Warming reverses top-down effects of predators on belowground ecosystem function in Arctic tundra

Abstract

Predators can disproportionately impact the structure and function of ecosystems relative to their biomass. These effects may be exacerbated under warming in ecosystems like the Arctic, where the number and diversity of predators are low and small shifts in community interactions can alter carbon cycle feedbacks. Here, we show that warming alters the effects of wolf spiders, a dominant tundra predator, on belowground litter decomposition. Specifically, while high densities of wolf spiders result in faster litter decomposition under ambient temperatures, they result, instead, in slower decomposition under warming. Higher spider densities are also associated with elevated levels of available soil nitrogen, potentially benefiting plant production. Changes in decomposition rates under increased wolf spider densities are accompanied by trends toward fewer fungivorous Collembola under ambient temperatures and more Collembola under warming, suggesting that Collembola mediate the indirect effects of wolf spiders on decomposition. The unexpected reversal of wolf spider effects on Collembola and decomposition suggest that in some cases, warming does not simply alter the strength of top-down effects but, instead, induces a different trophic cascade altogether. Our results indicate that climate change-induced effects on predators can cascade through other trophic levels, alter critical ecosystem functions, and potentially lead to climate feedbacks with important global implications. Moreover, given the expected increase in wolf spider densities with climate change, our findings suggest that the observed cascading effects of this common predator on detrital processes could potentially buffer concurrent changes in decomposition rates.

Keywords: Arctic; aboveground–belowground; decomposition; predator; trophic interactions.

Fig. 1.

Interactive treatment effects of altered wolf spider densities and warming on Collembola densities (A; individuals per cubic centimeter) and surface-active intermediate predators (B; log average total abundance per plot) within the experimental plots. Blue circles are from ambient temperature plots, and red triangles are from the experimentally warmed plots. Points are mean treatment effects, and error bars are SEs. Full model results are shown in Table 1 and SI Appendix, Table S5.

Fig. 2.

Effects of altered wolf spider densities and experimental warming on decomposition in the litter layer (A; percent litter mass remaining from the initial amount), percent N remaining in litter (B; in relation to initial N content of litter), and total available soil N (C; milligrams of NH₄⁺ per 10 cm⁻²) over the peak 6-wk summer season. Blue circles are from ambient temperature plots, and red triangles are from the experimentally warmed plots. Points are mean treatment effects, and error bars are SEs. Solid lines in A represent a significant interaction between the treatments, and dotted lines in B indicate a nonsignificant interaction. Full model results are shown in Table 2 and SI Appendix, Tables S3 and S5.

Fig. 3.

(A) Conceptualization of the hypothesized pathways by which warming alters the cascading effects of wolf spiders on decomposition via fungivorous Collembola. Solid lines denote direct effects of the wolf spiders, and dashed lines indicate indirect effects of the wolf spiders. Positive trophic effects are shown in green, and negative effects are shown in brown. (B) Mean (±SE) effect sizes of high wolf spider densities on Collembola, decomposition (belowground litter mass loss), percent N remaining in litter, and available soil N (total milligrams of NH₄⁺ per 10 cm⁻²) under ambient temperatures (shown in blue) and experimental warming (shown in red) across the five experimental blocks. The log₁₀ ratios are comparisons between plots with low vs. high densities of wolf spiders; positive log ratios₁₀ indicate a positive effect of high wolf spider densities on the response variable.