High survival promotes persistence in a reintroduced population of Common Crane (Grus grus)

Abstract

Reintroductions are increasingly used for re-establishing populations, though the documented success rates are variable. Success is typically measured by the survival and breeding success of released individuals and subsequent establishment and growth of the population. However, few studies quantify demographic rates of reintroduced populations of long-lived species, with slower dynamics and lower reproductive rates, and the demographic rates of subsequent generations, which determine population growth and persistence, are often unknown. We document the success of the reintroduction of the Common Crane Grus grus to south-west England in the UK, using survival analysis and population modelling to determine whether the overarching aim of establishing a self-sustaining population is likely to be achieved. Between 2010 and 2014, 93 Common Cranes were hatched from wild eggs, custom-reared and soft-released into suitable habitat. Survival of all age-classes was apparently higher than survival of healthy wild individuals, particularly in juvenile age-classes, and was higher in females than in males. Productivity was relatively low, but increased with age. Assuming that the demographic rates of subsequent generations are at the levels estimated for the captive-reared population, slow positive population growth is predicted over the next 30 years. Sensitivity analyses showed that the persistence of this population is sensitive to even a slight decrease in values of adult survival, and growth is impacted to a lesser extent by juvenile survival and levels of productivity. This study highlights the ability to achieve persistence in a reintroduced population despite low levels of productivity when survival is exceptionally high. It also illustrates the need for long-term monitoring to establish reintroduction success in animals that have slow life histories: 9 years after initial releases, the Common Crane population remains dominated by first-generation birds and productivity is skewed by transient age-structure and the late onset of reproductive success.