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:Thus, herbivory in leaves or roots can induce whole-plant changes in allelochemistry ( Bezemer et al. , 2004; Kaplan et al. , 2008a ), resulting in indirect effects on co-occurring species ( Bezemer et al. , 2003; Soler et al. , 2005, 2007 ; van Dam et al. , 2005 ).
Category: