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Nonetheless, researchers are calling for a better understanding of the functional traits of plant species to help predict ecosystem processes under changing environmental conditions (Diaz et al., 2004; Arndt, 2006; McGill et al., 2006).

Given the central role of microbes in ecosystem processes, we contend that microbial composition might be at least as important as plant composition for building predictive ecosystem models. Here we discuss two major hurdles to including microorganisms in ecosystem models. The first is a general idea that microbial composition does not matter to ecosystem processes. The second is that microbial composition may be too diverse to model. To address the first hurdle, we outline three conditions that would need to be true for changes in microbial composition to matter to ecosystem processes. We then review recent studies to assess whether particular microbial groups may be more or less subject to particular disturbances. We address the second hurdle by proposing a simple model of microbial process rates that incorporates information on community composition. The model is used to illustrate how empirical data could be used to predict microbial process rates under disturbance, even for relatively diverse communities. Because it is not feasible to add a parameter for each microbial population in a community, we consider when coarse information about microbial composition, such as the relative abundance of a few key clades within a functional group, could help decrease uncertainty about predictions of ecosystem processes.

A FRAMEWORK WITHIN THE BLACK BOX

Schimel (2001) points out that black box ecosystem models make two implicit assumptions: that microbial processes can be represented across a range of environmental conditions with one mathematical function, and that microbial processes are never limited by the abundance of any microorganism. These assumptions are implicit because no major ecosystem models include parameters that explicitly represent components of microbial community composition. At best, some models include total microbial biomass as a parameter [e.g., Harte and Kinzig (1993)], but many widely used models such as CENTURY (Parton et al., 1987), MEL (multiple element limitation) (Rastetter et al., 1997), and TEM (terrestrial ecosystem model) (McGuire et al., 1993) contain parameters related to microbial processes but not the microbial communities themselves. Ocean biogeochemistry models have only just begun to incorporate explicit parameters that capture plankton community composition (Moore et al., 2002; Le Quéré et al., 2005; Salihoglu and Hofmann, 2007).

These models implicitly assume that changes in community composition will not affect ecosystem processes, because there is no mechanism



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