干旱生态系统占地球土地面积的约40%，是20多亿人生活的家园，但它们在气候变化和人类行动面前仍然很脆弱。Kéfi等人利用数值模型及来自西班牙、摩洛哥和希腊的地中海生态系统的数据发现，植被斑块大小分布遵从一个“幂次法则”。随着动物吃草对植被造成的压力的增大，斑块大小分布将偏离该“幂次法则”，接近向荒漠条件过渡的状态。所以，斑块大小分布可能是荒漠化的一个有用的预警。本期封面所示为西班牙Belchite的El Planerón自然保护区的一个干旱景观（在封面照片的上部），下部所示为这一景观退化的情况（Letter p. 213）。在另一篇论文中，Scanlon等人用卫星影像表明，卡拉哈里盆地的树丛大小分布也遵从一个无标度（scale-free）“幂次法则”。这种现象可以用与现有树木附近的有利环境相关的正反馈来解释（Letter p. 209）。在一篇News & Views文章中，Ricard Solé对这两篇论文都进行了讨论。封面照片：Sonia & Michaël Kéfi/ Yolanda Pueyo/ Santiago Beguería Portugués
Nature 449, 213-217 (13 September 2007) | doi:10.1038/nature06111; Received 27 June 2007; Accepted 24 July 2007
Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems
Sonia Kéfi1, Max Rietkerk1, Concepción L. Alados2, Yolanda Pueyo1, Vasilios P. Papanastasis3, Ahmed ElAich4 & Peter C. de Ruiter1,5
Department of Environmental Sciences, Copernicus Institute, Utrecht University, PO Box 80115, 3508 TC Utrecht, The Netherlands
Pyrenean Institute of Ecology, Avda. Montañana 1005. Apdo. 202, 50192 Zaragoza, Spain
Laboratory of Rangeland Ecology, Aristotle University, 54006 Thessaloniki, Greece
Département des Productions Animales, Institut Agronomique et Vétérinaire Hassan II, Rabat, Morocco
Soil Center, Wageningen University and Research Center, Droevendaalsesteeg 4, 6708 PB Wageningen, The Netherlands
Correspondence to: Sonia Kéfi1 Correspondence and requests for materials should be addressed to S.K. (Email: kefi@geo.uu.nl).
Humans and climate affect ecosystems and their services1, which may involve continuous and discontinuous transitions from one stable state to another2. Discontinuous transitions are abrupt, irreversible and among the most catastrophic changes of ecosystems identified1. For terrestrial ecosystems, it has been hypothesized that vegetation patchiness could be used as a signature of imminent transitions3, 4. Here, we analyse how vegetation patchiness changes in arid ecosystems with different grazing pressures, using both field data and a modelling approach. In the modelling approach, we extrapolated our analysis to even higher grazing pressures to investigate the vegetation patchiness when desertification is imminent. In three arid Mediterranean ecosystems in Spain, Greece and Morocco, we found that the patch-size distribution of the vegetation follows a power law. Using a stochastic cellular automaton model, we show that local positive interactions among plants can explain such power-law distributions. Furthermore, with increasing grazing pressure, the field data revealed consistent deviations from power laws. Increased grazing pressure leads to similar deviations in the model. When grazing was further increased in the model, we found that these deviations always and only occurred close to transition to desert, independent of the type of transition, and regardless of the vegetation cover. Therefore, we propose that patch-size distributions may be a warning signal for the onset of desertification.
