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:The invaded range of the species
is well known (e.g. Brown et al. 2011), but the native
range of the species in Asia has until now been poorly
understood (Poutsma et al. 2008).
Category: