Activity density measures the flow of individuals and biomass through a landscape. A variety of methods can quantify it: from pitfall traps to malaise traps, camera traps to radar. Activity density is an important correlate of the ability of organisms to disperse through and perform work on their ecosystem. A key mission of NEON, the National Ecological Observatory Network, is to quantify and monitor invertebrate activity density across North America over a 30-y period. NEON does so using arrays of pitfall traps, sampled biweekly through the growing season. Here we report on the first analyses of 120 NEON pitfall arrays sampled over up to 5 years, from Alaska to Puerto Rico, with habitats including Deciduous and Evergreen forest, Grasslands, and Scrub.
We use these data to evaluate a model that assumes Activity Density has two components: biomass (constrained by the ecosystem’s Net Primary Productivity) and velocity (constrained by temperature and the permeability of the environment). All three drivers interact and play out differently based on habitat. Permeability—the ability of an insect to move unimpeded across the soil surface—tends to decrease with NPP as plants inhibit invertebrate movement even as they produce more food. Likewise, temperatures experienced by an insect tend to decrease with NPP, given that shading inhibits radiative heating.
Results/Conclusions
To explore the relationship between density, body size, and biovolume, we used image analysis to quantify the abundance and size distribution of invertebrates for a subset of traps. Mean size increased with biovolume, and density was a positive decelerating function of same. We are currently testing if local biovolume declines are thus biased toward large individuals.
Across 120 arrays in North America, average activity biovolume varied 31-fold from 0.09 ml/day in the scrub of central Alaska to 2.8 ml/day in cropland of Colorado. Despite widely ranging bioclimatic variables, with NPP varying 153-fold and annual growing season temperature varying 2.3-fold, NPP and temperature, did not shape activity biovolume uniformly across the continent. Instead, as predicted, these drivers behaved differently with habitat type. For example, in open scrublands, activity biovolume increased monotonically with temperature--reflecting the impacts of insolation on soil surface temperatures--and decreased with NPP as more plant barriers to lateral movement accumulated.
Finally, across the arrays, there is little evidence for consistent trends in activity biovolume over the 5-year sample period, although these data remain preliminary.