PS 5-68 - Geographic gradients in a functional trait: drivers of body size and size diversity of ground invertebrate communities

Abstract: Body size is a key functional trait governing how an animal community transforms resources and conditions into performance, abundance, and fitness. Our goal was to use the NEON network of pitfall traps to explore how an ecosystem’s plant productivity, temperature, and seasonality accounts for the range of body size across 99 ground invertebrate communities. Our approach combined image analysis to quantify body size of individuals, then characterize each community by its mean and CV in body size (mm²). Here we show that the 19-fold continental variation in mean body size failed to co-vary with latitude, while common ordinal sub taxa grew smaller (e.g., myriapods), to larger (e.g., acari) from Puerto Rico to Alaska. Communities with a larger mean size arose when winters were longer and Gross Primary Productivity was high. The diversity of body sizes in a community (measured as the Coefficient of Variation) varied 9-fold and decreased with latitude (r²=0.24) consistently across common orders. Size-diverse communities were less likely in ecosystems with long winters (suggesting constraints on the time to build, r²=0.34) and those with high invertebrate activity (and hence trap catch, r²=0.12). In conclusion, body size distributions appear to arise from conflicting combinations of constraint (i.e., the ability to build large bodies) and performance (utility of large size in surviving long winters). As warming promotes growing season length, populations of larger, rarer individuals may benefit.