Measures of exoskeleton aid in predicting elemental flows in arthropod predator-prey interactions

Total elemental content of prey is sometimes used as a measure of the nutrients available to predators during trophic interactions. Yet, different chemicals into which elements are incorporated can have different digestibility. This is especially true when considering the structural components of animals such as their skeleton or exoskeleton. We fed a variety of arthropod prey to spiders, measured the ingestion of carbon and nitrogen by the spiders, and then tested which measure of prey, total elemental content or metabolizable (i.e., non-exoskeletal) elemental content, was a better predictor of ingestion. Exoskeleton content of arthropods varied from 14 – 44 % and prior studies have shown that it is indigestible to spiders. Metabolizable dry mass content of prey (i.e., the dry mass of the non-exoskeleton part of prey) was a better predictor of dry mass consumption by spiders than was the total dry mass of the prey. Similarly, metabolizable N and C content of prey were better predictors of N and C consumption than were the total N and C content of the prey, respectively. Exoskeleton content of prey is relatively easy to measure and average values for a variety of arthropods are present in the literature. Using measures of exoskeleton content of prey to partition elements into structural versus metabolizable components may aid in better predicting how much of prey tissue will be discarded versus metabolized by predators.