Clicking on the picture above will open an interactive animation illustrating conditions under which desertification can be induced by disturbances such as clear cutting or grazing. The animated simulations have been obtained by numerically solving a mathematical model of vegetation growth on a computer ( Hardenberg et al., 2001). For further information on vegetation patterns press here.
Operating the animation
The two frames represent different responses of a vegetation spot pattern to biomass reduction denoted by lighter green hues. The animated simulations in frame A where obtained at precipitation rates higher than those of frame B. To view the vegetation response to a disturbance of a given strength (percentage of biomass reduction) proceed as follows:
Click a desired disturbance level on the bottom bar (the chosen value will appear in orange). Choose the desired frame and press the big arrow at the bottom of the frame. You can follow the time evolution in slow forward or backward motion by repeatedly clicking the little arrows at the bottom of the frame.
Running the animation
It is recommended to choose first the lowest disturbance level (30%), run the animations at both frames one after the other, and repeat this process at increasing disturbance levels.
Understanding your observations
The animated simulations in frame A correspond to a system whose only stable state consists of spotted vegetation. The animated simulations in frame B correspond to a system at a lower precipitation rate where stable spotted vegetation coexists with a stable state of bare soil. In the first case (frame A) the vegetation recovers from any disturbance, while in the second case (frame B) disturbances which are strong enough (60% and higher) shift the system to its alternative stable state - the bare soil. Coexistence of stable stable states as in Frame B therefore implies vulnerability to desertification.
This behavior resembles a ball rolling over a surface (see illustration figure below) containing a single well, representing the spots pattern in frame A, or a double well, representing the two stable states in frame B - spot pattern and bare soil. In the single well case the ball will return to the bottom of the well no matter what the disturbance strength is. In the double well case, disturbances exceeding a threshold value (denoted by red arrow) will shift the system from one well to the other.
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