Micronutrients are drivers of abundance, richness, and composition of soil insect communities in tropical rainforests

Abstract

Communities of soil insects in tropical rainforests are among the richest and most complex, but the mechanisms structuring them remain mostly unknown. Identifying whether nutrient availability plays a relevant role in the assembly of these communities poses several challenges due to the diverse nutritional requirements of insects. We investigated the importance of nutrient availability accounting for the abundance, richness, and composition of soil insect communities in tropical rainforests. We sampled soil insects in 72 1-m² sampling points at two sites in French Guiana, counted all specimens, and characterized each assemblage using DNA metabarcoding. We then determined the importance of nutrient availability by measuring 19 nutrient concentrations and collected 18,000 specimens from 2634 operational taxonomic units (OTUs). Despite an extraordinary diversity and spatial heterogeneity, the concentrations of sodium, potassium, and magnesium positively correlated with either the abundance or the richness of the communities. These micronutrients were also important predictors of the composition of the assemblages. However, we found different relationships when analyzing the data separately for Blattodea, Coleoptera, Hemiptera, Hymenoptera, and Orthoptera, the most abundant insect orders with the most OTUs. Our results demonstrated that the availability of micronutrients played a large role in species selection during the assembly of the soil insect communities in these tropical rainforests, in contrast to the null impact of macronutrients. By accounting for the response at lower taxonomic levels, we argue that part of the unexplained variance might arise from contrastingly different responses to micronutrient availability among the most diverse orders. The high unexplained variance, however, also suggests that processes such as stochastic population drift and biotic interactions likely play complementary roles in structuring insect communities in the soils of tropical rainforests.