Defining and measuring species interactions in aquatic ecosystems

Abstract

Based on synoptic values of observed biomasses of pairs of species in aquatic ecosystems, it is possible to distinguish types of interaction forms between the species (e.g., predation, competition, mutualism) and relate these to characteristics of their theoretical, prototype representations. For example, a distinguishing feature of observed predation was the counterclockwise rotation of time trajectories in phase space predicted by prey-predator theory. To characterize interactions, the phase portraits of the interactions were first constructed, i.e., sets of trajectories were identified by plotting biomass values for one species (or functional species group) on the x axis and the simultaneously observed value for the other species on the y axis. To characterize the phase portraits they were described by 14 ``form factors,'' each capturing particular features of the interaction patterns. In addition seven theoretical, or prototype, interactions were characterized. Comparisons, clustering, and multivariate calibration were performed with principal component analysis and partial least squares techniques.