New Zealand Journal of Ecology (1997) 21(2): 129-140 ©New Zealand Ecological Society these conceptual extremes should be seen as being endpoints of a range of possible situations, and not as discrete alternatives. It is therefore unlikely that an observed field situation would be completely described by any one of the models alone. More recently developed models use the conceptual extremes as components: they interpret the concepts more broadly, and combine them to produce integrative models that better describe observed data. The Declining-productivity model is an example of one such general model that has been applied to semi-arid rangelands worldwide. This model is examined, and its applicability to vegetation dynamics in Central Otago is discussed. Finally, the present state of knowledge of the vegetation dynamics of Central Otago is considered with reference to two frameworks for the management of semi-arid lands: degradation gradient assessment and the state-and-transition model.