An Attrition

1. Wearing or grinding down by friction.
2. The gradual reduction in a tangible or intangible resource due to causes that are passive and do not involve productive use of the resource.
3. (human resources) A gradual, natural reduction in membership or personnel, as through retirement, resignation, or death.
4. (sciences) The loss of participants during an experiment.
5. (theology) Imperfect contrition or remorse.
6. (dentistry) The wearing of teeth due to their grinding.
7. (linguistics) The loss of a first or second language or a portion of that language.

(employment reduction by natural causes): natural wastage

accretion