Dictionary.com
definitions

underpopulated

[uhn-der-pop-yuh-ley-tid]
adjective
  1. having a population lower than is normal or desirable.

Origin of underpopulated

First recorded in 1880–85; under- + populate + -ed2
Related formsun·der·pop·u·la·tion, noun
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for underpopulation

underpopulated

adjective
  1. having a low population rate
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012