Warming favours small-bodied organisms through enhanced reproduction and compositional shifts in belowground systems

Background/Question/Methods

The increased prevalence of smaller-bodied species under warmer conditions (community downsizing) is an ecologically critical consequence of climate change.  The mechanisms leading to ecological community downsizing is purported to arise from either top-down (non-random species loss) or bottom-up (metabolic and resource addition) trophic cascade responses. The main assumption in both top-down and bottom-up thermal cascade scenarios is that communities are size structured with respect to trophic level. Here I use an experimental mesocosm study of 100 intact soil monoliths from a northern peatland to observe changes in belowground soil biodiversity under three manipulated climate variables: elevated temperature, elevated atmospheric CO₂and lowered water table. Changes in species richness, abundance, community composition and body size spectra were used to determine whether climate change factors led to community downsizing, and elucidate any underlying mechanisms. I look for evidences of a correlated extinction risk in top predators and a corresponding trophic cascade resulting from non-random predator loss using predator : prey ratios.  I also examine whether bottom-up thermal cascades occur and whether this is manifest in an altered body size spectra through increases in the abundance of small-bodied species and / or increased developmental rates as measured by juvenile : adult ratios.

Results/Conclusions

Warming was the primary driver of compositional shifts in belowground fauna communities, with the strongest effect among the smaller-bodied, non-sexually reproducing species. Increases in abundance driven by enhanced reproduction in low trophic level species rather than an increased extinction rate among large-bodied predators was the basis of the observed downsizing. Warming-induced changes in belowground systems are expected to have cascading effects on microbial populations that could affect rates of decomposition and C release.