Sentence:  The congregation perishes when it is no longer possible to replenish that core from the neighborhood ; moreover , residential mobility is so high in metropolitan areas that churches have to recruit constantly in their core <WSD>stratum</WSD> in order to survive ; they can lose higher - and lower status members from the church without collapsing , but they need adequate recruits for the core stratum in order to preserve economic integration . 
Meaning of the word marked with <WSD> tags:  one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)