Asian wild horses, or Przewalski's horses, are considered to be the only surviving wild horses -- that is, horses that have never been successfully domesticated (American mustangs, for example, are domesticated horses that have once again become feral). Przewalski's horses went extinct in the wild during the 1960's, but were successfully reintroduced by selectively breeding wild horses surviving in zoos and preserves before releasing the horses back into the wild.
The model program to re-establish the wild horse suffered a serious blow in the winter of 2009/2010, as an extended drought followed by extreme snowfall limited the food available for grazing. Mongolian herders in the Great Gobi B Strictly Protected Area lost an average of 67% of their stock. Such weather conditions, referred to locally as "dzud," occur periodically and were not necessarily related to global climate change.
However, scientists Petra Kaczensky and Chris Walzer of the University of Veterinary Medicine of Vienna have used statistics on the geographical ranges of wild horses and wild asses to demonstrate that restricted ranges can "easily result in local population crashes such as the one seen for the Przewalski's horses." From the press release Don't Put All Your Eggs in One Basket -- or All Your Horses on One Pasture:
The severe effect of this localized catastrophic event was largely due to the small size and limited range of the present-day Przewalski’s horse population. A large and continuous population would be much more robust as it could counteract local population lows or extinctions via re-colonization. The dzud winter of 2009/2010 is a textbook example of how vulnerable small and spatially confined populations are in an environment prone to fluctuations and catastrophes.
The findings suggest that programs to protect endangered species, especially in the face of increasingly frequent extreme weather events related to global climate change, must adopt strategies to open wider ranges to allow species to migrate away from threats and into more habitable locations, as well as to introduce populations to diverse ranges.
The study, The Danger of Having All Your Eggs in One Basket—Winter Crash of the Re-Introduced Przewalski's Horses in the Mongolian Gobi, is published this month in PLoS ONE.