Jaco,
I understand what you are saying about the Nyala vs Bushbuck scenario as I was brought up with the same understanding of how they compete. Recently I was talking to one of the old school PH's in our area who owns a property not far from us, and we discussed this very topic. He has been a PH for 30+ years, and have done it all, so I respect his opinion and experience.
When he inherited the property from his dad, they use to have plenty of Bushbuck, but sadly the numbers are almost non-existent these days. He has no Nyala there. What has happened over the last several years, is that the annual quota for Kudu, had not been reached by a long way, causing a serious over-population of the specie. His opinion is that it has had the exact same effect.
I don't think that its a Nyala vs Bushbuck problem , but could very well be caused by say Kudu as well.
My personal theory, is that I believe the Nyala ewes have the same feeding height as Bushbuck, causing them to compete heavily for food. Where we hunt Nyala, I have seen over the last 7 or so years the numbers of Bushbuck dwindle drastically. It would not take you much more than a day to look for a good Bushbuck ram. These days, you would be lucky to find one there, and its usually with the guy who's not looking for a Bushbuck.
The only place you do find Bushbuck in numbers in this area these days is next to the river where the Nyala can't reach.
"The Higher the Nyala concentration in a specific area the lower the bushbuck concentration will be."
Although I fully agree with your statement above, I feel that its not only the Nyala that will lower your Bushbuck numbers, but also other species.