Environmental Engineering Reference
In-Depth Information
location of desertification in space (Thornes, 1990, 2007)
(Figure 24.8). What has not yet been studied or mod-
elled is any 'contagion' effect of desertification, though
given the catena concept and patch dynamical growth
approaches (Tilman and Karieva, 1997), it is clearly an
important problem for the near future. Often in Mediter-
ranean environments, one can observe a whole hillside
fairly uniformly covered with a single shrub species (such
as
Anthyllis cytosoides
) and the following question arises.
If a small locus of erosion breaks out somewhere in the
middle, will it propagate outwards in an unstable fash-
ion, moving the whole hillside to the erosion attractor?
This evolution has been suggested by Kosmas for the
case of the bush
Sarcopoterium
, on the island of Lesbos,
Greece (Kosmas, personal communication; see also Kos-
mas
et al
., 2000). Alternatively, given that different species
have hydrologically different characteristics, if a different
species invades such a slope, could it shift the system
across a threshold to a different attractor? We shall refer
back to this question in the fourth section of this chapter,
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Figure 24.8
Cellular model of grazing-vegetation interaction of Thornes (2007): (a) initial, randomly generated distribution of
biomass; (b) strong clustering of cattle in the free-range grazing model after only five iterations of the cellular automaton model; and
(c) the net result for the central region after 10 iterations of the cellular automaton model. The lighter patches are areas of no or very
little biomass. The marginal numbers indicate the locations of the cells in the central region (Reproduced with permission from
Thornes, J.B. (2007) Modelling soil erosion by grazing: recent developments and new approaches.
Geographical Research
, 45, 13-26).
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