Small‐scale spatial and temporal variation in the demographic processes underlying the large‐scale decline of eiders in the Baltic Sea

Abstract

The application of uniform conservation schemes often fails to account for small-scale spatial variation in the drivers of population decline. Demographic comparisons of imperilled populations across locations are therefore crucial for successful conservation, but progress is hampered by lack of long-term data from more than a single population. The recent large-scale decline of eider ducks (Somateria mollissima) in the Baltic Sea is ideal for determining to what extent mechanisms underlying population decline can be extrapolated over larger areas. We utilized stochastic demographic methods incorporating both environmental and sampling variation to assess small-scale spatial and temporal variation in the population dynamics of eiders at Söderskär (eastern range-margin) and Tvärminne (core breeding area), situated 130 km apart. The stochastic growth rate models accurately predicted the observed differences in the rate of decline between sites and time periods. At Söderskär, established breeder survival had by far the greatest elasticity, whereas elasticity was more evenly distributed among vital rates at Tvärminne. Although the study sites showed the single largest difference in fecundity, stochastic life table response experiment analyses revealed that reduced adult female survival at Tvärminne mainly determined the observed difference in growth rates between sites. In contrast, reduced fecundity primarily differentiated the past population increase from the present population decline at Söderskär. Our results demonstrate that different mechanisms may be associated with population decline across adjacent geographic locations, and indicate that dispersal of first-time breeders may be important for population dynamics. Safeguarding adult female survival and/or fecundity should be prioritized in management efforts.

Site‐selection bias and apparent population declines in long‐term studies

“…; Öst et al. ). Incorrect conclusions about monitored populations can arise from short time series (Krebs ; White ); inconsistent methods (Hayward et al.…”

Increased male bias in eider ducks can be explained by sex-specific survival of prime-age breeders

“…However, this estimate did not consider population age structure or the fact that individuals across different ages contribute disproportionally to population dynamics. For example, in their study on the decline of female eiders in the Baltic Sea area, Öst et al [ 19 ] showed that the population growth rate of females is far more sensitive to small proportional changes in the mortality of older, experienced breeders than to those of younger age classes. Such a pattern is typical for long-lived vertebrates, including many birds [ 12 , 20 ] and is due to the greater reproductive value of older, reproductive individuals that have high survival [ 21 ].…”

Evaluating models of population process in a threatened population of Steller's eiders: a retrospective approach

“…Öst et al. () found that common eiders breeding along the eastern edge of their range had significantly lower reproductive success than a population breeding in the core of their range. Low fertility may also be the result of negative impacts on survival immature birds once they leave the breeding grounds (birds younger than 1 yr old).…”