Understanding and predicting a species’ distribution across a landscape is of central importance in ecology, biogeography and conservation biology. However, it presents daunting challenges when populations are highly dynamic (i.e. increasing or decreasing their ranges), particularly for small populations where information about ecology and life history traits is lacking. Currently, many modelling approaches fail to distinguish whether a site is unoccupied because the available habitat is unsuitable or because a species expanding its range has not arrived at the site yet. As a result, habitat that is indeed suitable may appear unsuitable. To overcome some of these limitations, we use a statistical modelling approach based on spatio‐temporal log‐Gaussian Cox processes. These model the spatial distribution of the species across available habitat and how this distribution changes over time, relative to covariates. In addition, the model explicitly accounts for spatio‐temporal dynamics that are unaccounted for by covariates through a spatio‐temporal stochastic process. We illustrate the approach by predicting the distribution of a recently established population of Eurasian cranes Grus grus in England, UK, and estimate the effect of a reintroduction in the range expansion of the population. Our models show that wetland extent and perimeter‐to‐area ratio have a positive and negative effect, respectively, in crane colonisation probability. Moreover, we find that cranes are more likely to colonise areas near already occupied wetlands and that the colonisation process is progressing at a low rate. Finally, the reintroduction of cranes in SW England can be considered a human‐assisted long‐distance dispersal event that has increased the dispersal potential of the species along a longitudinal axis in S England. Spatio‐temporal log‐Gaussian Cox process models offer an excellent opportunity for the study of species where information on life history traits is lacking, since these are represented through the spatio‐temporal dynamics reflected in the model.
The UK great bustard Otis tarda reintroduction trial: a 5-year progress report R o b e r t J . B u r n s i d e , I a n C a r t e r , A l a s d a i r D a w e s , D a v i d W a t e r s L e i g h L o c k , P a u l G o r i u p and T a m Á s S z É k e l y Abstract The great bustard Otis tarda became extinct in the UK during the 19th century due to a combination of factors, including hunting, egg collection and changes in agriculture. In 2003 a 10-year licence was granted to begin a trial to reintroduce the species back to the UK. Here we report on the first 5 years of the trial and assess the progress made towards establishing a founder population. From April 2004 to September 2009 a total of 102 great bustard chicks were imported from Russia and 86 released on Salisbury Plain. Monitoring showed that post-release survival was 18% in the first year following release, and that mortality of released bustards was mainly attributable to predation and collisions. Estimated adult survival was 74%, although the sample size was small. All known surviving great bustards are faithful to the surroundings of the release site, returning throughout the year. A lek has been established where males have been observed displaying to females. The first nesting attempt was in 2007, and in 2009 two females aged 3 and 4 years successfully nested, fledging one chick each. Models incorporating the new demographic estimates suggest that at the end of the 10-year trial period the project can expect to have 8-26 adults as a founder population.
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.
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