Availability of nitrogen (N) for plant uptake is partially controlled by decomposition of thatch/leaf litter by soil microbes. Because live plants with high N content are preferentially fed upon by herbivorous insects, microbial decomposition can influence the population dynamics of herbivores by altering plant quality. Additionally, invertebrate predators are known to accumulate in areas of high thatch. Therefore, thatch may impact herbivore populations, both by its bottom-up influence on host-plant nutrition via the soil microbial community and by its top-down mediation of predator effects. We hypothesized that Spartina patches containing more thatch (high C:N) should have diminished plant quality and reduced herbivore populations compared to patches with less thatch (low C:N). To separate predator and microbial effects we manipulated both the C:N ratio of Spartina plants and thatch by adding carbon (sucrose), nitrogen (NH4NO3), and thatch in a completely randomized factorial field plot experiment. We measured abundance of the herbivorous insects Proklesia dolus and Proklesia marginata as well as plant quality and microbial nitrogen. We found that low C:N Spartina plots exhibit higher plant quality and herbivore abundance, whereas high C:N plots, those designed to enhance the microbial community, showed decreased plant quality and herbivore abundance compared to unmanipulated control plots. Thus, thatch may be driving herbivore dynamics on Spartina by indirectly altering both plant quality and predator aggregation. This study underscores an important linkage between above-ground food web interactions and below-ground soil processes.
Species 1: Homoptera Delphacidae Proklesia marginata
Species 2: Homoptera Delphacidae Proklesia dolus
Keywords: decomposition, nutrient cycling
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