Moth abundance changes and consequent bottom-up effects on birds in boreal forests

Many insect populations are declining, but we still don’t fully know how widespread or severe these declines are. Since insects are vital for the functioning of terrestrial ecosystems, evidence of widespread declines has raised concern about cascading effects across food web. Long-term data from Finland’s boreal forests make them an ideal setting to assess the impacts of climate and environmental changes on insect populations trends and food web dynamics. I studies three main questions: (I) how the biomass of moths has changed over time and which ecological and life-history traits could explain these trends, (II) whether moth biomass has a bottom-up effect on insectivorous forest birds at the functional group level, and (III) whether such bottom-up effects can be detected at the species level. I used advanced statistical modelling techniques to address the aims. My results suggest that: (I) there has been no decline in the total biomass of moth functional groups across Finland in the past 27 years. Instead, biomass remained stable for most groups, while several were increasing. There was also considerable geographical variation in abundance trends. (II) Abundance fluctuations of insectivorous forest birds were positively associated with fluctuations in moth biomass, particularly among early-season moths in the north-boreal zone, where seasonal constraints on breeding are most pronounced. These bottom-up effects were observed for birds that were residents or long-distance migrants. The strength of these associations declined towards the south, indicating regional variation in the strength of trophic interactions. (III) At the species level, moth bottom-up effects on birds were generally weak and inconsistent, with a few weak but biologically meaningful patterns emerging in the north-boreal zone. Weak bottom-up effects imply that forest birds are more flexible and resilient in their foraging than previously assumed, enabling them to adapt to specific prey fluctuations. This dissertation emphasizes how integrating long-term biodiversity datasets can reveal community-level trophic interactions and provide new opportunities for understanding the effects of global change on ecosystem functioning.