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Indirect bottom-up effects of microbe decomposers on herbivorous insects: linking ecosystem process with food web interactions.
Hines, Jessica*,1, Denno, Robert1, 1 Department of Entomology, College Park, MD
ABSTRACT- Availability of nitrogen (N) for plant uptake is partially controlled by decomposition of 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. On mid-Atlantic salt marshes Spartina alterniflora leaf litter, or thatch, has a patchy distribution, and is produced both on site and as an allochthonous input. Additionally, invertebrate predators are known to accumulate in areas of high thatch. Therefore, thatch may play an important role in impacting 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 the abundance of the herbivorous insects Proklesia dolus and Proklesia marginata (sap-feeding planthoppers) as well as plant performance and quality (height, biomass, and N content). We found that low C:N Spartina plots exhibited 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 non-manipulated controls. Thus, thatch may be indirectly driving herbivore dynamics on Spartina by indirectly altering both plant quality (via an enhanced microbial community) and predator aggregation. This study underscores an important linkage between above-ground and soil processes.
KEY WORDS: trophic structure, bottom-up, herbivore, decomposition