By Henrik von Wehrden
There is the story of Joseph interpreting the dream of the Pharao. It is a very old story, reaching us through the old testament, and when I was a kid that story was of great importance to me. Ultimately it led to the development of a recent paper which I led in collaboration with colleagues from Germany and Austria, and which you can find as an open access article here.
First, it’s about sustainability. You keep resources when they are abundant (in the seven good years), to later compensate for a potential famine (the seven bad years).
Second, it’s about the variability of drylands. I have seen my share of deserts and I have always been fascinated by how life prevails on the very edge of life itself. I remember traveling through the desert in Morocco when I was probably eight years old, and we came to a tree, which was packed with goats, which were chewing off all leaves. Around was nothing but barren desert, and I was greatly impressed by this.
Recent human development and use of natural resources has obviously triggered severe changes. Climate change is proposed to increase the impact of extreme events, and drylands (and most rangelands) are shaped by the amount of precipitation they receive. However, the amount of precipitation often fluctuates widely, making management of these regions a challenging endeavour. Traditional societies and cultures often depended on nomadism to compensate for temporal and spatially fluctuating resources; and often they also depended on rivers in distant highlands. Egypt is a good example of this, with one of the oldest cultures prevailing on the planet adapted to the Nile river. Other regions explore water hidden beneath the surface, making use of precious resources that may run dry in the future.
How are rangeland ecosystems adapted to grazing, which is the dominant use of rangelands? There is an ongoing discussion about why and when degradation occurs. I stumbled over this topic when I read a prominent paper in Science, which underlined the strong influence of precipitation variability on whether rangelands would end up degraded after times of drought, or not. The authors of that paper noted that some details had so far remained unresolved. While I was reading the paper I came to think of a recent product I was developing with Jan Hanspach at the time, namely is a global GIS layer of precipitation variability. Out of curiosity I started checking some of the papers quoted as puzzling in the Science paper.
Here is the hypothesis: In a nutshell, degradation should occur where precipitation has low variability, because there livestock numbers remain relatively stable (even in times of drought), and hence grazing pressure is continuously high. By contrast, in variable environments, livestock numbers are likely to go down during times of drought, given the ecosystem time to recover.
Regarding whether or not rangelands would end up degraded, a threshold of 33 % coefficient of variation of annual precipitation was proposed by Ellis and others in the 1980s – and has been criticized ever since. I was astonished that all papers we reviewed in our new paper (see above) seemed to fit the original proposal when tested against our global precipitation product. That is, degradation was restricted to environments with a low variability, or radiating around wells and rivers and also key resources such as wet meadows. After carefully examining the literature we found a really stable pattern, which has global consequences for livestock grazing and as such – global food production. From our review, it looks like indeed, ecosystems with highly variable rainfall are less prone to degradation than more stable ecosystems.
I strongly believe that humans need to be careful in managing stable rangelands, of which there are several across the globe. Artificially stabilizing variable rangelands, e.g. by wells and external food, may also pose risks. Degradation is a severe threat to ecosystem intactness. Despite their lower potential for degradation, regions with a high variability can be difficult to manage, because the possible collapse in livestock numbers can cause famines as we can frequently witness e.g. in the Sahel.
Thus the paper is a dream come true, yet with global consequences. If we do not learn to manage rangelands, both variable and stable ones, with a long term perspective, we will pay a high price, both regarding humans and the ecosystem.