| Summary: | Climate change has the potential to disrupt phenological synchronization between parasitic nematodes and their hosts, which may have implications for host regulation. Understanding the parasitic-climatic relations within ecosystems that are vulnerable to climate change is crucial for disease management and conservation. Svalbard, an inherently simple ecosystem with a high susceptibility to climate change, serves as an ideal site to study the dynamic between parasites and hosts regarding climate. Historically the parasitic nematode Ostertagia gruehneri has regulated Svalbard reindeer (Rangifer tarandus platyrhynchus) populations by reducing fat reserves, thereby lowering fecundity, and limiting population growth. However, in the last two decades, reindeer populations have more than doubled, raising questions about potential changes in the parasites’ life cycle. This study examines two potential factors that may explain the lack of regulatory impact on host populations due to changes in infection risk. First, a dynamic simulation model based on known parameters that govern nematode survival and developmental rates was used to examine whether the potential negative effects of warmer temperatures outweigh the positive effects of reindeer density, on seasonal abundance of the infective nematode stages in the pasture. Historical (1997), present (2022), and potential future (2070-2100) temperature data were examined alongside observed reindeer densities (1995-2021). Secondly, these simulated seasonal curves were compared to empirical grazing height data throughout the growing season (2022), to examine if a phenological mismatch may have occurred from reindeer feeding at higher stratums during peaks in nematode availability, as feeding near moss levels may increase infection rate. This study revealed that temperatures representing the warmest year (2022) had a modest adverse impact on the availability of parasites in contrast to the coldest year (1997) when the reindeer population was held stable in the model. However, ...
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