  A community's identity defines and shapes its internal dynamics. Our current
understanding of this interplay is mostly limited to glimpses gathered from
isolated studies of individual communities. In this work we provide a
systematic exploration of the nature of this relation across a wide variety of
online communities. To this end we introduce a quantitative, language-based
typology reflecting two key aspects of a community's identity: how distinctive,
and how temporally dynamic it is. By mapping almost 300 Reddit communities into
the landscape induced by this typology, we reveal regularities in how patterns
of user engagement vary with the characteristics of a community.
  Our results suggest that the way new and existing users engage with a
community depends strongly and systematically on the nature of the collective
identity it fosters, in ways that are highly consequential to community
maintainers. For example, communities with distinctive and highly dynamic
identities are more likely to retain their users. However, such niche
communities also exhibit much larger acculturation gaps between existing users
and newcomers, which potentially hinder the integration of the latter.
  More generally, our methodology reveals differences in how various social
phenomena manifest across communities, and shows that structuring the
multi-community landscape can lead to a better understanding of the systematic
nature of this diversity.
