[Pil-pc-oceania] Laws of Imminent Desertification: (Nature Journal: September 13/2007l)

[Deb Guildner]

What the world needs now is....
more positive feedback!  LOL!
Are we the ultimate 'grazer'??
As (an Adelaide wall) graffiti said in 1979: "things will get grazier before 
they get better".
(Those under 30 may not get that one...)

Deb
...................................................................

 Ecology: Scaling laws in the drier p151
The vegetation of arid ecosystems displays scale-free, self-organized 
spatial patterns. Monitoring of such patterns could provide warning signals 
of the occurrence of sudden shifts towards desert conditions.
Ricard Solé
doi:10.1038/449151a
Full Text | PDF
See also: Editor's summary :


Editor's Summary
13 September 2007

Reading the signs

--------------------------------------------------------------------------------

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.

COVER IMAGE Sonia & Michaël Kéfi/ Yolanda Pueyo/ Santiago Beguería Portugués

News and Views: Ecology: Scaling laws in the drier
The vegetation of arid ecosystems displays scale-free, self-organized 
spatial patterns. Monitoring of such patterns could provide warning signals 
of the occurrence of sudden shifts towards desert conditions.

Ricard Solé

doi:10.1038/449151a

Full Text | PDF (458K)

Letter: Positive feedbacks promote power-law clustering of Kalahari 
vegetation
Todd M. Scanlon, Kelly K. Caylor, Simon A. Levin & Ignacio Rodriguez-Iturbe

doi:10.1038/nature06060

First paragraph | Full Text | PDF (921K) | Supplementary information

Letter: Spatial vegetation patterns and imminent desertification in 
Mediterranean arid ecosystems
Sonia Kéfi, Max Rietkerk, Concepción L. Alados, Yolanda Pueyo, Vasilios P. 
Papanastasis, Ahmed ElAich & Peter C. de Ruiter

doi:10.1038/nature06111

First paragraph | Full Text | PDF (535K)



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

  1.. Department of Environmental Sciences, Copernicus Institute, Utrecht 
University, PO Box 80115, 3508 TC Utrecht, The Netherlands
  2.. Pyrenean Institute of Ecology, Avda. Montañana 1005. Apdo. 202, 50192 
Zaragoza, Spain
  3.. Laboratory of Rangeland Ecology, Aristotle University, 54006 
Thessaloniki, Greece
  4.. Département des Productions Animales, Institut Agronomique et 
Vétérinaire Hassan II, Rabat, Morocco
  5.. 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 at geo.uu.nl).

Top of page
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.