Comparative assessment of satellite- and drone-based vegetation indices to predict arthropod biomass in shrub-steppes

Abstract

Arthropod biomass is a key element in ecosystems functionality, and basic food item for many species, which must be estimated through traditional costly field sampling in normally just a few sampling points. Arthropod biomass and plant productivity should be narrowly related, as a great majority of arthropods are herbivorous, and others depends on these. Quantifying plant productivity with satellite or aerial vehicles imagery is an easy and fast procedure already tested and implemented in agriculture and field ecology. However, the ability of satellite or aerial vehicles imagery for quantifying arthropod biomass and its relationship with plant productivity has been scarcely addressed. Here, we used Unmanned Aerial Vehicle (UAV) and satellite Sentinel-2 (S2) imagery to establish relationship between plant productivity and arthropod biomass estimated through ground-truth field sampling in shrub-steppes. We UAV sampled 7 plots of 47.6-72.3 ha at 4 cm pixel resolution, and afterwards downscaling spatial resolution to 50 cm resolution. In parallel, we used S2 imagery from same and other dates and locations at 10 m spatial resolution. We related several vegetation indices (VI) with arthropod biomass (epigeous, coprophagous, and four functional consumer groups: predatory, detritivore, phytophagous and diverse) estimated in 41-48 sampling stations for UAV flying plots, and in 67-79 sampling stations for S2. VI derived from UAV were consistently and positively related with all arthropod biomass groups. Three out of seven, and six out of seven S2-derived VI were positively related with epigeous and coprophagous arthropod biomass, respectively. BNDVI and ENDVI showed consistent and positive relationships with arthropod biomass, regardless of the arthropod group and the spatial resolution. Our results point out that UAV and S2-VI imagery data may be a viable and cost-efficient alternative to quantify arthropod biomass at large scales in shrub-steppes. The relationship between VI and arthropod biomass is probably habitat-dependent and thus, future research should address this relationship including several habitats to validate vegetation indices as proxies of arthropod biomass.