freejung - 10:35 pm on Aug 13, 2010 (gmt 0)
The idea that some backlinks affect the viability of other backlinks, ouch, that makes my head hurt :).
I'm not sure I would exactly put it like that, but I suppose you could look at it that way. It should be noted that we're not talking about the "viability" (whatever that means) of the link, we're talking about what effect that link has on rankings. I think it's pretty well established that Google looks not only at individual links, but at metrics related to the entire backlink profile (that might include, for example, and this is pure speculation, the percentage of links of one sort vs. another sort, the growth rate of links over time, things like that).
If that's true, that the algo is looking at the entire profile as a whole as well as looking at individual links, then the effect of a single link would depend on the structure of the whole. That's what happens when you look at an entire system, not just at the sum of the parts.