The population sizes of species within these guilds in a savanna ecosystem such as Kruger, are mainly determined by the level of fulfillment of their dietary and habitat requirements as well as the impact of predators on their population.
These three key components (diet, habitat and predation) are in turn influenced by the 4 drivers of savannas (geology, climate, herbivory and fire).
In previous posts in the Science and Research Forum (Rockhound-Roan antelope numbers in Kruger National Park), details were given of how artificial water provisioning negatively impacted on the diet, habitat and predation rates of roan antelope.
Man-made waterholes erected in naturally dry areas, opened up these landscapes to water-dependent species such as zebra.
The increased zebra population competed with the scarcer roan for grazing.
Large areas of medium to tall grass preferred by roan were transformed into short grass stands by the constant presence of zebra and also wildebeest near these waterholes, degrading the habitat of the rare antelope.
More zebras also resulted in the increase of their main predator, the lion, which then started to predate at a disproportionate rate on the low numbers of roan in the park.
New research findings from Kruger also shows how vital climate is in determining the type of grass and therefore the types of grazers to be found in a specific area. Like all organisms, grasses need to find a balance between growth, maintenance, storage, reproduction and defense in order to thrive.
Grasses in savannas use mainly 2 strategies of utilizing metabolic performance.
Under the first strategy, grasses are geared towards maximizing growth under favourable or stable conditions such as high rainfall and high temperatures.
This is the RMP (Resource Responsive Metabolic Performance) Mode. Under the second strategy, grasses focus on maintaining themselves through harsh times such as cold winters and/or very dry periods.
This is called the SMP (Sustained Metabolic Performance) Mode.
Basically this means that in areas with relatively low moisture stress and high temperatures, grasses don’t need to worry about sustaining themselves through harsh periods and can focus primarily on production.
In contrast, grasses growing in areas with cold winters or long dry periods need to focus on surviving through the harsh times.
Grasses using the RMP strategy can focus on growth, leading to high nitrogen levels in their leaves.
Young leaves of these grasses are the most sought after because of the high nitrogen concentration.
These kinds of grasses are therefore preferred by the grazing guild of bulk grazers of short grasses (zebra and wildebeest).
The more the plant grows however, the more the nitrogen becomes diluted by structural material and thus becomes less favourable to this guild.
Grasses taking the SMP strategy focus on maintenance and storage and therefore possess high quality carbon favoured by the selective grazer guild of rarer antelope such as sable, roan, tsessebe and eland.
Roan and Sable - rare antelope and selective grazers with a preference for high quality carbon.
From climatic data obtained in Kruger over the past century, it is clear that average temperatures (especially night-time temperatures) are increasing, perhaps linked to global warming.
This has had a direct impact on the metabolic strategies employed by grasses in Kruger. In the past, cooler and drier parts of Kruger had a higher percentage of grasses employing the SMP strategy.
Due to increasing temperatures, more grasses are starting to convert to the RMP strategy.
The reduction in SMP-grasses corresponds with the decrease in grazers who utilize them (all the rare antelope).
The link can be seen even more clearly by looking at the current distribution of these rare antelope.
Although sable is predominantly found in the western nutrient-poor areas while roan and tsessebe are found in the nutrient-rich eastern parts of Kruger, the distribution of all these species are becoming more concentrated in the higher (cooler) or drier areas of their range.
Higher contours and drier areas will of course produce a higher proportion of SMP grasses.
Therefore, as the overall area of Kruger is becoming hotter, less and less areas will remain suitable for the rare antelope to utilize.
One would think that the higher temperatures and therefore higher proportion of RMP-grasses would suite high-nitrogen grazers such as zebra and wildebeest.
This is true up to a point. However, if the conditions become too favourable for grass production, the profuse growth will have a negative impact on the overall nitrogen quality by way of dilution.
This can indeed be seen in the population numbers of these two above mentioned grazers.
Historic data shows that their numbers normally increase in drier periods (when grass production is limited and kept at high nitrogen concentrations) and then decrease in wetter periods.
Furthermore, their long-term numbers initially increased with the increase in temperatures and later declined (due to continued increase in temperatures and nitrogen quality dilution).
The numbers of blue wildebeest specifically, have gone down more due to their critical requirement of short grass with a high nitrogen concentration.
The third grazing guild of bulk grazers tolerant of fibrous material seems to be unaffected by the temperature increase.
Buffalo and waterbuck are both water dependent and are therefore more sensitive to rainfall patterns.
Historic data indeed shows that their numbers increase during wet periods and decline sharply during droughts.
In nature, there is never a single answer.
Even though temperatures are increasing, sable and roan can still have high reproductive rates as can be seen in the Mokala National Park.
The key is that there are no predators of these antelope in Mokala.
It seems that a combination of climate change, habitat change and predation together resulted in the decrease of rare antelope in Kruger.
Sub-optimal nutrition probably resulted in lower vitality and therefore higher vulnerability to predators.
These new findings as set out above make it clear that the management of the impact of climate change on biodiversity will be one of the key areas of focus for conservationists in decades to come.
References:
Seydack, A.H., Grant, C.C., Smit, I.P., Vermeulen, W.J., Baard, J. & Zambatis, N., 2012, ‘Climate and vegetation in a semi-arid savanna: Development of a climate–vegetation response model linking plant metabolic performance to climate and the effects on forage availability for large herbivores’, Koedoe 54(1), Art. #1046, 12 pages. http:// dx.doi.org/10.4102/koedoe. v54i1.1046
Seydack, A.H., Grant, C.C., Smit, I.P., Vermeulen, W.J., Baard, J. & Zambatis, N., 2012, ‘Large herbivore population performance and climate in a South African semi-arid savanna’, Koedoe 54(1), Art. #1047, 20 pages. http://dx.doi.org/10.4102/ koedoe.v54i1.1047
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