A Hierarchical Model of Lotic Ecosystems

Abstract

This paper presents the structure and properties of a total stream model that simulates the dynamics of small, flowing—water ecosystems in the northwestern United States. Conceptually, the model is hierarchically structured, consisting of 7 basic processes: periphyton dynamics, grazing, shredding, collecting, invertebrate predation, vertebrate predation, and detrital conditioning. These processes are subprocesses of 3 echelons of higher level processes: detritivory; herbivory; primary consumption; predation; and the total ecosystem. The model has 14 state variables in the 7 basic processes, and is conceptualized in discrete time with a basic time step corresponding to 1 day. Behavior of the stream model relative to different schedules of energy inputs and to the practice of clear—cut logging was investigated and related to contemporary theory of lotic ecosystems. In general, model behavior indicated that the regulation of biological processes in streams is complex, the mechanisms of which vary seasonally and from process to process. If a process is regulated by food supply, its annual production tends to increase as predation increases, while mean biomass may or may not be affected appreciably. In contrast, an increase in predation tends to decrease both mean biomass and annual production in processes regulated primarily by predation and such life history phenomena as insects emergence. The stream model provided the stimulus that led to a mathematical expression for the rate of production at the level of the entire ecosystem, and model behavior suggests that this rate tends to remain constant along a continuum from small, first—order streams with no tributaries to larger rivers which eventually drain into the sea.