Supervisor: Vallo Tilgar, vanemteadur
Oponent: Ilpo Kojola, vanemteadur, PhD, Oulu Ülikool
Life-histories and population dynamics of large mammalian herbivores living in a variety of ecosystems have been found to be closely related to density-independent as well density-dependent processes. In ungulates, living in temperate and arctic environments, the negative effects of harsh winters on individual survival and condition are widely known. However, in parallel with the negative influence of severe winters, negative effects of warmer than average winters on ungulate populations have also been described. In this thesis, long-term data were used to explore the effects of weather and population density on body size, fecundity parameters, the timing of conception and on the population-growth rate in a wild moose (Alces alces) population in Estonia. In parallel, two different composite indices of winter weather were used - the maximal ice extent (MIE) of the Baltic Sea and the NAO (North Atlantic Oscillation). Given that in the strongly seasonal environments, the timing of reproductive events like conception can also have profound impact on individual fitness, the associations between the timing of conception and various components of maternal fitness such as litter size and foetal sex ratio were also studied. The results of this thesis reveal significant direct as well as time-lagged effects of winter weather on several life history traits and population dynamics of moose in Estonia. The relationships between climate and life-history traits are not always linear, suggesting that both higher or lower than the long-term mean of winter weather conditions can have significant impact on ungulate performance. The results of this thesis also demonstrate that winter effects based on the MIE index were consistently stronger than those based on the NAO index. Increase in population density had only a marginal negative effect on moose body size and fertility parameters, while it was significantly related to the advancement of conception date of female moose. The latter finding is opposite to the expected and may indicate that the effect of population density on conception date is mediated by the increase in habitat quality due to intensified forestry concurrent with increasing moose population abundance. In addition to environmental effects, a clear relationship between conception date and litter characteristics was revealed. Earlier-conceived females produced larger litters with higher proportion of male embryos than those conceived late. These adjustments are in accordance with the Trivers-Willard hypothesis if females that conceive earlier are in better condition.