begin to make less impact. Interesting strategies like guard dogs and rangers to help reduce human-animal conflict are also becoming more widely used. As well, scientific research and traditional indigenous knowledge is being used to help restore altered ecosystems.
If you live in, or visit a savanna, make sure you respect plants and wildlife, be careful with fire, and see where/how you can volunteer/donate for different organizations helping to protect and restore these habitats. If you visit the savanna, make sure you do your research and are visiting with a sustainable eco-tourism company. Last, you can eat less meat, and work on reducing your carbon footprint to help fight climate change.
Whether we live here or not, we can all work towards preserving this incredible biome and all the unique life that it supports, so that generations to come can experience all of its wonders.
Want to check out a savanna in real life? Here are a few options you could investigate:
Explore the Serengeti Plains of Tanzania in Africa while you stay in the remote, southern part of these world famous grasslands at Sanctuary Kusini. Incredible wildlife can be seen here all year round.
Head to Colombia in South America to experience Los Llanos (“the Plains”). Parque Nacional Natural El Tuparro is one of the few remaining pristine savanna regions in South America.
For something more hands-on, why not travel to the Everglades in Florida and help scientists find out more about this amazing grassland by doing some volunteer research? Everglades Hostel and Tours can help with both interesting work and a place to stay.