EXPEDITION REPORT - Biosphere Expeditions

2.5.4. Discussion
Ecological field studies rely on the ability to make reliable estimates of animal abundance,
but any estimate of abundance is influenced by sources of error associated with extrinsic
(environmental) or intrinsic (methodological) factors. Consideration of these potential
influences is very important for both study design and data analysis. The development of
effective methods to monitor abundance of large carnivores presents particular challenges,
as they are often secretive and widely dispersed.
The data gathered does not allow for an accurate estimate of true population densities for
any of the three large carnivore species at Okomitundu. On the one hand capture success
was low. On the other hand, radio-collared animals, male leopards in particular, did not
provide sufficient data due to being shot shortly after they were collared. Assuming that the
spoor density of large carnivores shows a strong linear correlation with true density
(Stander 1998), the above results at least reveal that cheetah density within the
Okomitundu study site is much lower than it was in the Omitara and Seeis study site. At
the same time leopard and hyaena densities are higher at Okomitundu than in the
previous study area.
It must also be noted that the four transects used for counting spoor frequencies were not
truly random. Due to the heterogeneous habitat they were selected to cover different
vegetation and geological types and to avoid very stony roads due to a low detectability of
carnivore tracks on rocky soils. We therefore increased the number of transects and the
total length of transects sampled throughout the year in order to improve data precision.
Looking at the total numbers of spoors detected during the expeditions, it is clear that the
ratio between cheetah spoors and leopard spoors changed markedly from 2005 to 2006.
The proportion of cheetah spoors to the total numbers of large carnivore spoors decreased
from 40.9% in 2005 to 28.3% in 2006, whilst the proportion of leopard spoors increased
from 35.2% in 2005 to 45.6% in 2006. With 24.6% cheetah spoors and 47.2% leopard
spoors this ratio was found again during the expedition in 2007. One reason may be that
the cheetah tries to avoid interspecific competition with the leopard and therefore switches
to areas with lower leopard density. Secondly, cheetahs may be more vulnerable to
changing habitat conditions such as drought, which result in limited resources, especially
food. Generally, the leopard and the brown hyaena are better adapted to poor habitat
conditions than the cheetah (Estes 1997). Usually brown hyaenas live in groups, which
occupy fixed territories, but they forage alone over long distances. Existence of a hyaena
den at Okomitundu study site explains why the proportion of hyaena spoors was almost
the same during all three expeditions (23.9% in 2005, 26.1% in 2006 and 28.2% in 2007).
Continued male leopard removal through trophy hunting around Okomitundu makes it very
difficult to assess the leopard density within the study site. Each time one leopard is killed
or dies of natural causes, its territory becomes available to other males. Usually more than
one male leopard would migrate to the area and try to take over this territory (Estes 1997,
Skinner & Smithers 1990, Marker et al. 1996, personal observation). This may lead to a
higher leopard density in the short term and possibly an overestimation in the long term.
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