Survival patterns and density-dependent processes in breeding mallards Anas platyrhynchos
Sammanfattning: Measuring and assessing vital rates such as births and deaths are prerequisites for understanding population dynamics. Vital rates may be affected by the density of individuals, even though the importance of density dependence on population dynamics has been debated for a long time. The mallard Anas platyrhynchos is one of the foremost game species in the Holarctic, with millions of birds in hunters’ bags annually. Still, basic knowledge about regulation of mallards’ vital rates is poor, and experimental studies on this topic are rare. In this thesis I have studied survival patterns and density dependence in mallards breeding in Sweden and Finland. Long-term ringing data from both countries were analysed for mortality patterns and causation, as well as for e.g. survival rate estimation. Most of the studies were, though, experiments run over two years involving manipulations of the density of nests, broods and/or adults, in southern and northern Sweden, comprising different biotic regions. Common response variables were survival of nests, ducklings and hens, mainly analysed with program MARK. About 90% of the recovered mallards in Finland and Sweden were hunting kills. However, survival rates were high, ranging from 0.66 to 0.81 for most groups (sex'age). The generality of density dependence was evident since such processes were detected in all studies. Consequently, depredation rate was higher in high nest density compared to low nest density. Survival of ducklings was density-dependent in both boreal and nemoral biotic regions, with food limitation being evident in the former region but not in the latter. In spite of their generality, density-dependent patterns varied within as well between years, and for nest predation rates also between landscape types. The findings about density dependence in breeding mallards in this thesis are novel since they are based on experiments. They are potentially of general interest for management because they embrace a variety of lakes in two geographically distant areas, eachbeing representative for large temperate areas in the northern hemisphere. Detection of density dependence at the local scale may be important at larger scales, too, following the principle of ‘ideal preemptive distribution’ in a source-sink dynamic system.
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