Abstract
The soil food web is often described as having three main energy channels: root, bacterial and fungal. Here we provide quantitative data using a sensitive stable isotope ratio mass spectrometry procedure with microcosms on species interactions in the fungal pathway. We measured 15N and 13C enrichment in microarthropods through grazing rare isotope enriched fungal mycelia. Experimental treatments were various combinations of 1, 2, 3, 4 microarthropods species. We used three fungivores (the collembolan Lepidocyrtus curvicollis, the Astigmata Tyrophagus putrescentiae, the Oribatida Oribatula tibialis), and the Mesostigmata predator Hypoaspis acquilifer. We collected individuals of each species separately, as well as their feces, and molt where available. All three fungivorous microarthropods consumed significantly more than their own body weight per day. The three fungivores differed in their consumption of the mycelium as it was not equally palatable to each. The Mesostigmata predator Hypoaspis also differed in its microarthropod prey preference. In multiple species combinations microarthropod behavioral interactions modified consumption and predation rates. Our selection of mites of different sizes, with varied preference for the mycelium, combined with differing predation rates on each mite, demonstrate that even three trophic level interactions with only five interacting species are not predictably simple. The interpretation of the stable isotope results and consumed-excreted weights indicate that: (a) behavior and microscopic observations should not be ignored in competition-predation interactions, and (b) functional guilds can take advantage of more diverse food opportunities. The reality of mixed diets complicates functional guild assignments that are reflected in 15N and 13C isotope levels at natural abundances in the environment. Microcosm experiments with this sensitive technique can help decipher the interpretation of rare isotope natural abundance values, as well as providing measured consumption, growth, and excretion rate values for modeling soil food web interactions.
Highlights
Soil food web models typically recognize three main routes energy can flow through the soil – roots, the fungal or bacterial pathway (Moore et al, 2005)
A number of studies employing stable isotopes focused on quantifying the bacterial energy pathway (e.g., Murray et al, 2009; Crotty et al, 2011b, 2012a), and here this study focuses on the fungal pathway
There were no significant differences in microarthropod weight when comparing unlabelled mycelium with highly enriched mycelium between the two experiments after 21 days (P = 0.073)
Summary
Soil food web models typically recognize three main routes energy can flow through the soil – roots, the fungal or bacterial pathway (Moore et al, 2005). It makes allusion to the appearance in graphs of successive reduced and increased biomass in the trophic levels in the system These increasing and decreasing consumption rates oscillate through time and ripple through generations and are one cause of population dynamics. Several additional papers discussed fungivory by soil microarthropods (e.g., Maraun et al, 2003; Schneider et al, 2004; Pollierer et al, 2009; Thiele-Bruhn et al, 2012), but few tried to quantify rates of hyphae consumption except for individual species, such as Collembola (Jonas et al, 2007), while others focused on feeding preferences (e.g., Ruess et al, 2005; Koukol et al, 2009; Semenina and Tiunov, 2011). The difficulty has been in designing microcosms with compatible species, and to develop a technique with sufficient sensitivity to work with the small number of individuals in a microcosm experiment
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