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(2013) Three types of spatial feedback loop in coupled human and natural systems. Vegetation pattern formation in semi-arid grazing systems. HilleRisLambers, R., Rietkerk, M., van den Bosch, F., Prins, H.
NETLOGO FOR LOOP GENERATOR
See how patterns vary for different random number generator seeds (i.e., different values of number-seed). Note how the pattern of growth changes for the different establishment rules. When you are ready to go, click setup, then go (if you wish to run one timestep at a time click step instead of go). To vary patterns between simulation runs change the random number-seed.ĭecide how long you want to pause simulation between each timestep by setting pause-duration. Next decide if you wish the colour of plants to reflect the point in time at which they establish (lighter color is later in the simulation) by switching age-colours? on or off. Centre: at the very centre of the simulated environemntįirst, decide which of the establishment and initial-veg rules you wish to use.One of two rules can be used to determine where the inital plant locates ( initial-veg):
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NETLOGO FOR LOOP PATCH
Random: could establish randonly with equal probability on any patch with sufficient water availability (and no existing plants).Wetter: more likely to establish on patches with greater water availability (and no existing plants) but potentially could establish on any patch with sufficient water availability.Wettest: on one of the patches with the greatest water availability that doesn't already have a plant growing there.One of three rules can be used to determine where a new plant will grow in each timestep ( establishment): Plants can only establish on patches that do not contain other plants. One new plant establishes on a patch with water available during each time step (except the initial timestep in which a plant is established regardless of water availability - this is assumed to be an initial condition not controlled by the rules of the model). HOW IT WORKSĪt each time step in the model water infiltration (and therefore water availability in the soil) is calculated based upon the location and density of plants. Through time the shape of the vegetation patch influences soil water conditions at the periphery which facilitates vegetation establishment (and therefore patch growth). Green dots indicate vegetation, blue squares indicate soil water availability (darker, more water), brown areas are dry soil not suitable for vegetation. This feedback loop is described in more detail in HilleRisLambers et al. Consequently, conditions for plant establishment are better near existing patches of vegetation, leading to the growth of the vegetation patch. In turn, the availability of water in the soil is determined by the density of plants nearby, as plants facilitate infiltration. Growth of plants is controlled by the availability of water in the soil. In this case the entity is a patch of vegetation. This model illustrates a positive 'growth' feedback loop described in Millington (2013) in which the areal extent of an entity increases through time. Do you have questions or comments about this model?