Biomagnification, Redirects and Back Link Penalties
I often find that the best sources of analysis in SEO, which is still a nascent industry, come from other academic pursuits. While these are regularly computer sciences (like latent dirichlet allocation) or mathematics (like volatility analysis), we sometimes find interesting lessons outside of those usual suspects – in this case, biology.
Biomagnification is a fairly simple principle that through a series of prey-predator relationships, toxic substances tend to accumulate in higher percentages among organisms higher in the food chain. You can see a visualization of this in the image to the left. As mercury accumulates in various organisms, predators consume those organisms and absorb those toxins. Unless the organism has a way of disposing those toxins, they can and will magnify in their accumulation.
This is the same when merging sites with one another. If you have many micro-sites or subdomains that have been received less-than-kosher links in the past, you may have been able to dodge some algorithm updates like Penguin simply because the sites’ link profiles were too small. Your rankings might not be that great either on these sites, but you have no evidence of a link penalty. However, redirecting multiple sites to another presents a potential for biomagnification because the combined link profiles may be enough to trigger penalties, filters or algorithmic devaluations.
The most interesting implication is this: even if your main site and all the sites/subdomains you intend to redirect to your main site appear to have no link penalties (manual or algorithmic), the combination of those sites could trigger one.
Of course, the simple solution is to be thoughtful when joining sites together. Determine what new link thresholds will be once you combine anchor text, root link domains, etc. to make sure you aren’t stepping over any lines. And, of course, if you are – remove the concerning links before doing the redirect to avoid the wrath of search engines.