Our study characterized the aboveground (ground- and canopy-dwelling) arthropod community response to an uncommonly long-term (24-year) nutrient addition experiment in moist acidic tundra. This experiment altered plant community composition, decreased plant diversity and increased plant production and biomass as a deciduous shrub (Betula nana) displaced graminoids, cryptograms and dwarf evergreens. A simple bottom-up view would suggest that the arthropod community in fertilized plots would, like the plant community, differ in composition, support fewer species, and hold greater abundance and biomass relative to controls.
Fertilization did alter arthropod community composition, but with negligible changes to total taxonomic richness. Also contrary to our hypotheses, we found that although arthropods were larger in fertilized plots relative to controls, total arthropod abundance and biomass was either unchanged or decreased by fertilization. Our results differ from the handful of terrestrial studies that have found bottom-up consumer community responses to nutrient addition—perhaps because unlike grasslands and salt marshes (where such studies have been performed) the dominance of the deciduous shrub Betula in fertilized moist acidic tundra decreases plant palatability. Additionally, by displacing insulating mosses and cooling the canopy microenvironment, Betula may displace arthropods keenly attuned to seek out warm microclimates. These results suggest that terrestrial arthropod assemblages may be more constrained by producer traits (i.e., resource edibility, structure) than they are by total primary production or diversity. The observed changes in overall community composition in experimentally fertilized tundra are consistent with observations of climate-change associated increases in greater woody shrub dominance across the arctic tundra. Our results suggest that such shrub encroachment will alter the composition of arctic arthropod communities, and potentially, arthropod-mediated ecosystem processes.