I am going to have to agree with Chau on this one. Once bacterial colonization is established, the flora remains stable over time, as long as regular feeding practices persist (Meroth et al, 2003). While other bacteria may occupy the starter, it would appear that they never grow to any significant degree. Moreover, aggressive bacterial strains will only takeover if the starter becomes neglected over time (Meroth et al, 2003), i.e. leaving out your starter for two weeks with out feeding it. This may in part explain Brian Spangler's observations with his particular culture.
Meroth C, Walter J, Hertel C, Brandt M, and Hammes W. (2003). Monitoring the Bacterial Population Dynamics in Sourdough Fermentation Processes by Using PCR-Denaturing Gradient Gel Electrophoresis. Applied Environmental Microbiology 69(1): 475–482.
This article is open access (meaning anyone can read it without subscription to the journal) and can be viewed here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC152404/