The case study we present in this paper illustrates how fundamental knowledge of environmental factors (rainfall variability) and ecosystem structure and function (plant functional type physiology) can be integrated within a modeling synthesis. Our results underscore the impact of short term climate shifts and year to year variability upon plant production, especially perennial grasses. Year to year variability in total plant productivity was found to be roughly 50% greater than the variability in "normal" rainfall, and variability in grass productivity was three times greater. From a management standpoint, perennial grass is one of the most important functional types because of its contribution to forage production and ground cover. Maintaining a stable grass cover is desirable in order to be able to project long term land use strategies and economic return. However, our modeling results suggest that maintaining stable grass cover may be very difficult in this semi-arid region, given the potential impacts of natural climate variability upon vegetation production. Furthermore, our results support the point of view that dryland agriculturists and pastoralists must be adaptive in terms of their response to climate variability.27

While these ecosystem-level predictions may be used independently to address issues relevant to sustainable development of these semi-arid regions, we suggest that the next step in assessing sustainability is the incorporation of the ecological impacts into higher level models that incorporate direct and other human impacts on these systems. This step will require further testing and evaluation of ecosystem-level models in the context of different management and land-use alternatives. Finally, at the highest level of integration, we propose the incorporation of both "natural" and human factors into a spatially explicit model of landscape elements and human land-use patterns. Only at this level will we have predictive tools capable of dynamic, integrated assessments of impacts of global climate change on human-dominated ecosystems. Although we are far from being able to do this, a number of programs around the world, such as the urban LTERs in the US, are striving to develop integrated approaches to link human and natural systems. We believe this task is essential in preparing for—and planning corrective measures to—the complex interactions between climate, human activities, and the well-being of the Earth .

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