Savanna herbaceous vegetation, Lake Mburo National Park, Uganda

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Termites and large herbivores represent important functional groups in savanna ecosystems. Termites affect vegetation far beyond their mounds. In addition, large herbivores feed selectively on termite mound vegetation or in the vicinity of mounds. Previous studies of savanna vegetation communities have focused on termites and large herbivores separately, although interaction effects may be predicted.

We studied the effects of large herbivores and large vegetated Macrotermes mounds on the herbaceous vegetation in Lake Mburo National Park in Uganda. We recorded herbaceous vegetation change over 3 years on savanna areas (with and without large herbivores) and on corresponding termite mounds (with and without large herbivores) in a randomized block design.

Termite mounds and savannas had significantly different plant communities, but large herbivore grazing exclusions did not result in significant shifts in plant communities during this study period. A canonical correspondence analysis separated species mainly along an axis from termitaria to savanna. Only a few species responded to grazing exclusion. Some erect species, such as Hyparrhenia filipendula and Themeda triandra, increased in cover, and creeping species, such as Cynodon dactylon, decreased, following the exclusion of grazers. Forbs dominated mound areas, while graminoids dominated the savanna areas. Fencing increased the cover of graminoids over time and led to gradual increase in the relative cover of graminoids compared with forbs.

Mound soil was higher in pH, calcium and magnesium and lower in sodium compared with adjacent savanna areas. Nitrogen and carbon soil content did not differ between the two habitats. Soil phosphorus increased following grazing exclusion.

This study shows that termites may exert a far more important effect on the herbaceous community than large herbivores in savanna areas, even if the biomass of large herbivores is relatively high. Thus, future studies on savanna vegetation ecology should focus increasingly on important insect groups in addition to the more conspicuous large mammal guild.

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Bibliographic Citations

1. Okullo, Paul, and Stein R. Moe. "Termite activity, not grazing, is the main determinant of spatial variation in savanna herbaceous vegetation." Journal of Ecology 100.1 (2012): 232-241. -
2. Okullo, Paul, and Stein R. Moe. "Large herbivores maintain termite‐caused differences in herbaceous species diversity patterns." Ecology 93.9 (2012): 2095-2103. -