With the exception of wildebeest and possibly impala, it looks like the numbers of most of the herbivores in the Kruger National Park (KNP) are on the increase. This has been revealed by the annual aerial census of Kruger, conducted from an aeroplane.
Each year, Kruger carries out two censuses – the fixed wing aerial census of large herbivores, and the helicopter count of elephant, buffalo and hippo. The census of all the large herbivores does not attempt to count animals over the entire area of Kruger, as occurred prior to 1998, but rather uses a sampling method known as distance sampling.
The park changed from the total count method to the sampling method in 1998, partly because an enormous number of flying- and man-hours are needed to carry out a total count, but also because weather conditions made it impossible to complete the total count for several years prior to 1998. The distance sampling method works by dividing the park into equally spaced strips. In each strip is a transect line, which the plane flies along.
People in the plane then look out the windows and record all the animals that they see, as well as their distance from the transect line the plane is flying along. Logically, the observers will see more of the animals closer to the plane than those further away, and so a sighting curve is used to allow for this.
Based on the sighting curve, the numbers of each species counted are estimated for the whole of the park. Judith Kruger, data analyst and programme integrator for science support in Kruger, explains, "Initially a 22 percent coverage was used for the entire park but because the north of the park has lower [game] densities, statistics showed that this area should be increased to a 28 percent coverage.
For the last three years the area north of the Olifants River has been flown using this increased coverage of 28 percent and south of the Olifants River has remained at the original 22 percent coverage." With the shift from total count to distance sampling, Kruger management has been fine-tuning the system with respect to the ways in which the observers are trained to count, and also in the statistical analysis of the data.
Judith comments, "We are now comfortable that this method is working well and giving us reliable estimates." Because not all of the animals in the park are not counted, the total number of animals in Kruger is obviously extrapolated from the percentage that is counted. However, scientifically and statistically speaking, because this number is an estimate, the scientists have to calculate what are known as upper and lower confidence limits.
Scientists then basically say that they are 95 percent sure (or confident) that the real number of animals on the ground in Kruger is somewhere between the upper confidence limit and the lower confidence limit, but is most likely to be the number published as the estimate.
For each game species these confidence levels are slightly different, for reasons such as how widespread the animals are in the park and how easy they are to spot and then count from the aeroplane. For this reason, while there is a clear decrease in wildebeest numbers, there is a small amount of uncertainty as to whether impala numbers have really decreased (from 100,947 impala in 2005 to 98,003 this year) on the ground, or if the apparent decrease is just because of the sampling method. Next year's census will shed more light onto this.