Method used for counting animals
An aerial survey is conducted annually in the Kruger National Park(KNP) to monitor the trends in large herbivore populations.
The large herbivores that are currently counted in this way are zebra, impala, kudu, warthog, waterbuck, white rhino, elephant bulls, giraffe and wildebeest. Certain other herbivore species such as buffalo, elephant and hippopotamus are counted in separate helicopter surveys. In the past (before 1998) the entire KNP was flown to obtain total counts for these herbivores. However, to complete this survey it took approximately 250 hours of flying time and in the last few years that this survey was undertaken the entire park could not be completed as a result of adverse weather conditions. For these reasons the census technique was changed in 1998 to a sample survey whereby the total number of animals of each species is estimated rather than being completely counted. The sample survey currently used is distance sampling.
Briefly this can be explained as follows. Equally spaced strips are flown from east to west across the entire park to obtain a 22% coverage. As the plane moves along the transect line the animals that are seen are recorded as well as their distance to the transect line. These distances are used to develop a sighting curve based on the fact that one expects to see more animals close to the line than further away. The sighting curve is then used to estimate the amount of each specie occurring in the KNP. The figure below shows how the animals are divided into distances classes when seen from above.