Liana Wait ・ For the High Meadows Environmental Institute
The African savanna supports one of the world’s last intact large-mammal communities. Savannas also are home to a diverse array of plant species, but human-driven declines in animal populations could disrupt the balance of both plant and animal species in this iconic ecosystem, according to a study featured on the Oct. 12 cover of the Proceedings of the National Academy of Sciences.
Princeton University-led researchers found that when herbivores — particularly large animals such as giraffes and elephants —are no longer present, a dense, fast-growing type of plant known as lianas rapidly take over local plant communities.
Lianas are woody vines most common in tropical forests (think: Tarzan swinging through the trees) that climb trees in search of the sunlight above the forest canopy. Lianas are often described as “infesting” the trees they climb because of their tendency to smother them. In rainforests, lianas are known to influence the abundance and distribution of trees. Lianas are relatively scarce on the African savanna, however, and relatively little is known about how the plants interact with other plants, or how animals interact with lianas.
The study authors wanted to know what would happen to plant communities if different classes of herbivores were excluded from the landscape and no longer present to keep lianas in check. Working at the Mpala Research Centre in Kenya —of which Princeton is managing partner — the researchers constructed a series of long-term “exclosure” experiments that prohibited access to different herbivores depending on their size, from elephants down to the Chihuahua-sized antelopes known as dik-diks. When herbivores were excluded, lianas rapidly increased in number and size, smothering trees and stunting their growth and reproduction, the researchers reported.
These findings coincide with the acceleration of human-mediated changes in the distribution of species, said co-first author Tyler Coverdale, a postdoctoral fellow at Harvard University, worked on the project as a Ph.D. candidate at Princeton in the research group of senior author Robert Pringle, a professor of ecology and evolutionary biology and associated faculty in Princeton’s High Meadows Environmental Institute (HMEI), which supported the study. On the African savanna, those changes are driven largely by the displacement of large animals by agriculture and poaching,
“We conclude that savannas may be more vulnerable to harmful liana infestation than previously thought, particularly if the extinction of large wild herbivores continues to accelerate,” Coverdale said. “I think this is ultimately a story about how human impacts can cause ecosystems to change in unexpected ways, and how we need to do better to understand and predict those changes so we can mitigate them.”
Co-first author Ryan O’Connell, a graduate student at Duke University who graduated from Princeton in 2017, initiated the study for his senior-thesis research advised by the paper’s joint senior author, Corina Tarnita, a professor of ecology and evolutionary biology and director of Princeton’s Program in Environmental Studies. The study was designed and initially carried out by O’Connell, Coverdale, Tarnita and Pringle.
The first type of exclosure the researchers studied excluded all herbivores; the second type allowed only dik-diks; the third type excluded megaherbivores such as giraffes and elephants, but permitted entry to small and mid-sized herbivores such as impala and zebra; and the fourth type of exclosure plot had no fences, allowing entry to all herbivores. The researchers found that the abundance of lianas and the severity of liana infestation were lowest in the unfenced plots and increased incrementally as more — and larger — herbivores were excluded.
Because a subset of the exclosures were constructed in different years, the researchers also were able to compare eight- and 17-years-worth of data, finding that liana infestations became progressively more severe as herbivores were excluded for longer periods of time. The researchers also modeled the long-term implications of liana infestation and found that, without herbivores, there are two possible outcomes: either lianas will coexist with trees, or replace them.
But the fact that lianas proliferated in the absence of herbivores was, at first, a little puzzling because not all herbivores can eat the plants, Coverdale said. The species of liana that was the focus of this study produces a toxic latex that can be fatal to livestock if ingested, but it wasn’t known whether wild herbivores also are affected.
The team used camera traps and DNA metabarcoding — which identifies the plant species contained in a dung sample by their genetic code — to investigate which wild herbivores regularly ingested lianas and the proportion of their diet the plants made up. DNA metabarcoding found liana DNA in the feces of 15 wild herbivore species, especially browsers such as impalas, elephants, giraffes and dik-diks. In contrast, liana consumption by livestock appeared to be rare, even in browsers such as goats and camels.
However, even among wild browsers, lianas made up only a very small proportion of the diet — up to only 5.2% in dik-diks, the researchers found. This could have two possible explanations: either liana abundance is too low to be a main dietary component, or lianas are available to herbivores but are not a popular food item.
To test these alternatives, the researchers transplanted large lianas from inside the exclosure plots to trees in openly accessible areas to test whether herbivores would eat them. After only 10 days, about 50% of the transplanted lianas had been consumed, showing that wild herbivores do readily eat lianas when they are available. Camera-trap footage showed that browsers such as impala, elephants and giraffes were the main consumers.
One of the strengths of the project was that the researchers attacked the problem from multiple perspectives, Pringle said.
“The paper brings together experiments and theory, field and laboratory data, and altogether it’s a really nice example of collaborative synergy brought to bear on an important problem,” he said. “It’s a complex system, but by using these various approaches in concert, we were able to pick it apart and derive some very elegant insights about how it works.”
Having determined that the removal of wild herbivores would disrupt the plant community, the researchers next wondered what would happen if wild herbivores were reintroduced.
In 2017, after 18 years of herbivore exclusion, the fences surrounding some of the exclosure plots were removed, simulating the reintroduction of herbivores after years of local “extinction.” Before the fences were removed, lianas covered approximately 50% of the canopy of any given tree inside the enclosure, but within two months of animal access the density of lianas was almost halved. This finding offers hope that, at least in some cases, ecological degradation can actually be reversed, Coverdale said.
“Part of what makes our findings so alarming is that — over just a few years — lianas came to dominate an otherwise healthy, intact ecosystem,” Coverdale said. “The silver lining in this case is that it appears that liana infestation is easily avoidable and reversible. To avoid the worst outcomes we documented, we need to preserve wild herbivores where they are and restore them where they’ve disappeared.”
Co-authors of the study from Princeton also include Matthew Hutchinson and Amanda Savagian, former and current graduate students, respectively, in ecology and evolutionary biology, and Tyler Kartzinel, a former postdoctoral fellow in the Pringle Lab who is now the Peggy and Henry D. Sharpe Assistant Professor of Environmental Studies at Brown University.
Other co-authors included Todd Palmer, an associate professor of biology at the University of Florida; Jacob Goheen, a professor of zoology and physiology at the University of Wyoming; David Augustine, a landscape ecologist with the USDA Agricultural Research Service; and Mahesh Sankaran, a researcher at the National Centre for Biological Sciences in Bangalore, India and lecturer at Leeds University.
The paper, “Large herbivores suppress liana infestation in an African savanna” was published Sept. 27 by the Proceedings of the National Academy of Sciences. This work was supported by the National Science Foundation, the American Philosophical Society, the Garden Club of America, Princeton University and the High Meadows Environmental Institute, the Universities of Florida and Wyoming, and a NatureNet Science Fellowship from The Nature Conservancy.
Around the world, savannas are threatened by human actions like logging, development, conversion to agriculture, over-grazing by livestock, and introduction of non-native plant species.Why is the savanna important to our ecosystem? ›
Savanna ecosystems cover 20 % of the global land surface and account for 25 % of global terrestrial carbon uptake. They support one fifth of the world's human population and are one of the most important ecosystems on our planet.Do savannas have high biodiversity? ›
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.What type of ecosystem is the savanna? ›
The African savanna ecosystem is a tropical grassland with warm temperatures year round and seasonal rainfall. The savanna is characterized by grasses and small or dispersed trees, along with a diverse community of organisms that interact to form a complex food web.How does human activity affect the savanna? ›
However, human impacts are causing widespread and accelerating degradation of savannas. The primary threats are land cover-change and transformation, landscape fragmentation that disrupts herbivore communities and fire regimes, climate change and rising atmospheric CO2.What is the greatest threat to the savanna in Africa? ›
Domesticated animals, or livestock, sometimes graze on savannas causing a shortage of food for wildlife. This overgrazing can have negative effects on the native plants as well. Poaching is a major threat for wildlife, especially in Africa.How can we save the savanna ecosystem? ›
The savanna ecosystem can best be saved by putting aside land as preserves for ecosystems to naturally create large population numbers, with little or no human development of buildings or roads or farms.Why do trees not grow in the savanna? ›
Savannas typically get very little rain – about 4 inches (100 mm) of rain – in the dry season, and they will often not get any rainfall at all for many months. This is a long time for plants to go without water, which is why you don't see many trees.Is the African savanna biodiverse? ›
A new study finds that, contrary to popular belief, grassy biomes such as grasslands and savannas are species-rich ecosystems every bit as biodiverse as rainforests — yet little attention is being paid to the fact that they're being destroyed at an even quicker pace.What are some environmental factors in the savanna? ›
Savanna ecosystems are heterogeneous environments characterized by the presence of trees, bushes, and grasses. Nutrient and soil moisture availability are usually the limiting factors affecting the biomass growth in savannas, and overall biomass is impacted by competition, fire, grazing, and harvesting.
Savannas cover 20% of Earth's surface, including much of Africa. They are defined by the stable coexistence of trees and grass. The stability of a savanna ecosystem was once thought to be determined by abiotic factors, such as rainfall, fires, soil, and nutrients.How is the savanna affected by climate change? ›
Climate change to have contrasting effects
Grass coverage will decrease in dry savannas, increasing coverage of shrubs and trees in previously open grasslands and rangelands - enhancing a phenomena increasingly observed today. Conversely, in wetter savannas, climate change may limit tree growth.
Animals adapt to the shortage of water and food through various ways, including migrating (moving to another area) and hibernating until the season is over. Grazing animals, like gazelles and zebras, feed on grasses and often use camouflage to protect themselves from predators when they are roaming in the open.What are savannas 2 examples? ›
Some examples of savanna habitat are the East African plains, the South American pampas, and the open woodlands of northern Australia. The savanna is home to large herds of grazing wildlife and the predators that follow them.Can humans live in the savanna? ›
Many peoples live in the savannahs: the Nubians in the upper Sudanese Nubia, the Kualngo and the Akan in the Ivory Coast, the Bushmen and the Hottentots in Namibia. The best known people of this habitat are the Masai.Why is the savanna hypothesis wrong? ›
Critics of the hypothesis often saw the savannah as open grasslands with sporadic tree growth. However, savannas can have a high tree density and can also be humid. The big difference between savannas and forests is the lack of grasses in the latter.What type of natural disaster is affected the savanna? ›
Tropical storms affect savannas if the savannas are near coastlines. Tropical storms and hurricanes increase in strength with an increase in global temperatures.What is the biggest threat to wildlife in Africa? ›
As Africa's forests, rivers, and land continue to disappear, it is clear that habitat loss is the greatest threat to wildlife. Ecosystems can change drastically from human activities like natural resource exploration, agriculture, and industrial developments such as pipelines, housing, and roads.How much of the savanna has been destroyed? ›
By 1996, savannas had a net increase of 2.26%. By 2016, savanna showed an overall decline to 9.34%. Extensive land cover changes were observed in Yunnan between 1986 and 2016. Fragmentation generally increased for all natural vegetation land cover types from 1986 to 2016, but was more severe for savannas.How can we save and restore our ecosystem? ›
Ways to restore them include reducing tillage, using more natural fertilizer and pest control, and growing more diverse crops, including trees. These steps can rebuild carbon stores in soils, making them more fertile so countries can feed their growing populations without using even more land.
In contrast to forest-grass boundaries with no fire vs. frequent fire (Bowman 2000), the fire regime is continuous across the savanna-grassland treeline. Several hypotheses have been proposed to explain the tree-less grasslands in South Africa, including anthropogenic disturbances, frost, fire and soil properties.What advantage would a plant have in being a tree in the savanna? ›
Trees, with their tougher leaves and roots, are able to survive better in dry periods because of their ability to withstand water stress.Does the savanna have poor soil? ›
Even though the savanna and the tropical rainforestes are VASTLY different in organisms and extent, they both have a climate that results in deep, highly weathered soils. The intense weathering causes these soils to be nutrient poor and low in organic matter.What is a savanna short answer? ›
savanna, also spelled savannah, vegetation type that grows under hot, seasonally dry climatic conditions and is characterized by an open tree canopy (i.e., scattered trees) above a continuous tall grass understory (the vegetation layer between the forest canopy and the ground).What is the main predator of the savanna? ›
At the top of the food chain as apex predators, African lions are strict carnivores, eating only meat, and are crucial to the health and balance of their ecosystem. Male kori bustards in breeding season are among the world's heaviest birds capable of flight.What causes a savanna? ›
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.How do plants survive in savanna? ›
Surviving the Drought
Well, plants in the savanna have developed defenses for this. Many plants have roots that grow deep in the ground, where the most water can be found. This defense also allows the plant to survive fires because the root is undamaged and can regrow after the fire.
Widespread degradation of soils in Africa has been a result of unsustainable farming practices including the continuous tilling of land and leaving land bare after crop harvests. Converting savannahs into farmland will leave these creatures homeless and result in more degradation of Africa's farmland.Which has more biodiversity savanna or tropical rainforest? ›
Hence tropical rain forests are the most species-rich biome on Earth. Because of a high plant diversity, tropical rain forests also contain huge collection of animals including various insects, amphibians, reptiles, mammals and birds.What is the savanna biome known for? ›
The savanna biome is often described as an area of grassland with dispersed trees or clusters of trees. The lack of water makes the savanna a difficult place for tall plants such as trees to grow. Grasses and trees that grow in the savanna have adapted to life with little water and hot temperatures.
Savannahs are flat grasslands with scattered trees and shrubs. They're usually found near to the equator (the halfway point between the north and south parts of the Earth). This part of the world gets more direct sunlight than anywhere else, so the temperature is warm, all year round!How does the savanna affect the economy? ›
They also make a significant contribution to the economies of countries, through revenue generated from the wildlife industry, commercial livestock farming, and the timber industry.How does the climate of savanna affect the vegetation and wildlife of the place? ›
The wet season comes during the summer period while the dry season comes during the winter. The climate during the dry season is disastrous to animal and plant life since most plants wither and dry up, leading to no food for the animals. Most of the rain in the Savanna biome is from the wet season.Is deforestation a problem in the savanna? ›
Deforestation is amplifying climate change effects in the Brazilian Cerrado savanna biome, making it much hotter and drier.What is the example of savanna? ›
Savannas are marked by large grasslands with scattered individual trees. These giant grasslands are found in several places on earth. The most familiar of the savannas is likely the African savanna. Almost half of the continent is covered with grasslands!What is the difference between forest and savanna? ›
Savannas are typically simulated as a mixture of tropical, broadleaved, deciduous trees (“savanna trees” hereafter), and mostly C4 grassland, while forests have mostly tropical, broadleaved, evergreen trees (“forest trees” hereafter).What country is the savanna in? ›
It covers Guinea, Sierra Leone, Liberia, Cote D'ivore, Ghana, Togo, Benin, Nigeria, Cameroon, Central African Republic, Chad, Sudan, Ethiopia, Somalia, and the Democratic Republic of the Congo, Angola, Uganda, Rwanda, Burundi, Kenya, Tanzania, Malawi, Zambia, Zimbabwe, Mozambique, Botswana, and South Africa.What are some natural disasters in the savanna? ›
As well, droughts increase the damage of wildfires, which prevents the growth of deciduous forests. Tropical storms affect savannas if the savannas are near coastlines. Tropical storms and hurricanes increase in strength with an increase in global temperatures.What are the major threats to grasslands? ›
Threats to natural grasslands, as well as the wildlife that live on them, include farming, overgrazing, invasive species, illegal hunting, and climate change.How does fire harm the savanna? ›
The researchers found that the frequency of fires determines whether forest or savanna will dominate an area more than other factors such as rainfall, seasons and soil texture, especially in areas with moderate precipitation. Regular fires prevent trees from establishing and savannas from turning into forest.
Some large scale disturbances, such as drought, grass fires and grazing are considered to be driving forces in savanna ecosystems. These factors are also hypothesized to allow for a long-term coexistence of trees and grasses which is one of the distinguishing features of savanna ecosystems.How are grasslands threatened by human activities? ›
Grasslands are threatened by habitat loss, which can be caused by human actions, such as unsustainable agricultural practices, overgrazing, and crop clearing.What are the two main threats of a biome? ›
The Threats Facing Biomes Today
Marine biomes are similarly threatened by pollution, particularly oil spills. The forest biome most at risk from human development is the rainforest, which has undergone significant deforestation due to logging, power generation, the expansion of agriculture and the paper industry.
Drought is a major driver of impacts to grassland and prairie ecosystems, and is likely to lead to increased wildfires and loss of wetland habitats – such as prairie potholes that are critical habitat for migratory bird species – as well as species migration and habitat shifts.Is fire good for grass? ›
Burning removes organic matter, dead leaves, blades of grass, and other natural material from resting on top of your grass. Organic matter can house harmful insects and disease. It can also hold onto important nutrients preventing them from reaching the soil.Why do people burn the savanna? ›
Fire, sometimes in combination with cattle or bison grazing, is used to control trees, woody shrubs and invasive species and keep grasslands healthy. After a fire, grazing animals are attracted to the lush re-growth of grass and concentrate their grazing in that burned area.How do fires help in maintaining savanna grassland systems? ›
These fires are usually lit by cattle farmers as part of their traditional management of the Savannas where their animals graze. Some fires are started to stimulate new growth of nutritious grass for their animals, others are used to control the numbers of parasitic ticks or manage the growth of thorny scrub.What would happen if the number of plants in the African savanna decreased over time? ›
With less vegetation, savannas can easily become deserts, creating new challenges for the plants and animals living there.