Sentence:  The congregation perishes when it is no longer possible to replenish that core from the neighborhood ; moreover , residential mobility <WSD>is</WSD> 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 preserve economic integration . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)