Beyond the Binary Web: Using Real-Time PCR to Quantify the Structure of a Spider-Dominated Food Web

Historically, food webs have been depicted as networks of binary interactions that show qualitative structure. Incorporating “weighted” data details interaction strengths and addresses a critique in food-web theory – that binary webs are too simple. Additional simplifications of published food webs include the aggregation of data across years/seasons and the aggregation of taxa into “functional groups”. This dissertation applies a molecular technique to the investigation of a food web to address the questions: 1) Is food web structure consistent across years and seasons? 2) How does foraging mode affect feeding interactions? 3) Are patterns of intra-guild predation (IGP) similar to interactions between predators and non-IGP prey? 4) Does examining interactions at a finer taxonomic resolution reveal divergent patterns in food-web structure? Real-time PCR was employed to examine predation within a model, spider-dominated food web. Eleven spider families were examined for the gut-content DNA of eleven potential prey. Food-web structure was analyzed using connectance, compartmentalization, weighted connectance, interaction evenness and whole-web specialization. The foraging mode of predators compared web-building to cursorial spiders and examined specialization. The examination of IGP compared the structure of IGP food webs to the structure of non-IGP webs. Examining interaction frequencies at a finer taxonomic resolution compared feeding patterns of families to those of genera with each family. Results showed the structure of the model food web was consistent across years but not seasons. Spiders displayed differences according to foraging mode - cursorial spiders showed wider diet breadth compared to web-builders. There were specialization differences between taxa, but this did not relate to foraging. Strong differences were seen seasonally when IGP interactions were compared to non-IGP interactions. Examining interaction frequencies at a finer taxonomic resolution revealed feeding patterns distinctly different from aggregating data to the family level. This dissertation demonstrates the importance of quantitative metrics in the examination of food-web structure. Temporal aggregation is sometimes sufficient to describe the structure of a food web, but this is not necessarily true for finer degrees of resolution. Finally, this research establishes that the aggregation of taxa, even at the family level, does not adequately describe food-web structure.