Positive inter-specific interactions (a.k.a. facilitation) can be a key to understand plant community structural change along disturbance gradients. Verbal models predict that facilitation by stress-resistant species maintains competitive species under intermediate to moderately-high disturbance, thereby maximizing species richness at intermediate disturbance. However, no mathematical models, to our knowledge, have examined this prediction. We built a mathematical model of multispecies plant community dynamics under grazing pressure. The model assumes that facilitation among plant species arises through grazing saturation, and species differ along a tradeoff between competitive ability and grazing-resistance. Analysis of a simple two-species model revealed four patterns of community change with increasing grazing pressure: (i) Two species experience catastrophic extinction at the same level of grazing pressure. (ii) Two species coexist at low grazing pressure, and the grazing-resistant species dominates at high pressure. (iii) Only the competitive species occurs at low grazing pressure, and is replaced by the grazing-resistant species at high pressure. (iv) No coexistence occurs. A complex model with more species confirmed that grazing-resistant species facilitated competitive species at intermediate to moderately-high grazing pressure, but the whole community could suddenly collapse to extinction at higher pressure especially when species have similar grazing resistance.