Reintroducing large carnivores to new habitats in Namibia, and hence
expanding their ranges, is one of our primary functions and important for their
long-term conservation. During 2002 three lions and two cheetahs were released
on the 26,000 hectare Kalahari Game Lodge (KGL), where free-ranging large
carnivores were resident more than a century ago. We faced many unknown factors
and risks during the planning phase of the introduction.
One of the more important concerns was whether there were sufficient prey
animals to support the lions and cheetahs. If there were too few prey animals
the predation pressure from three lions and two cheetahs in a confined area
might cause the prey populations to decline. We were particularly concerned
about the cheetahs and their impact on some antelope species. Cheetahs are
highly effective predators and in small, confined areas they are known to eat
themselves out of house and home. In areas where springboks are abundant, such
as at KGL, they form the main prey for cheetahs. Based on an estimated 4000 –
5000 springboks at KGL, we developed a mathematical model to calculate
predator/prey ratios, and to predict the predation impact on the springbok
population. The results suggested that two cheetahs will not have a negative
impact of the springboks, and we decided to go ahead with the introduction.
A few months after the cheetahs were released the KGL manager, Hennie Moller,
made a remarkable discovery. The cheetahs were not selecting many springboks,
instead, they killed the smaller steenbok and duiker. More than 60% of their
kills were of these two species. This discovery, exciting as it was, highlighted
the concern that our predator/prey model was obsolete. In fact, we had no
information on the population size of steenbok and duikers, and could not
predict the possible impact of cheetah predation. Obtaining reliable estimates
of steenbok and duiker numbers, therefore, became a matter of urgency.
Small antelope species, like steenbok and duikers, are difficult to count.
The traditional aerial survey approach, used for most larger antelopes,
unfortunately does not work for these smaller species. As an alternative we
decided to attempt a ground count of steenbok and duikers that relied on
“distance sampling” techniques along transect lines.
After much anxiety and head-scratching over the complicated statistics of
“distance sampling” we arrived at KGL late one Thursday afternoon in March.
Early the next morning we divided all the eager hands into three survey teams.
Each team, equipped with a vehicle, GPS, and data forms, consisted of a driver,
a data recorder, and two San (Bushman) trackers. This was an innovative approach
of combining age-old tracking skills of the San, with modern Global Information
System (GIS) technology, to survey steenbok and duiker populations. After a
brief training session and a practise run, the three teams set off in different
directions with much excitement.
The procedure was simple. Each survey team was allocated a number of transect
lines to complete. The vehicles drove slowly along the transect lines with the
San trackers scanning and searching for signs the quarry (see diagram).
Upon spotting (S) a steenbok or duiker, the is vehicle stopped, and the San
trackers walk to the area where they had first seen the antelope, searching for
fresh tracks. Once found (T), the trackers “back-track” the antelope’s spoor
(following the trail in the direction from where it came), reconstructing its’
behaviour, and finding the exact spot (X) where the animal stood/lay when the
vehicle approached. The coordinates of this position is recorded, using Global
Positioning System, and the perpendicular distance from this point to the
transect line (y) is measured and recorded.
For two days the survey teams searched, counted and measured, covering the
length and width of the 26,000 hectares. Immediately after completing the last
transect we entered the data on our laptop, and before the dust had settled, we
started the daunting task of data analyses. There were insufficient data for
duikers and they had to be excluded. Surprisingly, the analyses ran smoothly,
and Billy’s software only crashed twice. There was much excitement when the
results of the survey finally appeared.
We drove a total 243 kilometres along transect lines and encountered 46
steenboks. The average distance of steenboks from the transect line was 60.9
metres, ranging from 1 to 218 metres. The final “distance sampling” analysis
indicated that the steenbok population estimate for Kalahari Game Lodge is 375,
at a density of one steenbok on 70 hectares.
© Predator Conservation Trust.
