The 4 Drivers of Savanna Ecosystems
Last time we set the scene for the ecology of the Kruger National Park. We concluded that Kruger falls within the savanna biome and we defined some broad characteristics of savannas. In this post I am going to introduce the four main drivers of savanna ecosystems namely: climate, geology, fire and herbivores.Climate
The most important component which will determine if an area is or could become a savanna, is climate. This is clearly illustrated by the general latitudinal location of the planet’s savannas, in-between the tropical equatorial forests and the arid deserts.
Firstly, the average rainfall
an area receives has a major influence on the water that becomes available for plant growth. In general, savannas are found in areas with an annual rainfall of between 350 and 1 800 mm. The types of plants you may come across change as you travel from arid to moist savannas. Drier areas tend to have lots of fine-leaved trees such as Acacias
. Areas with higher rainfall normally have more trees with larger leaves. The general type of rainfall, whether intense and short thunderstorms (common in savannas) or extended light drizzle, also determine the amount of water run-off and therefore water availability.
Average annual temperatures
also play a major role, as hotter periods result in higher evaporation rates and higher water deficiencies. Likewise, areas which are prone to regular frost and general cold winter temperatures tend to inhibit the development of a woody plant component and never develop into savannas (for instance portions of South Africa’s Highveld grasslands).
Given above facts, just imagine how significant climate change could be to the inhabitants of savanna regions in decades to come...Geology
The types of rock underlying an area determine the potential type of soil that can be formed on the surface, through weathering over millennia. Soil characteristics
such as type and concentration of nutrients as well as water-holding capacity play a major role in determining the types of plants that can grow in an area. Especially in areas such as savannas which are not too dry or too wet, soil characteristics takes on a very significant role in determining plant communities. Due to soil differences, you may find plants adapted to dry environments as well as moisture-loving plants in the same area only a few meters from each other.
Furthermore, the age and structure of rocks result in the unique topography
of an area through the process of weathering. There are significant differences in plant communities on rolling hills compared to flat plains. Also, whether a hill faces north or south has a major impact on the climate in that small area and therefore on the plant life on that specific hill. Nutrient distribution tend to vary more on rolling hills than on flat plains due to leaching and the effect of gravity.Fire
The intensity and frequency of fire is a major force in the power struggle between grasses and trees in savannas. In general, plants of savannas have evolved to be relatively fire resistant. Trees may burn down to the ground but then re-grow as a multi-stemmed, lower growing individual. In this way, fires can influence the height structure of trees in savannas, with a cascading impact on the animals that live on and from these trees. Open areas with low tree density often indicate the presence of regular, intense fires.Herbivory
Lastly herbivores, whether mammals or insects, can have a huge impact on the plant life in an area. Mega-herbivores such as elephants can change the composition and structure of a landscape through their feeding activities. The tiny termite becomes a mega-grazer if we look at the impact of the whole colony on the surrounding landscape.
Of course, none of abovementioned drivers work in isolation. In each square millimeter of land in Kruger, these processes combine in unique ways to form a landscape of never ending variety. In future posts I will address these drivers in more detail and put them in context for specific Kruger examples.
For now I want you to think of these four drivers next time you visit Kruger. Try and imagine how these processes combine differently in the woodlands surrounding Pretoriuskop from the plains surrounding Satara. It could turn a quiet afternoon drive into an interesting detective game.
Climate, Geology, Fire & HerbivoryReferences:
Scholes, Robert and Walker, B.H. An African Savanna: Synthesis of the Nylsvley study.
Cambridge-UK: Cambridge University Press, 2004. Print.
Van Der Walt, P.T. et al. Bosveld: Ekologie en Bestuur.
Pretoria: Briza Publications, 2010. Print.
Photos of Clouds and Fire. Openphoto.net.
Online free stock photos. 27 June 2012.