Demography of a socially natural herd of Przewalski's horses: an example of a small, closed population

Owing to habitat loss and fragmentation, large mammal populations all over the world are becoming increasingly small and isolated. It is therefore a conservation priority to understand mechanisms influencing the demography of such populations, which can easily be driven to extinction. The Przewalski's horse Equus ferus przewalskii remains one of the world's most endangered species and reintroduced animals are still vulnerable. Over 9 years, we analysed factors affecting mortality and female fecundity at the individual level in a predator-free, closed population of Przewalski's horses, which grew from 11 to 55 individuals. Similar to other wild equids, the annual growth rate of the population was r=0.169. Typically, adult mortality was much lower than juvenile mortality, the latter being correlated with neither inbreeding coefficient of foals nor population density. We found no link between female fecundity and operational sex ratio of the herd, or inbreeding coefficient, lactation status and body condition of the mares. Although food therefore seemed not to be limiting in this population, density (number of horses ha(-1)) clearly reduced fecundity, especially in subadult mares. Thus, our results show that space can slow the growth rate of a population before resources become limited, a potential source of concern for increasingly shrinking habitats of endangered large mammals. Possible mechanisms causing this may be found in incest avoidance or other social parameters. Finally, in large herbivores, population density is said to exert influence in a sequential order: juvenile survival first, followed by fecundity of young females, then adult females, and adult survival last. Although we observed no link between density and juvenile survival in the studied population, our results otherwise support this hypothesis.