The social dysfunction and failure to mate successfully that occurs in some species when their population density falls below a certain threshold. This is believed to have caused the extinction of the American passenger pigeon (Ectopistes migratorius), probably because habitat destruction and hunting reduced the population density to a level at which individuals lacked the social stimulation necessary to prompt mating. The effect was first described in 1949 by W. C. Allee, O. Park, A. E. Emerson, T. Park, and K. P. Schmidt.
Subjects: Ecology and Conservation.