Size matters in African savannas

To mark the Savanna Science Network Meeting 2017 in South Africa, Deron Burkpile has written a blog about his recent study based in Kruger National Park. His paper, based on African savanna herbivores and plant richness, will also have a commentary paper about it in the next issue, so keep an eye out for that! Keep up with conference with #SSNM


Elephants, rhinos, giraffes, hippos. Along with the lions and leopards, these are the big wild beasts that are indelible in your mind after a visit to the Kruger National Park in South Africa. It’s tough to pay attention to the small antelope when large herds of elephants are breaking down trees or a huge rhino cow and calf are placidly grazing grasses. But, it turns out that these smaller herbivores, like the numerous impala, may be more important in the function of these ecosystems than we had previously anticipated.

Kruger is one of the flagship parks on the African continent. Set up in the late 1800’s by then President Paul Kruger, Kruger is now part of the Greater Limpopo Transfrontier Park that includes portions of Mozambique and Zimbabwe and is one of the largest terrestrial protected areas in the world. We went to Kruger to try to understand how different groups of herbivores, elephants vs. impala, wildebeest vs. warthog, impacted plant community dynamics different. We were also interested in a key ecological question – how does herbivory and productivity interact to determine the richness of plant communities.

Kruger_Park_Zebra

The Burkepile et al. study was based in Kruger – one of the largest National Parks in Africa (photo: https://goo.gl/jq4OJb)

To answer these questions, we used a six-year herbivore exclusion experiment that separated the effects of herbivores into larger herbivores (e.g. elephant, zebra, kudu) vs. smaller herbivores (e.g. impala, warthog, steenbok) across areas of the savanna that varied 10-fold in primary production.

We showed that the impact of losses of herbivores on plant richness will be strongly context dependent and will vary with the herbivores lost (e.g. larger vs. smaller, grazers vs. browsers), plant functional type (e.g. grasses vs. forbs), and environmental context. Similar to other studies of single species of grazers (e.g. bison, cattle, sheep), we show that the impact of herbivory on plant species richness depends strongly on habitat productivity.

However, our work adds significantly to the study of herbivory-productivity-richness relationships by addressing the complexity of these relationships using the diverse ungulate fauna of African savannas. Exclusion of smaller herbivores (e.g. impala, warthog) showed stronger effects on plant richness, particularly loss of forbs, at higher productivity rather than at lower productivity. In contrast, exclusion of larger herbivores (e.g. elephant, zebra, kudu) had stronger effects on plant richness, typically with increasing forb richness, at low rather than high productivity.

Ceratotherium_simum_Kruger_Park_03

The study faced regular disruption..due to damage caused by rhinos! (photo: https://goo.gl/cW5IAp)

Regardless of our exciting findings, doing experimental ecology around large animals can be exasperating. We had expected elephants to be the major animal causing destruction, but rhinos were, by far, the animals most intent on breaking our experiment. In fact, the whole experiment was designed to be about 40% larger than it ended up being. We lost 10 blocks of our exclosures due to persistent rhino damage at one of our core experimental sites. Despite persistent, some would say bull-headed, rebuilding of the exclosures over the first two years of the experiment, we finally threw in the towel and decided to write off that effort and abandon the site. That site also happened to be the most difficult and time-consuming to get to, so letting it go knowing all the effort that had gone into setting it up was a difficult decision. But, the rhinos would have broken our spirit had we continued.

Not only have we added some context to studying how herbivores and productivity interact to determine plant community dynamics, but we are also beginning to learn how the different groups of herbivores in this diverse herbivore community impact these ecosystems. Historically, most of the focus has been on the impact of the ‘megaherbivores’ like elephants, rhinos, and hippos. Yet, our work shows that the underappreciated mesoherbivores like impalas, which are the most numerous ungulate in Kruger, are massively important for the dynamics of this system. Size really does matter!

Deron Burkepile, University of California, Santa Barbara


See also, the Editor’s Choice blog for issue 105.1 – about another paper based on a study in Kruger National Park – Elephants trump fire in the Kruger by David Wardle.

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