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 stratum 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 <WSD>preserve</WSD> economic integration . 
Meaning of the word marked with <WSD> tags:  keep or maintain in unaltered condition; cause to remain or last