Functional and taxonomic diversity of grasshoppers differentially shape above- and below-ground communities and their function

Authors: LUCAS, JANE - University Of Idaho; JONAS, JAYNE - Colorado State University; LAWS, ANGELA - University Of Houston; Branson, David - Dave; PENNINGS, STEVEN - University Of Houston; PRATHER, CHELSE - University Of Dayton; STRICKLAND, MICHAEL - University Of Idaho

Submitted to: Functional Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2020
Publication Date: 1/8/2021
Citation: Lucas, J.M., Jonas, J., Laws, A., Branson, D.H., Pennings, S.C., Prather, C.M., Strickland, M.S. 2021. Functional and taxonomic diversity of grasshoppers differentially shape above- and below-ground communities and their function. Functional Ecology. 35(1):167-180. https://doi.org/10.1111/1365-2435.13682.
DOI: https://doi.org/10.1111/1365-2435.13682

Interpretive Summary: We found that grasshopper species composition and functional diversity (i.e. what food they primarily feed on) have strong but different effects on above- and belowground communities. Grasshopper diet shifted composition and function in soil communities, while grasshopper composition only changed belowground functions. These results demonstrate that documenting herbivore functional diversity may be a more important for understanding how grasshoppers affect both plants and belowground organisms. We also found that grasshopper presence may increase carbon storage belowground, which is a previously underappreciated pathway of soil carbon sequestration. Additionally, we found that grasshopper effects were often mediated by fertilization. This study demonstrates the complex interactions that connect above- and belowground ecosystems, and highlights the need for further research on the effects of top-down versus bottom-up effects in prairie ecosystems.

Technical Abstract: Herbivores are important ecosystem drivers in both above- and belowground ecosystems. Despite their importance, studies of herbivores often focus on the effects of individual species, potentially missing the effects of herbivore diversity. Here we examine how orthopteran herbivore species and functional (i.e., diet) diversity interact with nutrient availability to shape above- and belowground ecosystems. We stocked six species of grasshoppers (Orthoptera) with two different feeding modes (i.e., grass-only or grass and forb feeders) at varying taxonomic compositions (0, 1 or 6 species) in field enclosures and measured their effects on aboveground plant biomass, chemistry, and richness, as well as belowground microbial community composition and function. We treated half the plots with nitrogen fertilizer to examine how herbivore effects may be mitigated by nutrient availability. Aboveground, we predicted that grasshoppers would decrease plant biomass and richness, and shift the chemical composition of the plants. We also predicted that high diversity plots would have the strongest shifts in plant biomass and richness, but these effects would be offset by fertilizer additions. Belowground, we predicted that herbivore taxonomic composition and diet would differentially shift soil microbial community composition and function. We also predicted that fertilization would interact with herbivore effects to shift belowground community composition. We found that grasshoppers decreased plant biomass and richness, and this effect was not dependent on nutrient additions. Contrary to our predictions, we did not see the largest aboveground changes in our high diversity plots. Belowground, herbivore diet and fertilization, but not their interaction, shifted bacterial communities. Taxonomic composition did not influence bacterial communities. Grasshopper taxonomic composition and diet interacted with fertilizer to increase belowground levels of bioavailable C and microbial biomass. Fungal communities did not respond to any treatments. The differing effects of herbivore diet versus taxonomic composition highlight the need for studies that examine multiple diversity metrics when exploring herbivore-meditated effects on above- and belowground ecosystems. Combined, our results suggest that bottom-up and top-down controls are important factors to consider when studying the composition and function of grassland ecosystems.