Site-specific length-biomass relationships of arctic arthropod families are critical for accurate ecological inferences

Arthropods play an essential role in terrestrial ecosystems, not least by forming the food base for insectivorous birds. To better understand such trophic interactions, it is essential to monitor seasonal trajectories in arthropod biomass. Because obtaining direct measurements of the body mass of individual specimens is laborious, these data are often indirectly acquired by utilizing allometric length-biomass relationships based on a correlative parameter, such as body length. Studies on insectivorous birds have often used such relationships with a low taxonomic resolution and/or small sample size and/or adopted regressions calibrated in different biomes. Despite the scientific interest in the ecology of arctic arthropods, no site-specific family-level length-biomass relationships have hitherto been published. Here we present 27 family-specific length-biomass relationships from two sites in the High Arctic: Zackenberg in northeast Greenland and Knipovich in north Taimyr, Russia. We show that length-biomass regressions from different sites within the same biome did not affect phenology but did result in substantially different estimates of arthropod biomass. Estimates of daily biomass at Zackenberg were on average 24% higher when calculated using regressions for Knipovich compared to using regressions for Zackenberg. Our results illustrate that the use of allometric relationships from different sites can significantly alter the biological interpretation of, for instance, the interaction between insectivorous birds and their arthropod prey. We conclude that length-biomass relationships should be locally established rather than being based on global relationships.