Different ecologies on neighboring islands denote recent depopulation. Huge waves crossed the Pacific about 3,540 yrs. ago, as explained in Chapters 4 & 10. Once again, islands would be repopulated randomly.
Many low islands were completely depopulated about 3,540 years ago. Others would become populated after seal levels dropped 20 ft. at that time. Read about it at martiangrandpa.com.