you're reading...


Why lions can thank wildfires for the African Savanna

Article: S. Hoetzel, L. Dupont, E. Schefuß, F. Rommerskirchen, G. Wefer. (2013) The role of fire in Miocene to Pliocene C4 grassland and ecosystem evolution. Nature Geoscience. Vol. 6, 1027-1030. DOI:10.1038/NGEO1984

Background Information 

The African Savanna is known for its picturesque yellow grasses dotted with exotic shrub-like trees such as Acacia, teeming with impressive mammals including prides of lions, herds of giraffes, zebras, and elephants, leaping antelope, and gigantic rhinoceros. Anyone who has seen Disney’s The Lion King will know this grassland biome. However, these Savanna grasslands are geologically young and only evolved around 8 million years ago towards the end of the Miocene.


The Miocene is an epoch during the Neogene period that occurred roughly between 23 and 5.3 million years ago. Towards the end of the Miocene, the Earth started to cool down due to the formation of the Himalayan Mountains. Mountain building causes cooling because it increases the amount of exposed rock available for erosion, which ultimately leads to a reduction of the amount of carbon dioxide in the atmosphere.


This type of climate, dry with low CO2, led to the evolution and expansion of C4 plants in subtropical regions. C4 plants, such as most grasses, use a special type of carbon fixation (part of photosynthesis) that excels in warm regions prone to drought and low carbon dioxide. C4 plants are much more successful in these conditions than C3 plants, such as most trees.


Paleo-scientists have hypothesized that C4 plants, and grassland biomes, evolved during the end of the Miocene due to these arid and low CO2 conditions. However, the optimal decrease in CO2 happened much before the large expansion of C4 grasslands. This new study hypothesizes that wildfires were the initial trigger for the expansion of the African Savanna because they burned down forests and made room for grasses to grow.


The Findings


Hoetzel et al. (2013) used a sediment core collected off the coast of Namibia and analyzed it for the grass pollen Poaceae, pollen from desert plants, fire-charred particles, and biomarkers called n-alkanes (Figures 1 and 2). They used a section of the sediment core from the end of the Miocene to the beginning of the Pliocene (9- 2.5 years ago) in order to determine the role wildfires played in expanding grasslands. Marine sediments are excellent at preserving these biomarkers and pollen, allowing paleoceanographers to reconstruct what the conditions on earth were like millions of years ago.


The amount of C4 grass pollen increased by 30% starting about 8.4 million years ago, and then increased steeply up to 70% around 6.8 million years ago at the end of the Miocene. The n-alkane biomarkers also indicate a large rise in C4 grass plants over C3 plants. N-alkanes are derived from leaf waxes and C4 grasses typically have 31 or 33 carbons and an enriched δ13C. These n-alkanes can be used to determine the relative inputs of C4 grasses versus C3 plants. This rise of C4 grasses coincides with a spike in the amount of fire-charred particles as well as a prolonged dry period that would be optimal for wildfires.


These researchers noted that upwelling from the Benguela current (located off the coast of Southwest Africa) greatly increased during the end of the Miocene, which led to cooler temperatures, stronger winds, and an increasing aridification (dryness) of Southern African. This oceanographic event led to strong wildfire breakouts that burned down many of the woodlands in the area, allowing C4 grasses to grow in place of the trees. Grasses grow faster than trees, especially after wildfires. Thus, wildfires are hypothesized to have caused the initial expansion of the C4 grasslands by clearing space, and since the atmosphere was dry and had low CO2, C4 plants easily out-competed C3 trees are the region.


But the influence of fire did not just initiate the expansion of C4 grasslands in Africa; it created a positive feedback mechanism to continue its expansion. As grasslands out-competed tree biomes, the local region became even drier. All plants release some water back into the atmosphere in a process called evapotranspiration. Grasses put less water in the atmosphere than trees, which caused Southern Africa to become even drier, which caused more wildfires to occur, which burned down more C3 biomes, and led to a greater expansion of the grassy Savanna. This feedback continued until the local climate actually got too dry even for grasses, resulting in the expansion of deserts.


The expansion of deserts occurred around 5.8 million years ago. The sediment record shows that the amount of fire-charred material also decreased around this time, most likely since deserts do not have a lot of plants for fires to burn. However, C4 grasslands were already established and the subtropical climate even today favors their dominance over woodlands. The expansion of these C4 grassland biomes changed the ecology of Africa, allowing the animal diversity we know today to evolve.




Atmospheric conditions of low water vapor and CO2 during the late Miocene favored the evolution and expansion of the modern day African Savanna. However, wildfires caused by drought are now hypothesized to have triggered this expansion by clearing woodlands and allowing C4 grasses to quickly dominate this region. The increasing arid conditions also led to the growth of deserts, which is ultimately what stopped C4 grassland expansion. Thus, lions, cheetahs, and elephants can thank wildfires for giving them their home.




Figure 1: Present day vegetation map of the study area. The Benguela Current, and associated upwelling, is in blue. The dashed black line represents the 4,000 m ocean depth line. Black arrows indicate the modern prevailing wind direction. The fire symbols show the areas were large wildfires have occurred from October-Novemebe 2011.











Figure 2: The sediment record used to reconstruct Africa’s climate and vegetation between 9 and 2.5 million years ago. A) The grass pollen Poacaea shows an increase between 8 and 6.8 million years ago, demonstrating the expansion of grasslands. B) Stable carbon isotope ratios of grass waxes called n-alkanes also showed an increase in grasses. C) The concentration of fire charred particles occurred during the same time as grass expansion. D) Pollen from desert plants began to increase after the peak of wildfire frequencies, showing the expansion of deserts stopped the expansion of grasslands. E) Alkenones, a biomarker used to predict seas surface temperature, showed that the ocean (thus atmosphere) got cooler during this expansion due to an increase in upwelling from the Benguela Current.


No comments yet.

Post a Comment


  • by oceanbites 2 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 3 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 4 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 5 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 7 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 8 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 8 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 9 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 9 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 10 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 11 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 11 months ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com