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for such changes to influence model predictions. This assumption may be valid if microbial composition is resistant, resilient, and/or functionally redundant (these terms and others are defined in Table 8.1). Microbial composition is resistant if it is similar across a variety of environmental conditions—in other words, it is difficult to perturb from an original state. It is resilient if, when composition does change, it recovers quickly, whether by growth or by physiological or genetic adaptation. Finally, microbial composition may change in response to new environmental conditions but contain functionally redundant taxa such that ecosystem process rates are not altered. If these conditions are not generally true for microbial communities, then many current ecosystem models may fail at predicting the rates of microbe-driven processes under disturbance.

The idea that microorganisms are resistant, resilient, and functionally redundant is pervasive in ecology. Some microbial groups show a high degree of metabolic flexibility and physiological tolerance to changing environmental conditions [e.g., Meyer et al. (2004)], which could result in microbial communities that are resistant to change. These traits and others associated with microbes—such as high abundances, widespread dispersal, and the potential for rapid growth rates—have also led to the suggestion that microbial communities will be resilient to change (Fenchel and

TABLE 8.1 Summary of Definitions Used in This Chapter

Term

Definition

Functional group

All organisms that directly contribute to the rate of a particular functional process in an ecosystem

Functional redundancy

The ability of one microbial taxon to carry out a process at the same rate as another under the same environmental conditions

Functional similarity

The ability of two microbial communities to carry out a functional process at a similar rate, regardless of differences in composition

Microbial composition

The richness, relative abundance, and phylogenetic structure of taxa in an assemblage

Microbial taxon

A group of phylogenetically related microbes

Resilience

The rate at which microbial composition returns to its original composition after being disturbed

Resistance

The degree to which microbial composition remains unchanged in the face of a disturbance

Physiological trait

A physiological characteristic that determines the contribution of a microbial taxon to a functional process

Physiological response curve

The function describing the relationship between the process contribution of a microbial taxon and disturbance intensity



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