Written by Jason Sircely, 2014-2015 Sustainability Leadership Fellow and Postdoctoral Fellow, International Livestock Research Institute and Natural Resource Ecology Laboratory, CSU (currently Ecosystem Ecologist, International Livestock Research Institute and Natural Resource Ecology Laboratory, CSU).
Governments and non-governmental organizations (NGOs) seeking to improve the well-being of herders in the arid and semi-arid rangelands of East Africa and the Greater Horn of Africa often receive contradictory recommendations on how to address land degradation through changing grazing management.
Herders in the region face difficult challenges from frequent droughts, population pressure, conflict over land, livestock disease, and restricted pasture access. In some areas, overgrazing results in land degradation, which when severe compromises pasture productivity and can cause massive soil erosion (Figure 1). Not only are these problems of deathly seriousness to herders in the region, but poverty, conflict, and food insecurity in these drylands cripple development and threaten the stability of regional governments, triggering millions in international support, each and every year. Considering this long list of problems, how highly should changing grazing management to prevent or reverse rangeland degradation be prioritized in general, and where should degradation be a top priority?
Over the previous century, most observers considered overgrazing to be nearly universal, along with its consequences in terms of degradation (soil erosion, shrub invasions, reduced pasture quality).1 Progress in rangeland ecology in the 1980s and 1990s led to a new paradigm which held that the role of grazing in degradation is strongly affected by climatic conditions;2 this work continues to be confirmed by recent research.3
First, degradation is usually much more severe in more productive rangelands, such as the Borana Plateau in southern Ethiopia (Figure 1). In savannas receiving moderate rainfall (e.g., ~600 mm yr-1 in Borana) herders can maintain a high density of cattle, over-taxing grasses and compacting soils, especially if the land is not periodically rested from grazing. When the growth of grasses is reduced, woody shrubs can gain a foothold and eventually dominate the area. Since cattle generally need grass, invasion of shrubs inedible to cows threatens the production of milk, the main food for most pastoralists and a critical source of income and protein-rich nutrition. On soils prone to erosion, bare soil beneath shrubs is vulnerable to sometimes shocking losses of topsoil (Figure 1) and sedimentation of waterways. The switch from a more grassy to a more woody savanna is difficult (but not impossible) to reverse, through bush clearing, prescribed burning, and herding livestock that prefer to browse woody plants, namely goats and camels.
Yet in other rangelands, even heavy grazing can have little effect. For example, in the drier savannas of Turkana County in northern Kenya (~400 mm yr-1), not far from Borana, livestock do not appreciably affect the growth or condition of rangeland vegetation.2 In addition, rainfall is more variable in drier areas, so much so that dry rangelands can change in random and chaotic ways, depending on how much rain falls, where, on which day. Herders must move their livestock frequently, the animals spend little time in any given place, and the land is rested until the next flush of green growth sometime in the future. For all of these reasons, grazing often has little impact over and above rainfall in dry rangelands, meaning that grazing is unlikely to cause degradation.
Rangeland ecologists describe the difference between the wetter (Borana) and drier savannas (Turkana) in terms of their system dynamics.2 In wetter rangelands, termed ‘equilibrium’ systems, grazing (and other management) can strongly affect the condition of the vegetation, and therefore also the soil. In contrast, since rainfall is the primary control over the condition of drier rangelands, these are termed ‘disequilibrium’ systems. The significance of the difference is that wetter, more productive, equilibrium systems are generally more sensitive to grazing-induced degradation, while grazing does not often cause degradation in drier, less productive, disequilibrium systems.3
Certainly, though, dry rangelands can become degraded by overgrazing. Whether or not degradation occurs depends on many factors, one of the most critical being the local systems and rules for organizing who grazes which animals where, at what time of year, and for how long. Site-specific conditions (soils, plant species, location) can also be significant. Most ecologists agree that the truth lies somewhere in between the two extremes of herders always causing degradation, and herders never causing degradation.4 Both under- and over-estimating the role of grazing in degradation will likely confound effective rangeland management.
Should governments and NGOs therefore prioritize degradation more highly in rangelands with higher rainfall? Absolutely, at least in general. These productive lands are not only more ecologically vulnerable to degradation, they are often more densely populated, further exacerbating the risk of degradation. Meanwhile, attempts to change grazing practices in drier rangelands to improve the condition of vegetation and soils can generally be expected to accomplish little. Moreover, the financial resources used could be devoted to addressing more relevant and pressing issues.
However, all wetter savannas and all drier savannas do not respond identically to grazing and other management,3 and many of the problems in rangelands can be traced to other factors.5 Shrub encroachment, in particular, does not purely result from grazing, but is also accelerated by fire suppression, global increases in temperature and the concentration of CO2 in the atmosphere, and decreasing populations of shrub-browsing wildlife, all of which shift the competitive balance in favor of shrubs over grasses. The relative strength of each of these factors in driving rangeland degradation remains a matter of controversial debate.
In response, rangeland ecologists and their collaborators in the social sciences and conservation are building networks to monitor how rangelands are changing, where they are improving versus degrading, and assessing the mechanisms driving these changes. Some of the key efforts globally include GEOGLAM RAPP, the Global Agenda for Sustainable Livestock, and those of our Livestock Systems and Environment team at the International Livestock Research Institute (ILRI) and colleagues other centers of the Consultative Group on International Agricultural Research (CGIAR). These efforts benefit from integrating a variety of methodologies including field-based measures, herder and community interviews, mathematical modelling, airborne sensors, and satellite-based remote sensing to drive rangeland science toward reliable, actionable information on the extent and severity of degradation in rangelands, and where degradation can and should be actively combated through supporting herders to reduce the impact of grazing on vulnerable rangelands.
Figure 1. Wetter, ‘equilibrium’, savanna in Borana Zone, southern Ethiopia. Left: in good condition due to protection from wet-season grazing as a dry-season forage reserve, and Right: in poor condition due to open access grazing, shrub encroachment, and major soil erosion (note large gullies). In both photos, most shrubs had been selectively cut in recent years. Photos taken less than 300 m from one another. Photo credit: Jason Sircely.
1Lamprey, R. 1983. Pastoralism yesterday and today: The over-grazing problem. Pages 643–666 in Bouliere, F., ed. Tropical Savannas, Vol 13, Ecosystems of the World. Amsterdam: Elsevier.
2Ellis, J. and D. Swift. 1988. Stability of African pastoral ecosystems: alternative paradigms and implications for development. Journal of Range Management 41:450–59.
3von Wehrden, H., J. Hanspach, P. Kaczensky, J. Fischer, and K. Wesche. 2012. Global assessment of the nonequilibrium concept in rangelands. Ecological Applications 22:393–99
4Reid, R. 2012. Savannas of Our Birth: People, Wildlife, and Change in East Africa. Berkeley: University of California Press.
5D’Odorico, P., A. Bhattachan, K. Davis, S. Ravi, C. Runyan. 2013. Global desertification: drivers and feedbacks. Advances in Water Resources 51:326–44