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:Although these studies indicate that the invasion process is at least in part deterministic (Duncan et al., 2003), none identify specific mechanisms by which successful invaders establish and expand their new ranges (Ehrlich, 1989; Mack et al., 2000; Viet and Lewis, 1996).
Category: