A number of ccosystem characteristics can be used to measure ecosystem resilience, from vegetation structure to productivity. At the broad scale employed in this analysis, the biggest change in ecosystem function was delected when woody species increased at the expense of grasses, or vice versa. In such instances, changes in structure and standing biomass are very evident. This is the case when the fire regime or the degree of large ungulate herbivory changes, or when woody elements are removed mechanically. Replacement of one species of grasses by another, or a species of tree by another, seems to have a much smaller effect on structure, productivity or standing biomass. Yet, when detailed studies are performed, changes in ecosystem function are inevitably encountered.
In every ease, even at the level of functional group, the measured changes are complex and strongly influenced by environmental factors such as precipitation and fire. Resilience appears to be intimately connected to ecosystem function. When changes in function brought about by a disturbance are minor, the system has the ability to recover. The null hypothesis does not appear to be upheld by the evidence. Every savanna species appears to have unique physiological and demographic characteristics (Silva 1995), but it is these species differences, through changcs in the relative abundance of species as a result of disturbances, that given savannas their resilience. In other words, because every savanna species is functionally and demographi-cally unique, savannas are able to persist in time. This, in our opinion, is the significance of species diversity.
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