Terrestrial Biome Report: Savanna
Ecology and the Environment
By Billeh Scego, Kato Wong, Zain Khaki, Adrian Corcoro
Biome Description:
The savanna biome is a sloping grassland dotted with isolated trees, bushes, and shrubs. It can be found between a tropical rainforest and a desert biome and due to this they are warm year round and is known as a transitional zone. Just like other biomes, the Savanna has seasons however it only has two types dry (winter) and wet (summer) because compared to regions closer to the north and south poles, this region receives more direct sunlight throughout the year. The dry season is usually long, and characterized by thunderstorms and dry winds; this combination of thunderstorm and dry winds helps to spread fires. With the wet season, a lot of plants return and sprout13. These long periods of dry weather is what helps to make the savanna look the way it does today. Savannas can be found throughout our planet in South America, Asia, Australia and the most popular ones being in Africa. Savannas are usually around the equator due to more sunlight.
Savannas are categorized into three categories, derived, climatic, and edaphic. Derived savannas are ones that are engineered either by humans or elephants through the eating, and removing of vegetation and trees. Climatic savannas are when some trees can survive the length of the dry season. Finally the edaphic savanna is when the soil is not good enough to support plants throughout the year. Due to the lack of rainfall during parts of the year, there are frequent droughts and this leads animals to migrate. The plants in this area are best suited for the dry climate because of “water storage organs and long roots. Such as Acacia Senegal, Baobab, Bermuda Grass, Candelabra Tree, and Elephant Grass. There are also many types of grasses, like the lemon grass, red oats grass, Bermuda grass and Rhodes grass. Trees like the acacia tree, and the baobab tree can be found in the savanna as well. These trees that are in the savanna are also spread apart thus not forming a canopy. The trees not forming a canopy allow for the plants on the ground to get plenty of sunlight, these all contribute to the classic look of the savanna. There are many different types of animals in the savanna and each depend on each other and the environment. There are herbivores that graze the grass such as Zebras, Elephants, giraffes and wildebeests. The savanna is also home to predatory/carnivores mammals and birds such as lions, Cheetah. Wild dogs, hyena, Vultures and Eagles.
Climate:
As mentioned before, Savannas are warm temperatures year round with the only seasons being the wet and dry seasons. The warm dry seasons are characterized by thunderstorms and dry winds13. This warm and dry weather is known to cause many wildfires. These wildfires tend to chase many organisms away. However these wildfires do a lot more than just chase organisms away, they also affect the plant life. Melinda Weaver found that “Plants begin to die or shrivel to protect from water loss…”. Rivers and lakes tend to dry out around this dry and hot weather13. Things begin to sprout and return during the wet season13. Even though the dry season causes wildfires which go on to harm many plants, not all plants die. Many plants survive the wildfires and thrive as soon as the wet season comes around. The wet seasons bring back the rivers, lakes, trees, and animals. It was found that the savanna is able to be a savanna due to these dry and hot seasons accompanied by the wet season. This is due to the fact that the dry seasons help to keep the tall plants in check. By keeping these bigger plants in check, the savanna grass and other smaller plants are able to thrive. This is what makes the savanna the way it is; its lands are littered with grass with a tree here and there. The savanna “rarely falls below 60°F…”. The weather usually stays around the “80 and 100°F” range. During the dry season, rainfall is scarce and rarely exceeds “4 inches“ sometimes rain is absent for long periods of time. The wet season is a different story. Wet seasons can bring around “20–50 inches” of rain. This large source of water during the wet season “allows the grasses to grow thick and lush”. All of these factors work in conjunction to form the savanna biome.
Evolutionary adaptation of species:
The savanna biome is home to a diverse array of plant and animal species that have developed unique evolutionary adaptations to survive the challenges of this hot and dry ecosystem. Individuals who are successful in natural selection are able to pass on their trait of camouflage on to the next generation.
Camouflage: Like any biome, savannas have the same conditions that require flora and fauna to adapt in order to survive. Grazing animals such as zebras feed on grasses and often use this adaptation to protect themselves from prey, “Animals, like Gazelles and zebras, feed on grasses and often use camouflage to protect themselves from predators when they are roaming in the open”. The yellowish-brown color of the lion’s coat helps it blend in with the grass and bushes, making it more difficult for prey to see it. Some species, like the African bush elephant, have wrinkly, rough skin that looks like tree bark. This makes it simpler for them to conceal themselves from predators by enabling them to blend in with the savanna biome’s trees and bushes. Many creatures in the savanna biome also employ their behavior as camouflage in addition to fur and skin color. For instance, cheetahs follow their prey in the tall grasses, getting as close as they can before striking unexpectedly. In the savanna environment, several bird species employ flight to avoid predators or ambush prey.
Migration: Savannas are characterized by a distinct wet-dry season cycle, which influences the availability of food and water for wildlife. The migration pattern of savanna species is primarily driven by the search for food and water. Savannas are covered in lush vegetation and have plenty of water sources during the rainy season. Many herbivorous species, including antelopes, wildebeest, and zebras, migrate to these regions during this season in search of food and water. They drink from the running rivers and streams and feed on the new grass. However, as savanna vegetation withers and water supplies become limited during the dry season, many herbivorous animals are compelled to move to locations with more food and water. During this time, predators like lions and hyenas feed on the vulnerable prey and hunt them down. Even while certain species, such as elephants and giraffes, are not migratory, they are nonetheless able to adapt to the shifting weather patterns by covering shorter distances in search of food and water. Other species, such as birds, migrate completely out of the savanna biome during the dry season and then come back during the wet season when there is an abundance of food and water.
Burrowing: In the savanna habitat, which is characterized by wide grasslands with few trees and scant flora, many species have developed a common adaptation. Burrowing offers a more stable microclimate that can be cooler and more humid than the hot and dry surface environment, as well as shelter and protection from predators. Burrowing is a common adaptation for many animals in the savanna biome, including rodents, burrowing snakes, and various types of ants and termites. For burrowing, these creatures have developed particular anatomical and behavioral adaptations, such as powerful claws or digging tools, elongated bodies that are suitable for small tunnels, and the capacity to excavate underground chambers and tunnels.
Water Conservation: Essential for survival in the savanna biome, which is known for its hot, dry, and erratic weather. In this climate, both plants and animals have developed diverse water-saving techniques. Savanna biome plants have developed a range of water-saving techniques. The presence of deep roots that can access underground water sources is one such method. Other types of plants, including succulents, have evolved ways to store water in their leaves and stems. Some plants have also developed specialized leaves, such as tiny leaves or leaves covered in a thick layer of wax, that minimize water loss through transpiration. Savanna biome animals have water-saving adaptations as well. While some species may absorb water from the food they eat, others can concentrate their urine to prevent water loss. For instance, predators can collect water from the blood of their victims, whereas herbivores can get water from the plants they eat.
Nocturnal: Active primarily during the night and rest during the day. These species have developed adaptations to aid in their ability to move about, acquire food, fend off predators, and survive in low light. Animals that spend the most of their time in the dark have developed a number of adaptations, including huge eyes that let them gather more light, specific visual pigments that improve their night vision, and sensitive hearing or echolocation skills. Additionally, they might engage in specialized behaviors like hunting or foraging at night to take advantage of prey that is also active then or hiding during the day to evade predators that are active then.
C4 Photosynthesis Pathway: Efficient mechanism of photosynthesis and is advantageous in environments with high temperatures and low rainfall. Employed effectively by some plants to fix carbon dioxide and make glucose. It involves mesophyll and bundle sheath cells, two specialized cell types. Initially fixed into the four-carbon molecule oxaloacetate in the mesophyll cells, CO2 is then changed into malate and transferred to the bundle sheath cells. Malate is decarboxylated in bundle sheath cells to produce CO2, which is used in the Calvin cycle to synthesize glucose. Because the C4 pathway is more effective than the conventional C3 pathway, it allows plants like corn and sugarcane to thrive in hot and arid climates.
Deep roots: Give plants access to groundwater sources so they can absorb moisture that isn’t present at the surface. This offers a crucial benefit in a biome where water is frequently scarce and unreliable. Plants with deep roots can store water in their root systems to help them survive dry spells. The plants can use these reserves to keep up their metabolic processes and prevent water stress during dry periods. Not only does it help plants get water, but they can also stabilize the soil and stop erosion. By gaining access to nutrients that are further down in the soil profile, they can also facilitate nutrient uptake.
Spiny leaves: In order to protect plants from animals and to conserve water in the hot, dry conditions of the savanna, these leaves have evolved. These spines can have either long, thin, needle-like spines or short, wide, thorn-like spines. Acacias and aloes are two examples of savanna plant species that are well known for their spiky leaves. Spines prevent herbivores from eating the leaves by making them unpleasant or challenging to chew. By minimizing the surface area of the leaf that is exposed to the sun and wind, spiny leaves can help prevent water loss. In the savanna habitat, where water is frequently in short supply and transpiration rates can be high, this is crucial.
Thick bark: These plants have evolved thick, protective bark to survive in the hot and dry conditions of the savanna. The benefits of thick bark for plant species are numerous. It shields the underlying live tissue from physical harm from enormous animals like elephants and from fire. Thick bark also aids in moisture retention within the plant, which is beneficial in a habitat where water is sometimes in short supply. Some plant species native to the savanna, such baobabs and acacias, are widely recognized for having thick bark. The bark of baobab trees is incredibly thick, measuring up to 50 cm (20 inches). Baobabs can endure the lengthy dry spells of the savanna thanks to the massive amounts of water they can retain during the wet season thanks to their thick bark. The bark of acacia trees is more intricately layered, with a layer of dead tissue on top shielding the living tissue below. High quantities of tannins in this outer layer also render the bark unpleasant to herbivores.
Population Dynamics:
The features of the savanna biome have many qualities that influence population sizes, distribution of species, and population trends in general. One of these features is the seasonally dry climate. Plants in the savanna experience annual dry periods, when very little rain falls. This results in the need for greenery to develop wide-spreading, deep root systems in order to tap into moisture reserves that lie deep below the ground. While many types of trees can do so, certain plant populations such as grasses are forced to dry out and die. Certain grasses survive dry spells by developing the ability to exist as buds extremely close to the surface, while they wait for rainfall.
The presence of certain animals can also influence the spatial distribution and population sizes of various types of vegetation. High populations of grass-feeding animals can result in the phenomenon of overgrazing, leading to decreased populations of savanna grasses while leaving woody plants and trees, which have hard-to-reach leaves, to grow and increase in number. This can result in the forestation of savannas over time, as grasses are gradually driven to extinction and woody plants take over the environment.
The opposite is also true — in the Virunga National Park, populations of elephants ballooned between 1934 and 1959. This resulted in high consumption of woody plants and trees, greatly reducing those populations in favor of grasses closer to the ground, which were not the primary food source for the elephants. The gradual decrease in woody plants led to the deforestation of the savanna, transforming the once densely populated areas into open grasslands.
The presence of fire is a significant factor that affects many qualities of the savanna biome. Fires can start in savannas at any time throughout the year, most commonly started naturally by lightning strikes or artificially by humans. Trees found on the savanna can survive fires by developing a thick outer bark as they mature. However, dry vegetation such as grass, fallen leaves, and small shrubs can be consumed in the blaze. The impacts of fires can range from cleaning up the terrain to exposing smaller animals for ease of hunting.
Another significant phenomenon that impacts the savanna’s population dynamics is migration. Every year, during the dry season, a huge “Great Migration” occurs, when species that cannot reach water move to other regions. Migration can either be local, if only part of the savanna is experiencing a dry spell, or alternatively, species will often switch biomes entirely. One exception is the elephant — while species such as wildebeest and birds have no choice but to migrate, elephants have the ability to crack open savanna trees with thick bark, where water is commonly stored.
Community Interaction:
There are many different organisms and species within the Savanna biome, some big and some small. It is filled with organisms like gazelles, zebras, giraffes, hyenas, and lions that all thrive in the Savanna3. A noticeable interaction is with the herbivores and the maintenance of plant life and ecosystems. Herbivores actively help keep wildfires in the savanna on the downlow by consuming plants6. Organisms like elephants help push trees over which help with resprouting. This in turns helps smaller herbivores eating within the vicinity6. It is for this reason that elephants have come to be known as ecosystem engineers as they sculpt the environment around them7. Elephants also like to dig into soil and hit underground water in order to drink. This digging doesn’t just help the elephant, it also quenches the thirst of the organisms around it as well7. These holes that they dig also help to collect water7. This all helps to show how elephants are great organisms that not only eat their own fill, but help to pave the way for other organisms around them.
There are notable predator-prey interactions that are prominent in savanna ecosystems. For example, lions live in complex groups and prey upon antelopes, zebras, wildebeest. The predators compete with one another for these prey8. Michael et. al. talk about how large herbivores don’t really need to worry about predators, instead they worry about how much food they are able to consume. While the mid-sized herbivores get controlled by food quality, it is the small herbivores who are more controlled by predators6. A response to this size and predation scale, each sized organism has different ways of approaching self defense. Smaller herbivores will avoid being in the habitats where there are high levels of predation6. Larger herbivores will travel in big groups, and can usually wander into more dangerous territory6. On top of this, many herbivores aren’t able to store as much food, thus needing quality over quantity. Michael et. al. found that “… smaller-bodied ruminants, such as gazelles, are constrained to eating relatively high-quality forage”6. This means that once again another restriction is placed upon smaller herbivores. This is extremely important because it buys into another interaction within the ecosystem.
Smaller herbivores would have to interact with the primary producers by focusing on eating the more nutrient dense plants as they are not able to store as much. Resulting in smaller herbivores needing to depend on these more nutrient rich plants compared to other less nutrient rich plants. Michael et. al. noticed another interaction between organisms in the Savanna; “…islands of soil fertility and nutrient-rich forage, such as those created by termite activity…”. This means that these smaller herbivores can probably be found around these spots where termite activity is at its peak as it is nutrient-rich. You could almost say that you can find the smaller organisms by following the termites. This means that smaller herbivores are limited to where, how and what they eat based on their size; due to predators picking on them, and needing richer forage.
Plants also have evolved to defend themselves. One such way is to make a thicker covering, or have spines that hold chemicals that deter herbivores from eating them. Acacia trees have two noticeable defenses when it comes to being eaten. First, the leaves taste bad, so usually herbivores will stop eating after a bite. Secondly, the acacia trees shelter aggressive ants who in turn protect the tree. These ants will run out and sting any organisms that try to eat the tree. However, in order to survive, many animals have evolved adaptations that allow them to go around these defenses. One such way is being able to withstand the chemicals. Others learn to eat around the plants or trees defenses. Some have even gone so far as to only prey upon one type of plant instead of going after a variety. Weaver found that “each different mammal has its own preference for grass…”. While some predators of the savanna do compete, others prey on specific mammals.
Ecosystem functions of the Savanna:
Ecosystem functions in Savanna are dominated by the cycle of wet seasons followed by long dry seasons, with constant high temperatures. Both play a vital role in the savanna’s biodiversity, fostering rapid decomposition, seasonal fires, and vegetation growth. There are many examples as well of what human diversity can do that disrupts or helps the ecosystem function of the savanna.
The Ecosystem function for the Savanna is the most productive during the period and following season of the wet season. It is crucial that the annual rainfall is from about 50.8 to 127 cm (20–50 inches) per year while the rainfall is concentrated for six or eight months of the year, followed by a long period of drought where fires can occur. Priority of the water is available for the plants which can rival or exceed that of the average forests. If the rain were well distributed, many of the areas would become tropical forests! This results for those savannas to result from the climatic conditions to be called climatic savannas, the opposite which is called edaphic savannas is when the soil conditions are what the savanna is based on and are not entirely maintained by fire.
“The values for the aboveground biomass at its seasonal maximum range from 0.5 to 11.5 metric tons per hectare in drier regions to 5.5 to 20.8 metric tons per hectare in more humid regions.” (M.B Smith) The belowground biomass values are measured less often and are on average typically large or larger than the aboveground values. The primary productivity on the other hand is less evaluated, 3.6 metric tons of dry matter per hectare per year have been recorded in Sengal, West Africa; the values are increased to 21.5 to 35.8 metric tons per hectare per year in humid areas farther south. (M.B Smith)
The quality of vegetation as food for animals is majority of the time high, the proportion being 15 percent to more than 90 percent is grass which is more digestible compared to wood vegetation in forest growth. Seeds and underground organs provide important foods for many animals during the dry-season of the savanna. The soil of the savanna is near enough that nothing other than water is accepted. The soil is deemed highly rapid for the drainage of water due to the thin layer of humus (organic portion created by decomposition of plants and animal matter) which provides easy vegetation with nutrients, sometimes the vegetation can get so good that it can be classified as forests.
Dried grass and dead wood are quickly decomposed, primarily by termites, and what Kato said, also burned, which the mineral nutrients are released into in the air to be reused in subsequent production. This rapid pace of the mineral nutrients being released in the air and being reused helps the high productivity, diverse, and abundant faunas of savannas. Due to the high competition of resources in the Savanna, the most sought after source of food is, of course, grass; due to this, in some areas like the Serengeti plains or Kenya’s Laikipia plateau, the dominant grasses on well-drained soils are the Rhodes grass and red out grass, in East African savannas, star grasses and Uganda savannas the lemon grasses. (UC Museum of Paleontology)
The Ecosystem services that the Savanna provides for the people is water, grazing and browsing, food and fuel for millions of people. In Australia, the savanna is divided into four regions based on rainfall distribution. Top is the north tropical zone, the coast parts on the sides which are called typical savanna landscape, and the southern parts are the drier parts. The Savannas are mainly used for grazing, mining, tourism, conservation and subsistence grazing and other activities for people in the area. It is also a good source of minerals such as gold, gems, copper, bauxite, nickel, and iron ore which contribute to the state economy. Some areas are only conserved for national parks and Aboriginal land which tourism improves and helps the Savanna into integration of mining, grazing, conservation parks, and Aboriginal land. In turn the savanna has a lot to offer to people and adds value and variety of benefits.
As shown above, every land use is interlinked to support each other. Of course any unbalance affects the entire interlinked chain. (Kaur K) Causing negative effects on the savanna and people.
Human Impacts:
Human impacts can both be negative and positive for the savanna, causing a butterfly effect in some cases, and help distribute to the ecosystem. Some examples that it affects the savanna are poaching, fire, population control, and overgrazing for crops.
This affects the animals of the savannas through their wellbeing of habits constructed through the environment, an example of how it affects animals is through farming which can be harmful to wildlife. This is a critical problem if large farms take over grazing or hunting lands as animals need the nutrients to survive. When farms are cleared, they cut down trees which destroys the habitat that animals and other plants depend on to survive. Livestock animals also sometimes graze on savannas causing a shortage of food for wildlife, this overgrazing causes to have a negative effect on the native plants as it increases the shortage for the wildlife.
Poaching is a huge threat to wildlife, especially in Africa. Poaching for large grazing mammals like elephants and rhinoceros are killed for their tusks and horns. These parts are sold around the world for on average, a high and lucrative price. The loss of animals due to poaching can alter the entire ecosystem.
Fires in the savannas are a highly common occurrence, naturally by lightning strikes, but now in most regions, humans significantly cause the savanna burning. Fire primarily consumes grasses, leaf litter, and other dead plant material that quickly dries out after the rains are over. “Although fires are important in the creation and maintenance of savanna vegetation in all regions, some disagreement exists concerning the extent to which fire should be considered a natural phenomenon, as well as to what extent it is primarily responsible for the distribution and character of savanna vegetation. (M.B Smith) Sometimes firebreaks will happen, an example being Australia having constant firebreaks nearly all season in significant portions of rainforests and cypress pine to regularly spread uncontrollably without frequent fires.
Still the fires help naturally for the fires to reveal smaller prey like lizards, turtles, and rodents to big predators, create areas that later develop a cover of fresh green grass, which attract game and wallabies, and encourage plants to produce edible tubers.
Human conservation and restoration actions:
There are many ways individuals can work to make a difference and put forth a change. One such action is taking out trees that are altering the character of the biome. For example, in Brazil’s savanna-like ecosystem, the Cerrado, the open character is changed by the influx of non-native pine species. Sarah Sax interviewed Gustavo Paiva, someone related to the Tikre company16. They are focused on restoring lands by cutting down pine trees and replacing them with smaller native plants16. At first this may seem counterintuitive, but Paiva explained to Sax that the pine trees were actually an invasive species to the savanna16. Paiva goes on to say how if they let the pine trees linger, the native plants will be harmed, and possibly lower the biodiversity of the savanna16. In other words they are removing harmful trees and replanting native ones that help support the biome. If successful, not only will it protect what is already there, but instead heal the savanna and make it stronger than ever.
A third project across a large area of African savanna is the Great Green Wall Initiative17. The Great Green Wall initiative has a rumored pledge of more than $14 billion; these funds are being put forth into restoration of land, including savannas17. Some of the goals of this project is to restore about 8,000km of a savanna17. However the restoration of the savannas is just the beginning, the project desires to also invest in the continual maintenance of the restored savanna17.
A really cool and important project is being done by Minnesota Valley National Wildlife Refuge. They are girdling oak trees; this involves removing or cutting bark without taking the whole tree out22. This stops the nutrients from reaching the tree, and slowly kills it without having to actually cut it down22. It was found that many of the burr oak trees in the area are over 100 years old. In order to protect these Burr oaks, the girdling has been focused on the Pin oak trees. The goal is to protect the development of the Burr oaks by decreasing the competition for nutrients22. As a bonus, since Pin oak trees are really fast growers, the grindling also seems to help the plants that are lower and closer to the ground22. This project also does not remove the dead Pin oak trees. They are left behind to become habitats for native animals22. A noticeable animal that takes advantage of this resource is the red-headed woodpecker22.
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