The increasing of elephant populations is a major concern to wildlife managers and ecologists in many protected areas around Africa.
By: Fred de Boer and Edward Kohi
The increasing of elephant populations is a major concern to wildlife managers and ecologists in many protected areas around Africa. There is a perceived threat that the continuous growth of elephant populations could have a detrimental effect to other species in the system.
Elephants are bulk feeders, so they need a lot of forage, and sometimes push over, uproot, or ring bark trees. Especially bulls are known to fell trees. Such feeding habits could have negative impacts on the vegetation composition and structure. Indeed, there are several known cases where an increase of elephant density has been linked to the decrease of certain species. Large trees could locally disappear and patches of woodland might be lost if elephant populations continue to grow.
For instance, in several studies baobab density is known to decrease with increasing elephant densities, and in Addo National Park, mistletoes and aloes are known to decline in numbers as elephants increase.
It is often found that some plant species are preferred whereas other species are not consumed at all. The difference in preference for certain species can often be explained by a different quality of the leaves, such as through differences in the nitrogen content of the leaves. However, some species also have so-called secondary compounds in their leaves, which are a sort of toxic chemicals aimed at deterring animals from consuming them. These are compounds like tannins, which are also found in black tea leaves, responsible for the relatively bitter taste of some tea. The plants produce these compounds to prevent herbivores from eating plant parts.
Other plant species, such as mopane and some acacia species seem to be more tolerant of elephant impact, depending on their regeneration potential and their recruitment. Even though elephants can hammer these species and decrease the maximum height, some of these tree species are remarkable resistant and can sustain this impact well. Some scientists even refer to "hedging" when discussing the impact of elephant on mopane, as elephants continuously come back to mopane patches and browse the trees to a certain height and large branches are broken off from the main stems.
It is only recently that some scientists realised that these elephant impacts could also have positive effects for some species. In order to understand the impact elephants have on the system, a large study, known as the Tembo project, is underway in the Associated Private Nature Reserves (APNR) and the Kruger National Park to investigate this in more detail. Edward Kohi and Fred the Boer are two of the scientists involved in the project.
Elephants Change the Structure of Trees
According to our first analysis of fieldwork results we find that elephants indeed change the structure of trees. Tree height is reduced under elephant browsing, as branches are removed, and some trees are pushed over or partly uprooted.
However most of these trees continue to grow and produce new leaves, albeit at a lower heights. Hence the forage availability is affected as more leave material becomes available at lower heights that can now be used by elephants and other smaller browsing species like steenbok, impala or kudu.
It is not only the forage height that is affected, but also the number of leaves, the regrowth and the quality of the leaves. Mopane, but also other species, are known to resprout rapidly after browsing. In the wet season for instance, the defoliated leaves can sometimes be replaced in two weeks time. The new leaves are often of a better quality than the previously browsed leaves. The nitrogen and phosphorous contents of these new leaves are often higher. These are two nutrients preferred by herbivores. The concentrations of secondary compounds in these leaves are also often lower. So herbivores could indeed benefit from elephant impact, as the forage availability at lower heights and the forage quality is improved.
But do they? In an experiment we tested several of these predictions, such as whether the nutrient content increases after defoliation. Indeed this seems to be the case, forage quality increased, especially at lower browsing heights. We also experimentally simulated the impact of elephants, by creating several plots in which we cut back the mopane trees extensively to encourage denser growth, other plots where we pushed over trees and yet other plots were we totally removed all trees.
We were interested whether other herbivores species like the smaller steenbok and duikers are attracted to these patches in comparison with intact control plots. We measured the visits of the other species by dung counts and spoor counts. Preliminary analyses showed that some herbivores species indeed seem to benefit from the better forage availability at lower levels. Impala, Steenbok, and even Kudu reacted strongly and were relatively more abundant on plots which simulated elephant impacts. Even elephants were more frequently recorded in the plots where the branches were cut back compared to the control plots.
This is a phenomenon that has also been observed elsewhere. Elephants seem to return to browsed patches and apparently benefit from the different vegetation structure and higher forage quality at these plots. Habitat selection for most herbivores is mainly determined by forage quality and availability, but the risk from predators also plays an important role. Hence, on the clear cuts where we removed all trees, some of the smaller species were more frequently recorded, especially impala.
Impala are known to prefer feeding on the transition zone from woodlands to grassland or open areas. Maybe impala benefits from the clear cuts that elephants can create, facilitating the detection of predators? The impact of elephants on other herbivores species yields a complicated picture. Some species certainly seem to benefit, either by more forage provided at lower feeding heights, or forage of a higher quality for other species.
Elephants are also able to create clear cut areas where other species can then easier detect predators. The question whether populations of browsing ungulates also grow under an increasing elephant population remains to be answered though. However, that elephants only have negative impacts on the savanna systems by removing large trees is a one-sided vision and we are able to prove that the reality is far more complicated.
Elephants have a huge impact on the vegetation and thereby on other species that use this vegetation and we are just at the beginning of unravelling these patterns and processes to understand the role that elephants play as one of the species in these large savanna ecosystems.