Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Many phenotypic traits that are critical to microbial fitness and expected to play important roles in structuring microbial communities are highly variable among closely related microbial taxa (Oda et al., 2002; Davelos et al., 2004a; Al Dahouk et al., 2012; Schlatter et al., 2013).
Category: