Trophic positions of soil microarthropods in forests increase with elevation, but energy channels remain unchanged

Abstract

AbstractMountain forests are at risk as the consequences of climate change will likely lead to altered tree species boundaries. Characterizing food webs along elevation gradients in primary forests may help to predict the potential consequences of such changes, for example with regard to the decomposition of dead organic matter. Here, for the first time, we studied trophic variations in two species‐rich microarthropod taxa, Collembola and Oribatida, along an elevation gradient of primary forest at Changbai Mountain, China. Samples were taken at seven elevations of 150‐m elevational difference between 800 and 1700 m. At each elevation, Collembola and Oribatida were extracted from litter samples of eight subplots. We applied three state‐of‐the‐art methods to elucidate trophic positions and basal resource use at community level: Bulk stable isotope analysis of nitrogen (Δ15Nbulk) and carbon (Δ13Cbulk), compound‐specific stable isotope analysis of amino acids (CSIA‐AA), and dietary routing of neutral lipid fatty acids (NLFAs). Trophic positions calculated using Δ15Nbulk and CSIA‐AA (TPCSIA) in both taxa increased similarly with elevation by about half and one third of a trophic position, respectively. Stable isotope mixing models and linear discriminant analysis bootstrapping using δ13C of essential amino acids indicated fungi as the most important resource at all elevations for both taxa. Also, proportions of marker NLFAs changed little across elevations in both taxa; overall high proportions of linoleic acid indicated high fungal contributions, but in Collembola the contribution of bacterial markers was generally higher than in Oribatida. Δ13Cbulk did not respond linearly to the elevation gradient; however, changes in elevation differed between Collembola and Oribatida. A strong linear relationship between δ15N of phenylalanine and δ15N of litter indicated litter as the basis of energy channels in both taxa. Overall, food web functioning likely changes with changing forest types along elevation gradients, with microarthropods switching from feeding closer to the base of the food web at lower elevations to feeding at higher trophic levels at higher elevations, potentially compromising their role in litter decomposition and nutrient cycling.