Complex sentence: In the years prior to and also after John Robinson's death, Plymouth Colony had lost about a quarter of its residents as dissatisfied Strangers (non-Puritan community members) had moved to other areas of New England, went back to Old England, or to Virginia.
Split into simpler sentences: In the years prior to and also after John Robinson's death, Plymouth Colony had lost about a quarter of its residents. These were dissatisfied "Strangers" (non-Puritan community members) and had moved to other areas of New England or went back to England, or to Virginia.