How do grazing herbivores like hippos affect the influence of fire? Following a recently published article, Izak Smit demonstrates the need for interdependence between herbivore and fire management.

Grazing animals and fire are in direct competition – both of them consume grass. Previous continental-scale studies suggest that grazing animals have the competitive advantage in drier and more nutritious landscapes, whilst fires dominate in wetter and nutrient-poor landscapes. Areas with intermediate rainfall (500-800 mm per annum) are particularly interesting, because it seems as if either grazers or fire can get the competitive edge. There are very few studies in natural systems and with native ungulates to show if a system may switch from a fire to a herbivore driven system (or vice versa) in this rainfall interval. One of the best examples is the Serengeti where about 70-80% of the area used to burn annually after the blue wildebeest population was decimated by rinderpest, but since the wildebeest numbers recovered, annual fire extent has dropped to about 40% (Holdo et al., 2009; Archibald and Hempson, 2016). A central question for practitioners and academics alike is whether management actions can switch a system from a fire to herbivore-driven system? This question is of particular interest as we contemplate the impacts and aftershock of the extinction of megaherbivores and as the  concept of rewilding gains momentum.

Fig 1
Fig. 1: Grazers “switching off” fires: Grazers keep grass too short to burn in a heavily grazed area next to a river in Kruger National Park (Photo: Izak Smit)

The 20 000 km2 Kruger National Park, South Africa, used to cull large herbivores (hippo, buffalo and elephants) to keep their numbers within predetermined intervals. However, since the mid-1990s culling was discontinued and, as a consequence the biomass of these animals has more than doubled (Fig. 2). Using long-term data on fire occurrence in the park (i.e. fire scar maps), as well as census data of these large herbivore species between 1969 and 2015, it was possible to answer the question posed above.

Fig 2
Fig. 2: Combined biomass of buffalo, hippo and elephant in the KNP. Note the increase in biomass since the mid- 1990s when culling was discontinued.

We found that, as herbivore biomass increased after culling was discontinued, fires decreased, especially close to rivers (Fig. 3). This is most likely due to the direct competitive effects of herbivores consuming the grass, leaving less grass biomass and shorter grass; hence reducing fire occurrence (see Fig. 1). We expect fire intensity will also decrease with more herbivores, but we did not have data to test this.

Fig 3
Fig. 3: Average annual proportion of area burnt in KNP during culling (1969-1994) and post-culling (1995-2015) eras at different distances from perennial rivers (bars and left y-axis), as well as percentage reduction in average annual area burnt comparing high herbivore density years with low herbivore density years (line graph and right y-axis). Note how fire occurrence has reduced, especially close to rivers, since culling ceased.

However, the effect of increasing grazer population was not similar everywhere. Grazers reduced fires more in areas close to rivers as opposed to areas further from rivers (Figs. 3-4). This is likely because hippo, buffalo and elephant are water-dependent grazers and spend considerable time foraging close to rivers. We also discovered that grazers had a stronger effect in drier than in wetter landscapes (compare Figs. 4a and 4b). The most likely reason for this is that drier landscapes are less productive and hence grazers can have a large effect in grass biomass (and thus outcompete fire), whereas in more productive higher rainfall landscapes, grazers cannot ‘keep up’ with the grass growth leaving more grass for fires to consume. As such, the results suggest that grazing gradients, like distance to water, can result in herbivore and fire driven areas co-occurring within the same open system. For example, on average less than 1% of arid landscapes within 5km from rivers burnt annually in the presence of high herbivore biomass (predominantly herbivore-driven system), as opposed to 25% annual area burnt in higher rainfall landscapes further than 5km from rivers (predominantly fire-driven system), all within the same protected area.

Fig 4
Fig. 4: Average annual percentage area burnt for years with lower grazer biomass versus years with higher grazer biomass (bars and left y-axis) at increasing distances from rivers across a rainfall gradient, as well as percentage reduction in average annual area burnt comparing high grazer density years with low density years (line graph and right y-axis). Years with high grazer biomass burn less than the same areas during years with lower grazer biomass. Furthermore, arid landscapes and areas closer to rivers burn less than mesic landscapes and areas further from rivers.
Fig 5
Fig. 5: Fire burning in (left) landscape with few herbivores (adequate fuel for fire) and (right) similar landscape but heavily grazed (less fuel – this fire went out shortly after taking the photo) (Photos: Sally Archibald)

Previous studies have suggested that since elephants push trees over, they facilitate more grass growth and hence can increase fire occurrence (i.e. through reducing the competitive effect of trees on grass). However, our study showed that the net effect of elephants on fire was negative in KNP, most likely due to the fact that elephants are mixed feeders that can consume large amounts of grass when good quality grass is available (Fig. 6).

Fig 6
Fig. 6: In KNP elephant seem to be a net suppressor of fire (through consuming grass) rather than a net facilitator (through removing trees that compete with grass) (Photos: Izak Smit)

 

Our results reiterate how important it is to take a ‘systems’ view in conservation, with many processes interconnected – when you manage one process (e.g. herbivores), you may also affect another process (e.g. fires). In conclusion, within the intermediate rainfall range (500-800mm per annum), management can influence the competitive effects between herbivores and fires. Furthermore, we propose that grazing gradients – in this case distance to closest perennial river – are critical as it allows both fire and herbivore-dominated systems to occur within the same system along the gradient. This increases management possibilities, as management actions that induce grazing gradients can have positive implications for maintaining biodiversity in protected areas.

Fig 7
Fig. 7: Herbivores influence grass structure both at local scales (e.g. grazing “halo” around termite mound, as shown in photo) as well as larger scales (e.g. gradients radiating away from rivers, as illustrated in this study) (Photo: Izak Smit)

Read the full article, Herbivore culling influences spatio‐temporal patterns of fire in a semiarid savanna in Journal of Applied Ecology.