The effects of taxonomic aggregation on network analysis

In their search for ever better trophic models, ecologists have often tried aggregating the number of species in order to better focus upon the variables that most interest them. Previous attempts at aggregating food webs have yielded varied results. We studied a series of different taxonomic aggregations on the same trophic network model of the Chesapeake Bay. The original 50-compartment model, which served as the control configuration, exhibited the highest value for the ascendency index. As expected, in those systems with fewer compartments the ascendency declined in monotonic fashion. The ascendency dropped precipitously for systems with fewer than 40 compartments and achieved its lowest value for systems with less than 29. Systems with the same number of compartments but different aggregations of species yielded different values of the ascendency. The aggregation of bacteria and ciliates resulted in a precipitous drop in the information of the network, revealing perhaps the significance of the microbial loop. Direct and indirect trophic impacts were also affected by the nature of the aggregation, and the impacts seemed to be exaggerated whenever species were lumped into single compartments.