Written By Craig Ferguson
Recently there have been a number of emotive queries about the low number of animals tourists are seeing in the Kruger National Park. The Park Management's official response seems to regard their concern as a false perception when in fact there is published evidence of dramatic declines in some herbivore numbers since 1986 when they reached an all-time high.
In 2003 a collection of papers covering the history of ecological research and management in the Kruger National Park was published under the title The Kruger Experience: Ecology and Management of Savanna Heterogeneity edited by Professors Johan du Toit, Head of the Mammal Research Institute, University of Pretoria and Kevin Rogers of the Centre for Water in the Environment, Wits University and Dr. Harry Biggs of South African National Parks.
The chapter entitled 'Rainfall Influences on Ungulate Population Dynamics' by Professor Norman Owen Smith and Joseph Ogutu both of the Centre for African Ecology, Wits University argues a very strong case for the rainfall patterns of the last few decades being the overriding factor causing some of the larger herbivore population fluctuations observed in Kruger.
Past management practices may also have contributed As far back as 1947 Stevenson-Hamilton had observed 'there are annual curves of altering abundance and want which affect herbivore and carnivore conversely'. He attributed these fluctuations to rainfall and predators. Various researchers who have worked in the park supported the idea over the years, including Smuts in 1978, Joubert 1974, Whyte 1985 Whyte and Joubert 1988 and Mills, Biggs and Whyte 1995.
The general conclusion was that the high rainfall in the years preceding the census year had a negative effect on the growth of wildebeest and zebra populations with the buffalo, waterbuck, kudu, giraffe, warthog and impala numbers increasing. The increased vegetation cover in the wetter years was thought to have made the zebra and wildebeest more vulnerable to predation especially by lions. Lions were culled at one stage in the park in order to reduce the zebra and wildebeest losses until later research showed that the lions simply replaced themselves and the populations of zebra and wildebeest continued their declines until the next dry cycle began.
To try and understand what effect rainfall has on the animal numbers in Kruger, Owen-Smith and Ogutu used the 1965-1997 aerial census data for 12 large herbivore species and compared them to variations in the annual rainfall over the same period. They were unable to use the census figures from 1998 onwards as the census method changed from a theoretical total count to a number derived from counting a small percentage of the park, making any direct comparisons statistically questionable.
Elephant numbers were not compared as they were artificially manipulated by culling during the period studied. Rhino were also excluded as their numbers were growing exponentially and were obviously not being affected by the rainfall. The accompanying table from their chapter gives the average census total and the ranges for the 13 years between 1980 and 1993. Included is a final column with the 2004 estimates (using a different census method) for simple comparison.
Owen-Smith and Ogutu's results demonstrated 'the pervasive influence of rainfall variability on the dynamics of ungulate populations in Kruger'. They showed that cyclical rainfall with a cycle of 18 years was clearly evident from the five-year running average and that the herbivore populations responded predominantly to the combined rainfall over the previous few years. The effects of the dry season rainfall (between April and September) which usually constitutes about 20 percent of the annual rainfall was shown to be critically important for all species except giraffe.
Animal deaths were concentrated at the end of the dry season when animals were in their worst condition and the availability of green forage was at a premium or non-existent. The timing of the rainfall can also affect the dependence of the animals on waterholes where the predators may lurk. The rainfall in the 1970's was substantially greater than anything recorded since 1920 and the subsequent dry phase extending through the 1990's extended well beyond the expected nine years.
There were extreme droughts in 1963/64, 1972/73, 1982/83 and 1991/92. Although there are no census figures for the high rainfall 1950's available for comparison, it would appear that by the early 1980's all the large herbivores had reached record highs. These record numbers meant that in the subsequent dry cycle which was the driest on record, especially with regard to the dry season rainfall, their declines were even greater than would otherwise have been predicted.
The zebra population increased from 14 000 in the late 1960's to peak 31 000 in the low rainfall of the late 1980's. Wildebeest went from a high of 14 000 in the late 60's down to 8 500 in the late 70's and back up to 14 600 in 1986. Giraffe increased until 1986 (4 500) and maintained their numbers. Buffalo (32 000), kudu (11 000), waterbuck (4 500), warthog (4 000), eland (900), sable (2 200), tsessebe (1 100) and roan (380) all grew towards peaks during the wet 1970's and then declined steeply after 1986. Impala were also most abundant around 1985 (220 000).
The dramatic decline in herbivore numbers recorded after 1986 came four to five years after the 1982/83 drought and coincided with the beginning of a period of low dry season rainfall (extending through 1994) as well as a higher lion population (±2000 up from ±1200) which had increased in response to the increased overall prey numbers. Predator numbers usually lag a few years behind their prey. An outbreak of anthrax in the northern section of the park was blamed for the plunge in buffalo numbers (32 000-15 000) and decline of the kudu (11 000-2 500) after 1990.
Lion predation contributed to the roan population crash (450-43) after 1986. The lions had followed the zebra which moved into previously waterless areas as a result of the water provision program throughout Kruger. Kruger Park management initiated a water provisioning program in the 1930's with the idea of supporting the herbivores (especially the rare ones) during droughts and to minimise the effect of annual rainfall variability. On completion, 325 boreholes and 50 dams had been constructed with the intention of rotational closing never being implemented. Less than 20 percent of the park was more than five kilometres from water.
The excess of water favoured the water dependent species over the independent ones and provided a focus for lions and hyenas that were now assured a regular meal. With too much artificial water the grazing areas had begun to merge and the drought reserves were depleted. The effect of predation also became more pronounced especially on smaller populations and where lion numbers were high to begin with. Both the zebra and wildebeest population increases were correlated with the borehole numbers over the last 20 years.
The program achieved its aim of lessening the effect of rainfall variability but favoured the 'wrong' species. It had exactly the opposite effect to what was intended and 184 boreholes have since been closed in selected areas especially on the northern plains. In answer to those concerned visitors to the Park, yes there has been a dramatic decline in the number of plains game since record highs in 1985.
This is reflected in the 2004 census figures, which although not directly comparable to earlier figures give an indication of what the authorities believe to be present. The declines must however be taken in the context of large herbivore population fluctuations observed over the last 40 years and are not necessarily permanent. Although Skukuza has recorded above average rainfall for three of the last four years and the 2004 dry season was unusually wet, the last few years' dry seasons have not had much rain. This means there is no likely prospect of an immediate recovery.