Stable isotopes of saproxylic beetles reveal low differences among trophic guilds and suggest a high dependence on fungi

Deadwood stores about 8% of global carbon stock, and its decomposition is a key factor in forest ecosystems. Deadwood-associated (saproxylic) organisms constitute a food web that sustains a substantial part of biodiversity globally. After fungi, saproxylic beetles are the most prominent agents of structural deadwood decomposition in forests. They are often classified according to their presumed link to the deadwood decomposition gradient, generally as feeding on fresh wood, decayed wood, fungi, or predators. These classifications are, however, based on ecomorphological characters (e.g., trophic morphology, habitat use) while information on their diet is globally limited. Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios represent potential useful tracers to improve knowledge on the trophic ecology of this model group and the whole decomposition food web. We performed stable isotope analysis on 121 beetle species (530 samples) from a mixed-deciduous forest in Central Europe in order to (1) characterize drivers of saproxylic beetles' isotopic variability with respect to potential food sources along the wood decomposition gradient and in relation to the potentially key intrinsic factors such as phylogeny and body size and (2) to assess how isotope information matches with two trophic guild classifications based on ecomorphological characters which are commonly used in ecological studies. The analysis revealed a clear pattern of δ¹³C increase and simultaneous C:N ratio decrease across potential food sources along the gradient from fresh to decayed deadwood and fungi. Beetle phylogeny and body size explained a significant part of their isotope variability, with values of δ¹³C being lower in smaller species. After filtering out these effects, the δ¹³C values reflected the position of beetle species on the decomposition gradient only loosely. Fungi-feeding guilds had higher δ¹³C values than the guilds dependent on fresher deadwood, but otherwise the guilds were indistinguishable. Deadwood consumers did not differ from predators. The isotopic niches of different feeding guilds largely overlapped, and the large observed variation suggests that not only fungi feeders but species from most guilds may depend considerably on fungi and that mixed trophic strategies may be more common in the decomposition food web than currently acknowledged.