The role of immigration and reinforcement in the population dynamics of a long‐lived bird: implications for the conservation of threatened species

Abstract

Understanding population dynamics requires knowledge of the differential effects of survival, productivity and dispersal on population growth. This is particularly important for the conservation of small and recently established populations, where stochastic births and deaths may result in negative growth and even extinction. Here, we investigated the population dynamics of a small population of Eurasian cranes Grus grus in the UK and the effect of a population reinforcement in population growth. We also estimated the probability that the conservation status of cranes improves in the future. We developed stochastic population models to assess the population dynamics and the effect of adding 90 individuals between 2010 and 2014. The best-supported models suggest that the crane population is self-sustaining with an annual adult survival of 0.88, but suffers from low productivity. In addition, much of the population increase has been driven by immigration of birds from continental Europe. We found that population reinforcement resulted in a 50% increase in the projected population size, from 178 to 275 breeding pairs over the next 50 years. We showed that the relative contribution of immigration to population growth declined from 43%, when the translocated birds were not considered, to 29%, when they were included in the breeding pool. Moreover, after the population reinforcement, the probability of the population improving its conservation status increased from just above zero to 32%. In light of the recent increase in translocation programs worldwide, our study highlights the need to consider population dynamics to successfully predict the increase in population size when management strategies, such as reintroductions and reinforcements, are planned.

Management Actions Promote Human-Wildlife Coexistence in Highly Anthropized Landscapes: The Case of an Endangered Avian Scavenger

“…For instance, Serrano et al (2021) showed that natal dispersal distances of Egyptian vultures were negatively related to natal breeding population density and population trends (social cues). Hence, increasing the breeding success with low adult mortality rate, as our case (Zuberogoitia et al, 2009), would help to increase the viability of Egyptian vulture populations, not only in the target area but also in the neighboring regions since they play an essential role in demographic processes such as source-sink dynamics (e.g., Hernández-Matias et al, 2013;Lieury et al, 2016;Weegman et al, 2016;Sanz-Aguilar et al, 2017;Soriano-Redondo et al, 2019). Conservation programs that succeed in reducing and palliating the main threats to target species and identify and engage with all stakeholders to implement solutions, particularly in areas of conflict with high human density, should be regarded effective.…”

Population reinforcement and demographic changes needed to stabilise the population of a migratory vulture

“…The number of birds in each age class was estimated for each year by random binomial draws with the survival probability of birds from the previous year. Although immigration can be an important driver of population dynamics in small populations (Lieury et al., 2015; Schaub & Ullrich, 2021; Soriano‐Redondo et al., 2019), and has been shown for other raptors in the Balkans (Demerdzhiev et al., 2015), we did not include an immigration component in our population model. The Egyptian Vulture population in the Balkans is small and fragmented (Velevski et al., 2015), and >500 km from the nearest healthy source population that could provide a significant number of immigrants (Balaban & Yamac, 2018; Katzenberger et al., 2019).…”

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

“…Sergio et al 2011). After visualizing survival estimates from a model with age in years as a factor (0-9 years), we collated the data into 10 different age-class groupings based on various fledgling, subadult and adult classes (Mathews & Macdonald 2001, Soriano-Redondo et al 2019a) and ran models testing the effect of the groupings on survival, with constant encounter probability. Models were ranked using Akaike's Information Criterion adjusted for small sample size (AICc; Burnham & Anderson 2002) and the age structure in the top model was selected for use in subsequent models.…”

“…However, largely as a result of overexploitation, Common Cranes were extirpated in the UK in the 17th century (Mathews & Macdonald 2001, Stanbury 2011. In 1979, a pair naturally re-established in Norfolk, east England, and by 2015, this population had increased to 20 breeding pairs (Soriano-Redondo et al 2019a). Being a long-lived species with slow maturity, expansion of this population was likely to continue to be slow without active intervention.…”