Nitrogen Content in Riparian Arthropods is Most Dependent on Allometry and Order

William D. Wiesenborn

Abstract


I investigated the contributions of body mass, order, family, and trophic level to nitrogen (N) content in riparian spiders and insects collected near the Colorado River in western Arizona. Most variation (97.2%) in N mass among arthropods was associated with the allometric effects of body mass. Nitrogen mass increased exponentially as body dry-mass increased. Significant variation (20.7%) in N mass adjusted for body mass was explained by arthropod order. Adjusted N mass was highest in Orthoptera, Hymenoptera, Araneae, and Odonata and lowest in Coleoptera. Classifying arthropods by family compared with order did not explain significantly more variation (22.1%) in N content. Herbivore, predator, and detritivore trophic-levels across orders explained little variation (4.3%) in N mass adjusted for body mass. Within orders, N content differed only among trophic levels of Diptera. Adjusted N mass was highest in predaceous flies, intermediate in detritivorous flies, and lowest in phytophagous flies. Nitrogen content in riparian spiders and insects is most dependent on allometry and order and least dependent on trophic level. I suggest the effects of allometry and order are due to exoskeleton thickness and composition. Foraging by vertebrate predators, such as insectivorous birds, may be affected by variation in N content among riparian arthropods.

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