What will happen if we lose megaherbivores: Africa’s massive lawnmowers?

Posted on August 25, 2016 in Comment by . Share this article

Emma Jardine an ecologist and botanist currently completing her PhD with our Associate Director, Professor Colin Osborne. In this blog post, she looks at how hippos, wildebeest and white rhino help maintain grasslands in their habitats, and gives her view on what could happen without them.

You can read more about Emma’s work at grassissometimesgreenerblog.wordpress.com.

white_rhino_uganda_15222537106The word lawn conjures up images of highly artificial and neatly manicured areas of grass in the grounds of a house or park. However, lawns also exist in natural ecosystems with grazing mammals providing the fertilizer and acting as the lawn mower. Grazing lawns (areas dominated by short, palatable, prostrate grass species) are observed across African savannas. Frequent grazing and high nutrient input from animal dung releases short grasses from competition for light and nutrients where they would otherwise be replaced by taller bunch grasses.

Hippos, large herds of wildebeest and also the white rhino, have all been implicated in the formation and maintenance of grazing lawns but the white rhino is thought to exert a particularly large effect on the structure of savanna grasslands. Rhinos (often referred to as megaherbivores) consume enormous quantities of grassy vegetation including tall, low quality forage grasses that mammals of smaller body size cannot subsist on. The subsequent establishment of shorter, more nutritious grass species allows for the recruitment of smaller bodied, short grass specialist mammals into areas, which then maintains the lawns created by rhino. Significantly shorter grasses have been observed in areas grazed by rhino and 20 times more grazing lawn patches in areas where rhino density is high. Such is the impact of these megaherbivores on savanna heterogeneity they have been described as keystone species and ecosystem engineers.

During the first four months of 2016, 13 white rhino have been illegally poached in the Eastern Cape of South Africa. These animals were killed for their horns, which are used as medicine in parts of Asia. Over the past 25 years rhino poaching has increased across all of Africa, with the first half of this year seeing 363 rhinos being poached in South Africa alone. White rhinos are now classified as near threatened by the IUCN with just an estimated 20,000 animals left in the wild. Their possible extinction is a tragedy in terms of species loss but may also have cascading effects for the function of the ecosystems from which they are being removed.

The removal of just 4% of the rhino from around wallows can result in a significant increase in grass sward height. Rhino removal in mesic savannah can also lead to less use of short grass patches by smaller animals like zebra, impala and wildebeest. The removal of rhinos from savanna ecosystems may also alter other processes determining ecosystem assembly such as fire area and intensity. Fire areas are shown to increase following rhino removal probably because their grazing reduces the fuel load for fires. Also the short grass patches characteristic of grazing lawns act as natural firebreaks, limiting the size and spread of wild fire.

The possible extinction of these charismatic and majestic megaherbivores has implications and their loss could permanently alter the diversity of savanna ecosystems as we know them. The paleo-ecological record evidences large changes to plant communities following the late quaternary megafaunal extinctions. Africa is the only continent with extant megaherbivores and their conservation is vital to maintaining savanna ecosystem structure and function.

(originally published on grassissometimesgreenerblog.wordpress.com)