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:To quantify habitat indication for each native and non-native species, we used an objective method that we developed in previous studies of effects of invasions, land management practices, or environmental conditions on plant species composition in a variety of ecosystems in MS (Brewer 2008; Brewer and Menzel 2009; Brewer 2011; Brewer et al. 2012).
Category: