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Spatiotemporal agent-based modelling to analyze sustainability issues at the landscape level: the grazing herbivores metaphor

Guerrin F.. 2015. In : Weber T. (ed.), McPhee M.J. (ed.), Anderssen R.S. (ed.). MODSIM 2015 Book of abstracts: Partnering with industry and the community for innovation and impact through modelling. Canberra : MSSANZ, p. 81-81. International Congress on Modelling and Simulation. 21, 2015-11-29/2015-12-04, Broadbeach (Australie).

This model aims at simulating herbivores grazing in a rangeland landscape. The first aim is to find a balance between the herbivores and the vegetation dynamics guaranteeing sustainability: maintain a healthy animal population and a green landscape. Two opposite processes threaten this equilibrium: overgrazing, leading to desertification, reversible; under-grazing leading to shrub invasion, irreversible. Both processes may ultimately lead to the population extinction by starving and pasture invasion by shrubs. The model implementation with the NetLogo simulation platform (Wilensky, 1999) comprises two types of agents: “Patches” standing for land plots; “Turtles” standing for herbivores. Patches are characterized by their color: shades of green for grass, red for shrubs. Herbivores are characterized by attributes like birth date, age, previous location, destination, pathway, travelled distance, ingested feed, body weight, calving dates. During simulation each turtle iterates the following: find a destination, move, graze, gain and lose weight, age and, possibly, reproduce or die. Simulations have been made to check variants of the system's structure and behaviors based on a reference landscape comprising 1,225 patches (1 ha each) and 1,225 turtles standing for cattle (1 head/ha).Simulation assessment criteria are the herbage biomass, herbivore population size, individual body weights, birth and mortality rates, land-use patterns and landscape fragmentation obtained after a 5-year time period. The following issues have been explored by simulation experiments: •Heterogeneity of landscape at initialization: starting with patches uniformly green or with different greens in a relatively narrow range makes almost no difference. However, with higher heterogeneity the system's performances decrease in terms of population size, pasture area and shrub extension. Homogenous landscapes become more heterogeneous and conversely. Whatever their initial state, all landscapes converg

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