Add contributor of the day/week/month to keep the top contributor's list fresh.
What you will find is that often the same people will stay at the top of lists like this because they joined early on and your competition will go stale. Leave them there, but have a second list of top contributors in the day/week/month to get your secondary contributors going.
Thank you for the suggestion, we do really like the idea of finding more enticing ways to provide top user contributions. By time period as you suggest is one way we could do this. We also like the idea of using topics as verticals so that there are subject-matter experts recognized for their contributions and knowledge.