Philopatric predisposition to predation-induced ecological traps: habitat-dependent mortality of breeding eiders

Abstract

Because population size is sensitive to changes in adult survival, adult survival may be buffered against environmental variability. Philopatry may be adaptive in changing environments, but it could also constrain breeding habitat selection under changing conditions such as shifting predation regimes. Habitat preference and quality could become decoupled in long-lived philopatric species that evolved in stable environments when suddenly faced by increased adult predation risk, as dispersal may be triggered by past reproductive failure. We evaluated whether the Baltic eider (Somateria m. mollissima) population may currently face a predation-induced ecological trap. Eiders are philopatric and nest on open and forested islands. We hypothesized that open-nesting females would be disproportionately affected by increased predation. We compared female annual survival in these two habitats in 1996-2010. We also tested for effects of time trends, winter severity (NAO), female body condition, and habitat-specific predation pressure on survival. Our results revealed the lowest survival recorded for this species (Φ = 0.720), and survival on open islands was significantly lower (Φ = 0.679) than on forested islands (Φ = 0.761). Nonetheless, only 0.7 % of females changed breeding habitat type despite ample availability of alternative islands, and breeding phenology in both habitats was similar. Female survival increased with body condition, while it was unrelated to winter climate and stable over time. Open islands had a higher predation pressure on incubating females. Breeding philopatry results in a predator-mediated ecological trap for open-nesting eiders. Our results contribute to explaining the drastic decline of the Baltic eider population.

Trends and drivers of change in diving ducks

“…At Tvärminne Ekroos et al (2012b) documented high female mortality (about 72% apparent annual survival) among breeding birds, contrasting with the situation at Söderskär. A re-analysis of the vital rates at Söderskär and at Tvärminne (Öst et al unpubl.…”

“…No data on vital rates exist from this population, but a population at the western entrance of the Gulf of Finland (Hanko, Tvärminne, see Ekroos et al 2012b) which shows roughly a similar pattern, with a decrease starting in the last years of the 1990s (Kilpi & Öst 2002), provides data on vital rates.…”

Ecological traps: current evidence and future directions

“…For example, birds commonly experience traps caused by predation (e.g. [35]), but aquatic taxa may be susceptible to traps that arise in other ways (e.g. pollution [36]).…”

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

“…This information can, in turn, be used to assess the utility, if any, of a spatially uniform conservation policy. While the decline itself is an easily observable large-scale phenomenon affecting a large fraction of the Baltic/Wadden Sea flyway population of eiders, the causation of the decline has been open for debate over the last few years (Hario and Rintala 2006;Ekroos et al 2012b;Ottwall 2012). Based on long-term data on reproduction and survival from the central Gulf of Finland, Söderskär, it has been argued that the cause of the eider population decline should be sought in factors affecting juvenile recruitment and survival rather than adult survival (Hario and Rintala 2006;).…”