Body size structure of soil fauna along geographic and temporal gradients of precipitation in grasslands

Precipitation is a global driver of animal abundance and diversity in terrestrial ecosystems, but we know little on how it influences the body size structure of invertebrate communities, particularly soil fauna. It is unclear whether aridity limits the abundance of large-bodied soil invertebrates, and whether temporal precipitation changes can induce local shifts. The potential role of trophic identity in modulating such relationships is also unexplored. Here we investigate the community-weighted mean body mass of soil nematodes, globally widespread invertebrates, in a two-year manipulative experiment with experimental drought and increased rainfall, conducted in three North American grassland ecosystems ranging from arid to semiarid and mesic conditions. We predict that community-weighted mean increases along with precipitation, and that long-term aridity prevents body size shifts in response to within-site temporal changes in precipitation. Nematode community-weighted mean mass increased from arid to mesic conditions. Altered precipitation within sites had much weaker effects, and none detectable in the arid site. When analysing community-weighted mean mass separately by feeding group, only plant-feeding nematodes responded positively to rainfall addition in semiarid and mesic conditions, as well as to ambient interannual differences consistently across sites, presumably due to a tighter ecological linkage with primary production. Our findings point to aridity as an environmental filter against large-bodied soil nematodes, with limited potential for community body size shifts in response to extremely dry or wet years. However, the different responses of plant-feeding taxa from other functional groups indicate potential mismatches between morphologically similar taxa in multitrophic invertebrate communities.