| Summary: | The Baltic region is characterised by a strong seasonal climate. Climate change may bring profound ecological changes in the region. These ecological responses to a changing climate can be better understood if the effects of recent yearly variations are known. Other ways to explore possible consequences to future climate change are by using climate models and by looking at the response of species to a warmer climate during the past millennia. This thesis utilizes these methods including a range of study species: a mammal, migratory birds, and an insect. The mammalian species studied is the Baltic ringed seal (Pusa hispida botnica), a subspecies of the ringed seal. Ringed seals need ice as a substrate for breeding, including the construction of a breeding lair. It is shown (study I) that the projected changes in the ice climate for 2071–2100 in the southern breeding areas (the Gulf of Finland, the Archipelago Sea and the Gulf of Riga) are so large, that for most years successful breeding is unlikely. In the northernmost parts of the Bothnian Bay, the ice climate is still projected to be suitable for breeding for most years. By the end of this century, it is possible that the Bothnian Bay will be the only remaining breeding area for the Baltic ringed seal. Based on an extensive material of subfossil seal finds, study (II) suggests that ringed seals have probably lived continuously in the Baltic Sea for more than 10,000 years, even surviving the Holocene Thermal Maximum (a several millennia-long warm period). As the warm winters of the Holocene probably weren't as warm as the temperatures projected for the final decades of this century, the survival prospects of the ringed seal in the Baltic will probably be reduced in a way unprecedented in the history of the subspecies. The bird study (III) adds to the growing evidence that temperatures along migration routes have an effect on arrival times. We found negative correlations between temperature and arrival times in several of the ten studied long-distance Finnish ...
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