Fire and herbivory in African grasslands
Joshua Thoresen
Post-doctoral research fellow
Biological Sciences, University of Cape Town
Research team: Dr Marie-Liesse Vermeire, Dr Heidi-Jayne Hawkins, Professor Michael Cramer, Professor Jennifer Adams Krumins
Institutional affiliations: University of Cape Town and Conservation South Africa
The grasslands of Drakensburg. Photo by: J. Thoresen
The Drakensberg mountains are one of those places that feel different, outside of the world somehow, a place apart. Every afternoon in summer thunderstorms sweep through, lightening splits the sky, thunder echoes off the escarpment rolling on for longer than seems possible. Alpine grasslands progress to mountains so precipitous entire cliffsides can fall away. The constant rains leach nutrients from the soil, despite the wealth of vegetation there are few herbivores. Those that do persist, do so on unpalatable grasses. Wildlife exists in hints and moments, the recent footprints of a massive eland in the mud, a jackal crossing a path stops and stares before disappearing into the long grass. Baboons roam the mountains and the campsites, uncaring of where they scavenge their food: the roots of a shrub or the pantry of an unwary tourist. The rarity of wildlife makes a sighting all the more special, it also means there is little pressure against the growth of the grasses. Fire then takes its place. Naturally (and anthropogenically) burning every three years or so, it is a dominant force shaping the ecology of these grasslands.
A curious black-backed jackal stops and stares before disappearing again. Photo by: J. Thoresen
This wilderness is where one of our field sites is hidden away, atop a plateau 1,800 meters (5,900 feet) above sea level. A large fence excludes the rare herbivores from an experimental plot where patches of grassland have been burnt at different intervals for 30 years. The difference between these plots can be stark; regular or less frequent burns create a patchwork quilt of the grassland visible even on google earth. We spent a week here in January taking soil samples from burn plots within the exclosure to compare with the less controlled wilderness outside, where the ephemeral influence of herbivores may show up in our data. We are actively processing these samples and are excited to share results in the near future.
Humans have altered natural fire regimes over the last 60 thousand years and Drakensburg is no exception. Fire is used as a management tool here and elsewhere in Africa (and the world for that matter). Fire was used by hunter-gatherers to flush out prey from dense thickets and as we progressed to more agrarian lifestyles, fire was used to clear land for farming. The history of humanity is a history aflame which continues to this day. In some areas (like Drakensberg) this burning is perhaps necessary, in others it may not be. Farmlands all around the world are currently succumbing to a problem known as bush encroachment, where forest is trying to reclaim the land. Periodic burning is one way of preventing this, but it may not be the only way.
Our research will provide insights into these issues. Our overall objective is to evaluate and compare how fire and herbivory influence soil carbon stocks, nutrient cycling, plant successional trajectories and soil food webs (microbes and invertebrates) in the savannas and grasslands of South Africa.
The differences between burning treatments can be stark. The grassland on the left is burnt annually while on the right it only burns in a natural cycle, every three years or so. Photo by: J. Thoresen