The ongoing accessibility of digital material is challenged by the constantly changing environment in which it exists. In particular, application profiles are threatened by a number of factors such as loss of context, social change and linguistic change. In this paper, we draw on observations taken from a number of application domains to build simple mathematical models for community growth and change, to explore the impact of community structure on the sustainability model required for application profiles over time. Finally, we discuss the use of similar models in evaluating application profile sustainability in general, and lessons to be drawn for DCMI.