zehrila - 10:23 am on Jun 15, 2011 (gmt 0)
Competitors can effect your ranks, e.g if the you are fairly new in the business, they can sabotage you by doing mass spam linking, negative reviews. In case you are buying links, linking from your own sites, they can find out your back links and report! Some of these things are unethical though.
Its extremely hard to sabotage bigger sites unless you are really dedicated and have a lot of time and budget, in that case you should spend that budget and time building up your own portfolio.