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composition is sensitive and not resilient might produce process rates similar to the original community if the members of the community are functionally redundant.

Only if community composition is sensitive to a disturbance, not resilient, and functionally dissimilar to the original community do changes in community composition matter for predicting ecosystem process rates. In addition, the degree to which a community is resistant, resilient, and functionally similar will influence the degree to which community composition matters to a particular process. For instance, even if microbial composition is highly sensitive and not resilient to a disturbance, if all of the taxa perform a process at very similar rates (that is, they are nearly functionally redundant), then predictions of ecosystem process rates will not be improved much by including information about microbial composition.

This conceptual framework does not exclude the possibility that there is little change in microbial composition but large changes in ecosystem process rates. This scenario would suggest that the changes are a direct effect of environmental changes, the result of compositional changes in other organisms such as plants, or immediate physiological responses of microorganisms that are not accompanied by compositional changes. An example of the latter is that some transformations such as decomposition occur faster at higher temperatures. This is seen immediately, before changes in composition could be responsible (Fierer et al., 2005a).

RESISTANCE OF MICROBIAL COMPOSITION

To assess whether microbial composition is often resistant to disturbance, we reviewed studies that experimentally exposed microbial communities to various disturbances. We searched Web of Science for papers including “microb*” and “community composition” in their titles, abstract, or subject words. In addition, we specifically searched the journal Global Change Biology for global change experiments that assessed microbial composition. We did not limit papers by study system, but the majority of studies returned by these search parameters focused on terrestrial soils. We acknowledge that there are many manipulative marine studies that we did not pick up in our search. The disturbances we examined were limited to CO2 enrichment, temperature, fertilization with mineral nutrients, and enrichment with C substrates (including complex organics such as manure and potential toxins such as pesticides). We reason that these four disturbances are typical of those that ecosystems experience under human-driven global change. Finally, we did not intend the literature search to be exhaustive, but to be representative of these types of studies.



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