Temporal fluctuations in populations are influenced by a large number of factors, including density effects, dispersal, interspecific interactions, and a number of environmental variables. Within populations, individuals of different ages or stages often respond differently to such factors. Thus, differences among age groups in survival and reproduction combined with the structure of populations may have a large influence on how a population responds to changes in the environment. How different age groups, and consequently the total population, are affected by perturbations is influenced by the life-history characteristics of the species (such as age at maturity and longevity). This thesis addresses how such life-history variation within populations (i.e. age variation in vital parameters) and among species (i.e. differences in a set of vital parameters) affect temporal population dynamics in Arctic vertebrate species. To understand the population dynamics of species we need to disentangle all the sources of variation contributing to temporal fluctuations in populations. This can be difficult, particularly due to additional stochasticity introduced by random variation among individuals in their realized survival and reproduction as well as observation error in the data. A set of stochastic population models are therefore applied that incorporates age structure, density dependence, as well as environmental and demographic stochasticity. These models are formulated as hidden process models, which are designed to separate the effects of process variation in the estimates of population sizes and demographic parameters from the stochasticity introduced by observation error. Demographic variation in the magnitude of responses to environmental fluctuations is common among long-lived ungulates. This thesis shows that there is considerable age variation in the mean survival and reproduction as well as in the temporal variation in these vital parameters in the Svalbard reindeer. The temporal fluctuations in age-specific vital parameters cause large fluctuations in age structure and in population size. In particular, large declines coincide with years of high population size and thick layers of ice covering the feeding grounds, and are associated with low calf survival and low reproduction. This study shows that if we want to understand how climate change or other anthropogenic impact will affect future populations, it is crucial to consider the age structure of populations. A common pattern across species is that vital parameters whose variation would have large effects on long-term population growth are often more stable over time than vital parameters with lower potential impacts on population growth. Recent studies have suggested that this can be explained by natural selection favoring traits that buffer influential vital parameters against environmental stochasticity. Results obtained for the Svalbard reindeer in this thesis suggest that such a pattern across species may be generated by interspecific life-history variation reflected by trade-offs between survival and reproduction, independent of demographic buffering (involving selection for reduced variance) of influential vital parameters. Previous studies indicate that life-history variation among species can explain interspecific differences in stochastic population dynamics in terrestrial birds, mammals and plants. This thesis addresses whether this also applies for marine fish species. Fish species with a fast life history (i.e. early maturing, with short generation times, high natural mortality and high reproduction) are found to exhibit larger temporal variation in vital parameters and population growth rates than species with a slow life history. Results in this thesis therefore indicate that species with a fast life history are likely to exhibit relatively large and rapid fluctuations in population sizes because of a relatively large influence of environmental stochasticity. In contrast, population fluctuations in slow species are likely to be slower and dampened and more influenced by deterministic processes such as age-structure fluctuations. Such relationships between life-history characteristics and population dynamics can be useful for developing life-history based principles for sustainable management. ; Livshistorie og bestandsdynamikk hos Arktiske virveldyr Svingninger i bestandsstørrelser over tid er skapt av mange ulike faktorer, slik som tetthetsregulering, spredning, konkurranse mellom arter og tilfeldig variasjon i miljøet (miljøstokastisitet). Ulike aldersgrupper responderer ofte ulikt på slike faktorer slik at aldersforskjeller i overlevelse og reproduksjon kan oppstå. Aldersstrukturen i en bestand har derfor stor innflytelse på hvordan populasjonen reagerer på miljøforandringer. Effekten av endringer i miljøet på de ulike aldersgruppene, og dermed på hele bestanden, er påvirket av artens livshistorietrekk (for eksempel modningsalder og forventet levetid). Denne avhandlingen undersøker hvordan aldersstruktur innen bestander og variasjon i livshistorietrekk mellom arter påvirker svingninger i bestander over tid. For å forstå bestandsdynamikk må vi skille mellom de ulike kildene til variasjon som skaper svingninger i bestander. Dette kan være vanskelig, særlig på grunn av tilfeldig variasjon mellom individer i deres evne til å overleve og reprodusere (demografisk stokastisitet) samt observasjonsfeil i data. I denne avhandlingen blir det benyttet stokastiske modeller hvor dette blir tatt hensyn til. Bestandsstørrelser og vitale parametere som overlevelse og reproduksjon kan dermed bli estimert med et mål på usikkerheten i estimatene. Analyser av bestandsdynamikken hos Svalbardrein viser at overlevelse og reproduksjon i ulike aldersgrupper varierer i forskjellig grad på grunn av aldersavhengige responser til variasjon i miljøet. Hvordan dette kan påvirke bestandsvekst blir diskutert i sammenheng med naturlig seleksjon. Dette studiet viser også at dersom vi ønsker å forstå effekten av klimaendringer eller annen menneskelig påvirkning på bestander må vi ta aldersstruktur i betraktning. Tidligere studier på fugler, pattedyr og planter indikerer at en arts livshistorie har betydning for hvor mye variasjon i miljøet påvirker bestander over tid. I denne avhandlingen blir slike sammenhenger også funnet hos marine fiskearter i Barentshavet. Denne kunnskapen er nyttig med tanke på å utvikle forvaltningsstrategier som er basert på arters ulike livshistoriestrategier. ; PhD i biologi ; PhD in Biology