There is a process from maturity to mitosis that needs to be initiated by the community manager.
If you've done your job right, at some point the level of growth and activity should plateau. This is good. Only so many members can be interested in the community topic. Once the majority are in the community, growth will naturally slow. Check out Facebook's USA stats.
This is what happens in a mature community. The plateau isn't a problem, as long as you can sustain the community.
The problem is when you hit a decline.
The problem with mature communities
This happens more often than you might think. A successful community becomes too successful. The level of growth and activity increases until members no longer feel a sense of familiarity with one another. Information overload becomes a problem. It becomes difficult to follow what's going on.
Members lose a sense of connection with each other. The sense of community declines, followed by the level of activity. Soon the decreased activity leads to a downturn in growth and a death spiral.
Over time, an increasingly smaller number of members is responsible for an ever-greater amount of activity. More members feel left out etc, etc…
Moving from maturity to mitosis
If you're collecting data, you're tracking this. When you hit this dip (usually when the level of departing members isn't matched by newcomers, or when smaller number of members are responsible for an increasing amount of content) you need to advance your community to mitosis.
This means identifying popular topics or sub-interests within the community (a shared trait or a recurring popular theme) and building a new place within the community just for this. You can also build sub-groups for social circles that have developed within the community platform.
It means you need to shift your role from the maturity phase which is largely optimization of activities to mitosis, which means launching new, smaller, groups within the community where members again feel a strong sense of connection. This is the cycle part of community lifecycle.
It helps to have begun this in the establishment phase of the lifecycle to balance out the social density, but starting now is the next best thing.
Key tasks undertaken at maturity and mitosis
You wont change the plateau (that's an outcome of your success, congratulations!), but you will reverse the decline. You should be able to build a stronger and far more vibrant online community by managing smaller sub-groups.