To answer this question, we collected data in the famous Kenya Long-term Exclosure Experiment (KLEE). The KLEE was established in 1995 in a Kenyan savanna where cattle co-occur with wild herbivores. Using a set of exclosures, the KLEE separately excludes cattle, wild large herbivores in general, and elephants (and giraffes) specifically (Figure 1). The overall project has thus far generated over 120 peer-reviewed publications, making the KLEE the most scientifically productive field experiment ever carried out in Africa.
Figure 1. Exclosures in the KLEE use electric fencing to manipulate the presence and absence of three types of mammalian herbivores: cattle, wild mesoherbivores (15-1,000 kg, such as zebra and gazelles) and wild megaherbivores (>1,000 kg, elephants and giraffes). The following treatments are present: (1) no large herbivores, (2) wild mesoherbivores only, (3) wild mega- and mesoherbivores, (4) cattle only, (5) wild mesoherbivores and cattle, and (6) wild mega- and mesoherbivores and cattle. Pictures by T. Young and D. Kimuyu.
For me personally, it was a joy to return to Kenya (as I spent part of my childhood there) and conduct research in my favorite ecosystem, after having worked in the Arctic tundra two years prior. Mpala Research Centre where the KLEE is established is an amazing hub for savanna research. It is located smack in the middle of a Kenyan savanna providing a ‘living laboratory’ that allows researchers to manipulate the environment and conduct landscape-level, controlled experiments. I visited Mpala twice during my postdoctoral research at the Vrije Universiteit Brussel in Belgium and sampled soils and vegetation in the KLEE (Figure 2), in collaboration with Prof. Truman Young and Dr. Duncan Kimuyu.
Figure 2. My field assistant Buas taking soil and vegetation samples in the KLEE plots. Pictures by J. Sitters.
I had a great time during my research stays at Mpala. The staff and other visiting scientists were very welcoming and eager to help out. Driving to the KLEE plots in an old Land Rover while enjoying the magnificent views, stopping for a herd of elephants on the way, making a detour to enjoy the setting sun; it makes me nostalgic just typing about it (Figure 3). I did get slightly electrocuted several times by the KLEE fences and you can imagine the shock of a fence that needs to keep out elephants... One of the funniest scenes I remember: an elephant limbo-dancing under the fence that was supposed to keep him out (these creatures are surprisingly limber!). This then quickly turned into quite a frustration as we had to drive the elephant back out (and all of a sudden he wasn’t a contortionist anymore).
Figure 3. The beauty of the savanna at Mpala Research Centre in Kenya. Pictures by J. Sitters.
Our data from the KLEE plots indicate that where cattle graze, the soil becomes poorer because they do not sufficiently re-fertilise the soil where they graze. Cattle mainly deposit their dung at night when they are kept fenced-in to protect them from predators. However, when elephants are present, this soil depletion does not occur; the soil is even enriched. Elephants bring down trees in the savanna, which accelerates the return of nutrients to the soil. But more importantly, elephants slow down cattle grazing because they compete for the same available food. This also means that cows export less dung away from the place where they graze, while dung deposition of wild herbivores is stimulated, which in turn feeds the soil (Figure 4). Our results therefore suggest that a mix of cattle and wild herbivores can be sustainable, provided that the assemblage of wild herbivores includes the largest species such as elephants.
Figure 4. Illustration of the possible non-exclusive pathways by which elephants increase soil carbon and nutrient pools in savanna. By J. Sitters.
Special thanks to Dr. Dino Martins, Executive Director of the Mpala Research Centre, for allowing us to use his picture of the cattle and elephants for our poster image.