| Summary: | The shifting seasonal timing of key life history events, termed phenology, is one of the most widely observed ecological responses to climate warming. Such phenological shifts can have implications for the ability of populations to persist and the stability of species interactions. However, the relationship between phenology and climate is complex and re-quires standardized long-term monitoring data to elucidate patterns of change. This thesis investigates the phenological responses of arthropods to climate change by analyzing data from monitoring programs in high-Arctic Greenland and the Subantarctic Kerguelen Islands. The findings demonstrate earlier trends in arthropod activity, correlated with earlier snowmelt timing and warmer temperatures, although responses have slowed in recent years. Substantial variation in the magnitude and direction of phenological responses among arthropod taxa underscores the importance of functional roles and life-history traits to predict phenological responses, with body size and phenological niche influencing shifts in muscid fly species activity. As phenological responses are controlled by limits to phenotypic plasticity, this could give rise to thresholds in the linear nature of responses to climate change. However, this thesis finds little evidence of nonlinear patterns in arthropod phenology in response to environmental variation, although taxa appear to be approaching their limit in responses to earlier snowmelt. Furthermore, this research elucidates the usefulness of phenology in understanding the factors that drive the success and establishment of invasive species, exemplified by the introduction of a non-native blowfly species on Subantarctic Islands. This thesis contributes valuable insights into the complex phenological responses of arthropods to climate change.
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