The role of the search portal is evolving form a gateway to a destination by the advent of the community-based search. A community-based search delivers content from the knowledge community inside the portal that binds the traffic to the portal instead of allowing it access to external information sites. The Q&A mechanism available to users gives more relevant search results, hence resolving the search limitations inherent in the automated search engine. However, by monopolizing traffic inside the portal, it may of course deprive external sites. We examine how the evolution of the community-based search may affect information ecology on the web from a long-term perspective. The results show that the community-based search can be an efficient service if there is an increase in total web quality when the magnitude of network externality is high, there are sufficient references to external sites before knowledge sharing in the community, and the marginal disutility of users due to irrelevant information is low. However, in the opposite environment, this service causes the collapse of the web by lowering the quality of the content for both portal and external sites.