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:across different stages of their life cycle and thus have been proposed as possible causes of jellyfish blooms (Richardson et al., 2009; Dong et al., 2010; Purcell, 2012; Duarte et al., 2012).
Category: