This week’s issue of Nature magazine contains an article authored by Todd Scanlon, Kelly, Ignacio Rodriguez-Iturbe and Simon Levin. The article investigates the dynamic origin of power-law distributions of vegetation clusters at a series of sites within the Kalahari region of southern Africa. It is accompanied by a “News and Views” article by R.V. Sole, and an additional article written by Kefi et al. on a similar observation of vegetation pattern in Mediterranean drylands.
Here’s the editor’s summary of the two articles:
Arid ecosystems cover about 40% of Earth’s land area and are home to over two billion people, yet they remain vulnerable to climate change and human actions. Using numerical simulations, and data from Mediterranean ecosystems in Spain, Morocco and Greece, Kéfi et al. show that patch-size distribution of vegetation follows a power law. As grazing pressure increases, patch size deviates from the power law close to the transition to desert conditions. So patch-size distribution may be a useful early warning of desertification. The cover shows an arid landscape (top) in the El Planerón nature reserve in Belchite, Spain, and the lower panels show degradation in this landscape. In a separate paper, Scanlon et al. use satellite imagery to show that the size distribution of tree clusters in the Kalahari basin also follows a scale-free power law. This can be explained by positive feedback associated with preferential environments near existing trees. In News & Views Ricard Solé discusses both papers.
In our article, we find that positive feedbacks operating over non-local scales (i.e. beyond nearest neighbors) can lead to power-law distributions in the sizes of contiguous vegetation clusters. The spatial patterns that arise from these positive feedbacks do not arise under models of neutral spatial interactions.