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:…is variation among ecosystems, the most common eVect of exposing plants to UV-B is a decrease in the intensity of insect herbivory (Ballaré et al. 1996; Rousseaux et al. 1998, 2004; Mazza et al. 1999b; Zavala et al. 2001; for reviews, see Ballaré et al. 2001; Caldwell et al. 2003; Bassman 2004).
Category: