Underlying seasonal shifts determine the effect of long-term nutrient addition on arctic tundra arthropod communities

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In many systems, fertilization (press nutrient addition) experiments have elucidated the ecological controls on plant production. Less-studied is the response of animal consumers to nutrient addition. To that end, we characterized the aboveground arthropod community response to a 24-year fertilization experiment in moist acidic tundra (MAT) at Toolik Lake, Alaska in 2013. Fertilization of MAT doubled plant production and biomass, doubled the height of the plant canopy and increased standing litter biomass tenfold. Fertilization also changed the plant community compositionally from a relatively even mix of evergreens, shrubs, graminoids and forbs, to a community dominated by a single species of deciduous shrub and a forb. Dominance of deciduous plants in fertilized plots resulted in later plant canopy green-up (as measured by NDVI) but higher mid- to late-season peak greenness. Where we expected to find an equally dramatic change in the biomass, abundance and composition of the arthropod community, we instead found subtle changes. Arthropod communities in fertilized plots differed from controls, but only when the underlying seasonal turnover in the arthropod community was taken into account. Similarly, treatment differences in the total biomass and abundance of arthropods were not uniform across the season, as they also depended upon sampling date. The same pattern held for certain functional groups, including herbivores and predators. Even with this relatively species-poor arctic arthropod fauna, a compositional reshuffling likely buffered the aggregate responses of this arthropod community to nutrient addition. Our results suggest that the aboveground arctic arthropod community may be functionally resilient to increased nutrient availability, which is an expected consequence of climate change in the arctic region.