Consumer direct and indirect effects on biological soil crust function

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Shelby Rinehart, Biological Sciences, University of Alabama, Tuscaloosa, AL and Dror Hawlena, Ecology, Evolution and Behavior, The Hebrew University of Jerusalem, Jerusalem, Israel

Background/Question/Methods
Large and small-bodied consumers can influence ecosystem function by directly and indirectly affecting the productivity of vascular plants, algae, and other primary producers. However, to our knowledge, there have been no attempts to evaluate the impacts of meso-consumers on biological soil crusts— communities of cyanobacteria, fungi, moss, lichens, and algae that colonize the surface of soils. This is surprising, as biological soil crusts account for 4-13% of the global NPP of terrestrial vegetation. We sought to explore the direct and indirect effects of snail meso-consumers on biological soil crust function in the central Negev desert, Israel. Specifically, we evaluated the individual and combined effects of snail grazing, fecal deposition, and mucus deposition on biological soil crust carbon dynamics using two snail species that differed in their dietary preferences [i.e., crustivore (consume biological soil crusts) versus detritivore].
Results/Conclusions
We found that the individual effects of snail grazing, fecal deposition, and mucus deposition on biological soil crust function differed based on the identity (or diet) of the snail species. However, when we looked at the combined effects of snail meso-consumers in the field, these species (or diet)-specific differences diminished. Instead, we found that the addition of any snail to the environment resulted in biological soil crusts with lower carbon efflux rates, higher carbon fixation rates, and greater overall carbon storage. Our findings suggest that consumers can have profound effects on the carbon dynamics of biological soil crusts, and they highlight the need to develop a robust framework linking trophic dynamics to biological soil crust function. Ultimately, understanding how trophic interactions can affect biological soil crusts will inform predictions of ecosystem functions, like NPP and carbon cycling, across local, regional, and global scales.