Tree cover and biomass increase in a southern African savanna despite growing elephant population
Kalwij, J., de Boer, W.F., Mucina, L., Prins, H.H.T., Skarpe, C. and Winterbach, C.
Ecological Applications 20: 222-233. 2010
Elephants are similar to moose in that they damage trees. Apart from that the similarities are probably
limited. Northern Botswana, together with adjacent areas of Namibia, Zambia and Zimbabwe has the
world’s largest elephant population, well above 100 000 animals. In the Botswana part of the
distribution range Chobe River forms the only natural source of permanent water, and many elephants
gather around the river in the dry season from about April to September. This leads to a strong impact
by elephants on the vegetation and the whole ecosystem in the region, mainly within the Chobe
National Park, including a decrease in tree density close to the river. Rightly or wrongly this is considered
a problem by many naturalists and managers. Like in many other places with high elephant densities,
artificial permanent waterholes have been established far away from the river (more than 50 km) in
order to reduce the impact close to the river and spread out the elephants. Critics claim that the
provision of more permanent water would rather lead to a further increase in the elephant population
and to heavy impact in more areas.
In any case, clusters of artificial water points have become established at two places in Chobe National
Park , at Savuti, where Evenstad students have been involved in the evaluation of the effects (analysis
not completed), and at Nugadsaa where a South African team lead by Jesse Kalwij have studied the
effects on woody vegetation, and I have been involved in this study as well. At Nugadsaa both
permanent artificial water holes and natural pans holding water only in the rainy season have been
studied in plots located at different distances, 100 m, 500 m, 1000 m, 2000 m and 5000 m from the
water hole or pan. At the artificial waterholes we used the same plots as had been used in two previous
studies in 1997 and in 2000. In the plots we recorded, among other things, tree density, canopy cover
and canopy volume for large trees (taller than 3 m) and small trees (0.2 m – 3.0 m).
We had expected to find a general decrease in tree density, cover and volume over time at the artificial
water points. We further predicted that the effect should be strongest close to the waterholes where
browsing and other impact by animals would be most intense. An initial decrease was also discovered
between 1997 and 2000, but the overall result comparing the situation in 1997 and that in 2008 showed
a pronounced increase in tree density, canopy cover and canopy volume both at the artificial waterholes
and at the natural pans. Time was a stronger explanatory factor for the change than either distance
from the water or the difference between natural pans and artificial waterholes. The increase was
attributed to the small trees, less than 3 m, whereas density and size of trees taller than 3 m had
decreased.
The lack of response to distance from water or to the difference between natural pans and artificial
waterholes suggest that the change is unrelated to the provision of permanent water and the yearround presence by elephants and other water dependent animals. We have no good explanation to the
increase in woody growth over time. A sequence of wet year in the beginning of the 2000’s could have
contributed to establishment and growth of the small trees, and also have promoted hot fires,
potentially killing the big trees above ground, causing them to resprout as small trees or shrubs.
Increasing CO2 concentrations in the atmosphere has been suggested as a contributing reason for
increase in woody growth elsewhere, but the change we observed seems too fast for that. We can also
not exclude effects by the now permanent presence of elephants in the area. Elephants use large areas,
often doing most browsing away from areas where other game concentrate, hence responding little to
the distance from waterholes unless there are licks or extra high quality food to attract them in addition
to the water. The studied area was in fact within the range, a bit more than 10 km, of elephant bulls.
Elephants might cause a reduction in large trees but promote resprouting and establishment of small
trees and shrubs as long as there are low densities of smaller browsers, such as impala, feeding on the
seedlings and regrowth.