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:In grassland systems, grazing intensity can alter the composition and structure of both vegetation and the associated prey base (arthropods) for grassland-nesting birds (Fondell and Ball 2004; Sutter and Ritchison 2005; Little et al. 2013).
Category: