Savannas cover more than 10% of the world´s land surface and more than 50% of the African continent. They are a mixture of trees and grasses, where the former are discontinuous and the latter are continuous. They provide important habitat for a large range of animal and plant species. The tree-grass balance of the savannas make them socioeconomically important for the large human populations that live within and earn a livelihood by exploiting them.
“Despite their importance, the driving factors behind them are still poorly understood,” says PhD candidate, Erik Francis Acanakwo.
“We are uncertain of the response of woody plants to the loss of ungulates in the savanna.”
Examining the interactions
Erik has spent the past three years researching how termites and large herbivores, such as zebra, warthog and various antelopes, affect the savanna’s plant communities.
“A particular focus of mine has the interplay between biotic factors, in this case the effect of termites and large mammalian herbivores on woody plants .”
His results show that ungulates amplify the differences of tree community properties between on- and off termite mound habitats.
Vital part of savannas
Termite colonies are well-known structures of tropical savannas, where they can reach a height of up to 9 meters, and contain millions of termite individuals. With some species, like the ones in Erik’s projects, there are also plants growing on top of the mounds. The termites are very important when it comes to nutrient cycles, earth formation and quality, and as food for other animals
“Even though the termites are small, the collected biomass equals the one of large mammals in many areas,” Erik says.
“Understanding the interactive effects between termites and ungulates on woody plants can contribute to improved natural resource management strategies for savannas.”
The aim of Erik’s study was to investigate how tree species composition, species diversity patterns, and seed removal varied on- and off-mound with and without ungulates. He also investigated wood decomposition rates on active and inactive mounds with open and closed canopy vegetation microhabitats. He then compared these results to wood decomposition rates off mound.
Habitat types determine species composition
Erik’s results show that habitat types determine tree species composition.
“Species composition depended on whether tree communities are located on- or off termite mound, but in 10 years, the period 2005-2015, the exclusion of herbivores did not affect composition.”
In the absence of ungulates, stem density markedly increased both on- and off-mound. Ungulate exclusion determines species diversity patterns off-mound.
“While exclusion of ungulates made little change in species richness on-mound, richness was markedly increased off-mound in the absence of ungulates.”
No ungulates, less seed loss
Seed removal rates did not differ in the presence or absence of ungulates off-mound, but on-mound, seed removal rates were marked lower in the absence of ungulates.
“Smaller native seeds were removed more than larger seeds.”
Smaller native seeds experienced higher rates of seed removal relative to larger seeds, suggesting that all seed removal agents likely fed on small seeds, while a few larger agents removed the bigger seeds.
Wood decomposition rates
“Wood mass loss was more rapid in the closed than open vegetation microhabitat,” Erik comments.
“However, while wood decomposition rates did not differ much on active mounds, wood mass loss was higher in the closed than open vegetation microhabitat of inactive mounds.”
Termite mounds are refuges
“My findings suggest that although termite mounds act as refuges for woody plant species, in the absence of ungulates, stem density and diversity of woody plants increases in the landscape.”
While an increase in stem density would enhance carbon sequestration, the tree-grass balance is altered and this would affect organisms that depend on grass biomass