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semiarid savanna (Gillson and Ekblom 2009a). Therefore a TPC for rising tree abundance in
open, grassy habitats might be needed, particularly as woody elements are currently benefit-
ting from rising CO
2
at the expense of open grassy habitats (Bond and Midgley 2000, Higgins
and Scheiter 2012, Parr et al 2014).
Many of the debates about elephant culling centre around concerns over the loss of tree cover.
However, as discussed above, our perception of what elephant habitat normally looks like is
probably quite distorted, because of lack of information on savannas prior to the impacts of
the colonial era. There is a chance that habitat impacted by elephants is still returning to its
normal range of variability, and further palaeoecological work is needed to understand
savanna dynamics over long time scales, when elephant populations were likely to have been
much higher. Furthermore, rising levels of CO
2
are probably contributing to the widespread
and dramatic bush encroachment (increasing density of woody plants) that has been
observed in many rangeland habitats (Figure 2.5) (Scheiter and Higgins 2009, Wigley et al.
2010, Higgins and Scheiter 2012). As well as the loss of habitat open to grazers and other plains
adapted species, with implications for pastoralism, increased tree cover will have potentially
deleterious effects on wildlife tourism, which demands open habitat for good game viewing.
Looking to the future, rising levels of CO
2
are predicted to enhance tree recruitment, fur-
ther encroaching on grassland habitats and grazing resources, but massively increasing the
availability of browse, and thus, arguably, the capacity to accommodate elephants (Scheiter
and Higgins 2012). A modelling experiment by Scheiter and Higgins (2012) explored the
effects of increasing CO
2
on elephant carrying capacity in the Kruger National Park. The study
is based on the underlying premise that savanna trees and shrubs benefit more from rising
CO
2
than grasses, leading to increased tree cover at the expense of grassy habitats. This is
because savanna trees and shrubs utilize a different photosynthetic pathway and use CO
2
less
efficiently than grasses. Whereas savanna grasses can store and concentrate CO
2
in their
leaves, trees and grasses must use CO
2
at ambient concentrations, and are thus more respon-
sive to changing atmospheric CO
2
. The simulations suggest that the impact of elephants on
tree cover declines as CO
2
increases, because trees are more likely to recruit and therefore
recover more quickly from browsing.
The model suggests that, with current levels of CO
2
, increasing elephant numbers to 15,000
would dramatically open up Kruger's savannas and by 30,000 there would be very few trees
left. In contrast, if predicted future CO
2
emissions are used in the model, Kruger may be able
to accommodate as many as 60,000 elephants by 2100, with no appreciable effect on tree
cover (Scheiter and Higgins 2012). Of course, many other interacting factors are likely to
influence tree cover, but still the main message is that the 'carrying capacity' of the Kruger
National Park for elephants is likely to increase dramatically over the coming decades. The
work is important because it highlights some potential benefits of CO
2
increase—at least for
elephants—but it also stresses the tension between different conservation goals, not least the
trade-off between biodiversity conservation and carbon storage.
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