The range of Columbian WT used to extend from the Columbia River up to the Southern Puget Lowlands. Habitat alteration gradually reduced the range seen today. Considering that genetic tests show that they are virtually identical to the WTs found in Eastern Oregon, it is surprising that they are not more invasive. Studies on the population at the Julia Butler Hansen CWT refuge show that the males don't seem to disperse (like other deer populations) after becoming mature, they don't leave the island when they could easily do so. In my mind, if you have to spend millions of taxpayer dollars to keep a geographic strain (not genetically different ) of deer from going extinct, and they only show minimal response in population growth with all our management, then it may be time to let them have a go at it on their own.
Anyways, I found this regarding CWT/BT interaction. Looks like a large population of CWT keep the blackies out. It may have been that as their numbers declined across the original range of CWTs, BT overtook their territories:
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Interspecific Interactions
At high densities, Columbian white-tailed deer appear to exclude Columbian black-tailed deer (Odocoileus
hemionus columbianus) from lowland riparian habitats (Smith 1987). However, black-tailed deer readily
occupy riparian lowlands when densities of white-tailed deer are reduced (Smith 1987). This can lead to
increased competition and potential hybridization (Davison 1979, Smith 1987, Gavin and May 1988,
Whitney 2001).