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:…said, however, that M. senhousia, as well as other alien species, might initially increase local species number; however, through strong direct or indirect effects, it might lead to reduced species richness of native species, and a decrease in the abundance of remaining species (Ruiz et al., 1997).
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