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:Evidence has been presented for the importance of salt marsh plants in determining distribution patterns of the diazotrophic bacteria Bagwell et al. 1998; 2001; Lovell et al. 2000; Piceno and Lovell 2000a; b; Piceno et al. 1999 .
Category: