Aquatic resource subsidies are more important than invasive lupine for ground-dwelling arthropod communities around Lake Mývatn

The effects of invasive plants on arthropod communities are highly variable, shaped not only by the resources these species introduce but also by the baseline availability of those resources in the invaded environment. In nutrient-poor ecosystems such as subarctic Iceland, nitrogen-fixing invaders may enhance litter quality and boost primary productivity, potentially benefiting arthropod communities. However, the extent of these effects likely depends on the presence of external nutrient inputs, such as aquatic resource subsidies. Interactions between plant invasions and aquatic subsidies likely have significant ecosystem consequences, yet their combined effects on arthropod communities remain poorly understood. Here, we examined how an invasive nitrogen-fixing lupine, Lupinus nootkatensis, and aquatic resource subsidy from the deposition of midge carcasses affected arthropod communities in subarctic heathlands. In 2013, we set up 2 × 2-m plots in native (“Heath,” n = 12) and lupine-invaded (“Invaded,” n = 12) heath vegetation and manipulated aquatic subsidies (control, inorganic KNO3, and midge carcasses) in experimental plots for 6 years (2013 – 2018). We sampled arthropods using pitfall traps and soil-core extractions via Berlese-Tullgren funnels. In both the Heath and Invaded plots, aquatic resource subsidies altered arthropod community composition, with the KNO3 and midge-addition plots containing more beetles and control plots containing more harvestmen and velvet mites. Increasing beetle densities were positively correlated with grass cover, whereas harvestmen and velvet mites were correlated with native heath shrubs (e.g., Betula nana and Vaccinium uliginosum). Lupine invasion had minimal effects on arthropod composition, as only the beetles were found in higher abundances in the Invaded plots. No other arthropod taxon responded to lupine invasion. These findings suggest that aquatic resource subsidies can influence arthropod communities in nutrient-poor ecosystems, but lupine invasion may be less important in determining arthropod community structure in these subarctic ecosystems.