干旱生態(tài)系統(tǒng)占地球土地面積的約40%,，是20多億人生活的家園,，但它們在氣候變化和人類行動(dòng)面前仍然很脆弱。Kéfi等人利用數(shù)值模型及來自西班牙,、摩洛哥和希臘的地中海生態(tài)系統(tǒng)的數(shù)據(jù)發(fā)現(xiàn),，植被斑塊大小分布遵從一個(gè)“冪次法則”。隨著動(dòng)物吃草對植被造成的壓力的增大,，斑塊大小分布將偏離該“冪次法則”,，接近向荒漠條件過渡的狀態(tài)。所以,，斑塊大小分布可能是荒漠化的一個(gè)有用的預(yù)警,。本期封面所示為西班牙Belchite的El Planerón自然保護(hù)區(qū)的一個(gè)干旱景觀（在封面照片的上部）,，下部所示為這一景觀退化的情況（Letter p. 213）。在另一篇論文中,，Scanlon等人用衛(wèi)星影像表明,，卡拉哈里盆地的樹叢大小分布也遵從一個(gè)無標(biāo)度（scale-free）“冪次法則”。這種現(xiàn)象可以用與現(xiàn)有樹木附近的有利環(huán)境相關(guān)的正反饋來解釋（Letter p. 209）,。在一篇News & Views文章中,，Ricard Solé對這兩篇論文都進(jìn)行了討論。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: [email protected]).
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.
