Accelerating Towards Extinction
Once an animal species drops below 50 individuals, there is little likelihood the species will survive in the wild without help from humans.
But situations do vary. Populations have declined inexorably to extinction from as many as 112 individuals, as for golden plover.
The final slide to extinction is a slippery slope. As populations shrink, the annual drop in numbers becomes greater. Losses also grow larger as extinction nears. In the final years, changes to population size become increasingly volatile, fluctuating erratically.
Researchers found this pattern emerging consistently from yearly census data taken for ten vertebrate populations, including two mammals and five birds. The data were collected for the last 12 to 21 years before a population died out. The final count of animals before the closely-monitored populations vanished was on average eight and ranged from 2 to 26 individuals.
The results confirm what theorists speculated over 20 years ago, that there exists an “extinction vortex”. As a species dwindles, the normal dynamics affecting populations disintegrate and the species loses its resilience to inbreeding and unfavourable environmental conditions, among other survival challenges. The feedback accelerates the decline of a population that's on the final descent to extinction.
The data from populations going extinct also confirm the theory that the time left until extinction scales logarithmically as a function of population size. As extinction nears, the rate of a population's fall increases continually. With their study confirming that extinction can happen suddenly, the authors suggest intervening sooner rather than later to save a species.
William F. Fagan, E.E. Holmes and Gary Belovsky. 2006. Quantifying the extinction vortex. Ecology Letters. 9(1): 51-60.