Herbivores, particularly introduced ones such as sheep and rabbits, have reduced the levels of regeneration of a number of species of native trees and shrubs in the arid rangelands of southern Australia since European settlement. These reduced levels of regeneration mean that populations of some species are declining while others, less palatable to herbivores, are being maintained, hence gradually altering the composition and/or overall cover of vegetation. In the long term, this could result in loss of palatable species valuable to the pastoral industry for animal production, and/or the dominance of large areas by undesirable “woody weed” species. To investigate the effects of grazing, we develop a model which projects the long-term survival rate of a focus species, Myoporum platycarpum, under grazing and nongrazing regimes. The technique we are applying is an extension of the Leslie growth model, where the vital rates are linked to random rainfall events and deterministic grazing. The effect of grazing is incorporated into the model based upon data primarily collected from the TGB Osborn Koonamore Vegetation Reserve, 400 km north of Adelaide in South Australia within and without the exclosure. The results suggest that only with higher than average rainfall for extended periods will the species of interest, M. platycarpum, survive under grazing.