If people are participating in a community for extrinsic reasons, they're likely to participate more frequently.
If you run competitions, offer prizes, add gamification systems, people might participate more frequently (gamification studies typically show a small impact in larger, established, communities). However, the quality of contributions is low.
Worse still, you need to continually increase the rewards to maintain the level of contributions. Once the rewards are removed or become stale, the level of participation plummets.
When you begin offering rewards, you change the participant's motivational state. A member that might respond to questions because she likes to help people and believes in the community, now participates to get prizes.
You can do serious long-term harm by offering extrinsic rewards.
Studies show that extrinsically motivated people participate more (until the rewards become stale), but intrinsically motivated members create the best quality responses (and stay for longer).
It's the quality responses that attracts more people. It's the quality responses that increases all member's pride in the community. It's the quality responses that spur other members to share their own thoughts/learning.
Two action points here. First, resist the urge to add gamification systems. Second, it's your absolute duty to shine the spotlight on the quality contributions and mention their impact to the author. This self-efficacy, the impact that a member's great post has had on other members, is what will spur on more quality contributions.
If you want to master the psychology of community participation, sign up for our community management course. Enrollment closes on Feb 4th.