Population Dispersion

Populations show an inherent tendency to spread out or disperse in every direction until they encounter some barrier that stops further expansion, and this dispersal shows up as individuals moving into a population area (immigration) or out of it (emigration). Migration is a distinctive form of mass, round-trip population movement, undertaken and later reversed by the same individuals — Siberian cranes migrating from Siberia to Vedanthangal in Tamil Nadu to escape severe winters, then returning in spring, is the chapter's example, alongside fish that migrate from the sea into fresh water to breed (anadromous migration, as in salmon) and fish that migrate from fresh water out to the sea (catadromous migration, as in eels). Emigration typically occurs under natural conditions when a habitat becomes overcrowded, functioning as an adaptive behaviour that relieves pressure on the original site, prevents the habitat from being over-exploited, and lets the emigrating individuals colonise new areas elsewhere. Immigration, conversely, raises the size of the receiving population; if that population then grows beyond the habitat's carrying capacity, the result can be increased mortality among the …