Przewalski's Horses Die Over Gobi Winter - Climate Change or Effects of Captive Breeding?
Kaczensky, P., Ganbataar, O., Altansukh, N., Enkhsaikhan, N., Stauffer, C. and Walzer, C. 2011. The Danger of Having All Your Eggs in One Basket-Winter Crash of the Re-Introduced Przewalski's Horses in the Mongolian Gobi. PLoS ONE e28057 doi: 10.1371/journal.pone.0028057.
The Gobi is home to small populations of endangered wild Asiatic asses (Equus hemionus) and three very small populations of re-introduced Przewalski's horses (Equus ferus przewalskii). All living Przewalski's horses are descendants of the last few wild animals captured from the Gobi and bred in zoos and game preserves worldwide (van Dierendonck and Wallis de Vries 1996). Captive horses were re-introduced into the Gobi in 1992 and the population was self-sustaining by 2003. The population of Przewalski's horses hit a total of 138 individuals by 2009 but over the winter of 2009/2010, a particularly severe dzud occurred and the population crashed.
Winter mortality is common in large-bodied ungulates and particularly cold winters with heavy snow and/or ice are associated with higher than usual mortality. For example, approximately 23% of the reindeer population of Svalbard Norway died over the severe winter of 1975/76 (Reimers 1982). Therefore, some winter mortality is to be expected even in species that are well adapted to winter conditions and in most regions, some winters are worse than others and are associated with higher-than-normal mortality.
As the authors of the present report explain (Kaczensky et al. 2011), "large mammals re-introduced into harsh and unpredictable environments are vulnerable to stochastic effects (Asbjörnsen et al., 2005; Sæther et al 2002), particularly in times of global climate change and the associated increase in extreme weather events (Saltz et al., 2006; IPCC, 2007)." The winter of 2009/2010 was said to be the worst dzud to hit Mongolia in the last 50 years and most of the Przewalski's horse population did not survive.[Presumably then, the winter of 1967/68 (Sternberg 2010:73) was as bad or worse.] Was this a "textbook example of the risks faced by small and spatially confined species in unpredictable environments (Kaczensky et al. 2011) "-- made worse by climate change-as suggested by the authors or were other factors at work?
In describing their research approach, the authors state: "We used spatially explicit livestock loss statistics, ranger survey data and GPS telemetry to provide insight into the effect of a catastrophic climate event on the two sympatric wild equid species and the livestock population in light of their different space use strategies." They recorded births and deaths of individual Przewalski's horses from May 1 to April 30 each year from 1992 (the year of their introduction) to 2010, in order to generate a population per year. Rangers monitored the horses 1-2 times a week and an additional fifteen horses were followed by satellite telemetry between 2001 and 2008. In addition, between 2002 and 2010, they also monitored the movements of between seven and ten Asiatic wild asses via satellite telemetry in order to provide a comparison to a similar but Gobi-born species. It was found that the population of Przewalski's horses fell from 138 in 2009 to 49 in 2010, with most of the losses occurring during heavy snow storms that raged between December and March. The weather was most severe over the winter range for two out of the three distinct groups of re-introduced horses; the winter range of the third group was less severely hit and had only minor losses.
Not surprisingly, the youngest animals suffered the highest mortalities. However, zoo-born animals-even those who had lived for several years in the wild-apparently had lower survival under dzud conditions than Gobi-born horses and fewer stallions survived than did mares.[Perhaps due to the territorial nature of males? The authors offer no explanation.]
In contrast to the Asiatic wild asses, who moved west ahead of the storms during the dzud period and thus escaped the worst of their effects, Przewalski's horses in the eastern Gobi did not respond to the storms by moving over the landscape and many of them died.
The authors comment (pg. 6) that: "Whether autochthonous [indigenous or ancient] Przewalski's horses were more mobile than the present-day re-introduced animals is unknown. From other species we know that sedentary and migratory animals or subpopulations can coexist within the same species and habitat (Bunnefeld et al., 2011). Thus it is possible that during captive breeding either the behavioural tradition or the genetic component for exploratory movements was lost."
The authors make this statement in their discussion: "Our findings suggest that with the increasing proportion of Gobi born Przewalski's horses the re-introduced population may become more robust in facing future dzud conditions, although we have yet to understand the underlying adaptive mechanisms." In other words, the severe losses of Przewalski's horses were not a function of small population size per se but a result of the particular composition of the population: it is possible that eventually, a population of Przewalski's horses born and raised in the Gobi (rather than raised in captivity and released into the wild) will be more resilient to the dramatic swings in weather conditions that are normal for the Gobi, even if some years are more extreme than others-although it is also possible that the ability to respond appropriately to such severe events has been lost because of captive breeding. In addition, the results also suggest that the winter of 2009/2010 was not an extraordinary meteorological event for the Mongolian Gobi-there is no evidence presented that it was exacerbated by climate change.
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