Recent theoretical studies have shown that spatial redistribution\ud of surface water may explain the occurrence of patterns\ud of alternating vegetated and degraded patches in semiarid grasslands.\ud These results implied, however, that spatial redistribution processes\ud cannot explain the collapse of production on coarser scales observed\ud in these systems. We present a spatially explicit vegetation model to\ud investigate possible mechanisms explaining irreversible vegetation\ud collapse on coarse spatial scales. The model results indicate that the\ud dynamics of vegetation on coarse scales are determined by the interaction\ud of two spatial feedback processes. Loss of plant cover in a\ud certain area results in increased availability of water in remaining\ud vegetated patches through run-on of surface water, promoting\ud within-patch plant production. Hence, spatial redistribution of surface\ud water creates negative feedback between reduced plant cover and\ud increased plant growth in remaining vegetation. Reduced plant cover,\ud however, results in focusing of herbivore grazing in the remaining\ud vegetation. Hence, redistribution of herbivores creates positive feedback\ud between reduced plant cover and increased losses due to grazing\ud in remaining vegetated patches, leading to collapse of the entire\ud vegetation. This may explain irreversible vegetation shifts in semiarid\ud grasslands on coarse spatial scales
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