Their birthrate is also very low... way below the level 2.1 children per woman needed to maintain population levels.
Muslims in general have a birthrate above 4 children per female. The native German population is going to be cut almost in half every generation while the Muslim population will double, even as immigrants are pouring in and native Germans are "leaving in droves".
https://www.gatestoneinstitute.org/9154/germans-leaving-germany
http://www.express.co.uk/news/world/723962/Germans-leave-Germany-Angela-Merkel-migrant-policy