Understanding controls on carbon cycling in the Arctic is critical due to the rapid pace at which this region is warming and its potential feedbacks on the global climate. However, our knowledge of these dynamics is incomplete, because studies have focused on plants, detritus, and microbes and largely ignored their consumers. Microbial diversity, composition, and activity can be influenced by consumers (e.g., micro- and macro-invertebrates), and those consumers themselves are regulated by predators. Wolf spiders are dominant tundra predators that have cascading effects on detritivore abundances, decomposition rates, and N cycling, but the role of the microbial community in mediating these impacts is unclear. We tested the effects of wolf spider density (ambient, reduced, or high density) and warming (ambient temperature or warmed) on bacterial and fungal diversity and community composition through a fully factorial mesocosm experiment in arctic tundra over two summers. We deployed two sets of replicate litter bags at the soil surface and buried in the upper organic horizon and collected the litter bags after two and 14-months. Litter subsamples were saved for microbial DNA extraction before litter was assessed for mass loss. We prepared Illumina libraries by PCR amplification of bacterial 16S and fungal ITS and processed the sequences via QIIME.
Results/Conclusions
After two-months incubation, there were no treatment effects on fungal diversity, but both surface and buried litter in warmed plots had significantly reduced bacterial diversity. After 14-months, however, there were significant interactive effects of wolf spider density and warming on bacterial and fungal diversity. Specifically, under ambient temperature, bacterial diversity increased with increasing spider density, whereas in experimentally warmed plots, higher spider density reduced bacterial diversity both at the soil surface and in buried litter. Treatment effects on fungal diversity varied by location. At the surface, fungal diversity increased with increasing spider density, but this trend was opposite under warming. In buried litter bags, fungal diversity was reduced in low and high spider density treatments compared to the control, whereas under warming, fungal diversity tended to decrease with increasing spider density. Wolf spider density and warming also had interactive effects on structuring the composition of both fungal and bacterial communities after two-months and on fungal community composition after 14-months. Overall, our results are consistent with the idea that predators can have indirect, cascading effects on microbial communities and suggest that wolf spiders play an underappreciated role in influencing rates of carbon loss from arctic ecosystems.