WordNet (r) 3.0 (2006):

overpopulation
    n 1: too much population

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "overpopulation":
   avalanche, deluge, embarras de richesses, enough, extravagance,
   extravagancy, flood, inundation, landslide, lavishness,
   money to burn, more than enough, overabundance, overaccumulation,
   overbounteousness, overcopiousness, overdose, overlavishness,
   overluxuriance, overmeasure, overmuchness, overnumerousness,
   overplentifulness, overplenty, overprofusion, oversufficiency,
   oversupply, plenty, plethora, prodigality, redundancy, spate,
   superabundance, superflux