ABSTRACT
Summary A conceptual model was developed using a modified-state variable approach, which involved the recognition and suitable description of specific objectives, processes, and situations considered relevant to the simulation of ecosystem dynamics. Six primary ecosystem subsystems were selected: terrestrial abiotic, floral and faunal, aquatic abiotic and biotic, and topographic 486land use. These subsystems are embodied by a geographic structure defined as an ecological response unit (ERU). Subsystem characteristics of an ERU are considered homogenous, but between ERUs the characteristics may differ greatly. A set of ERUs may be arranged into any mosaic pattern, a quality that affords a unique response behavior to both the individual ERUs and the mosaic pattern. Discrete management events, such as controlled burning or livestock stocking, and continuous events, such as oil-shale mining, may be scheduled for a complex of ERUs. As the model operates “mechanistically,” the results are tractable, and they provide a framework for interpretation and discussion among resource managers. A computer-based simulation model of the conceptual model, named An Ecosystem Simulation of Perturbations (AESOP), has been developed to provide resource managers with a methodology for analyzing the future consequences of multiple-use management scenarios. The computer model provides pseudoquantitative ecosystem responses as a consequence of various management strategies. AESOP allows a user to specify many types of scenarios involving both ecosystem characteristics and management practices, which may vary in location, timing, and magnitude. AESOP was programed in the general-purpose computer language PASCAL because of the superior data-typing and program-structuring capabilities of the language, which allow model flexibility not possible with most models reported in the ecological literature. AESOP closely imitates the conceptual model through data-structuring techniques and derived mathematical functions representing ecosystem relationships. The block structure of the computer program provides ease in understanding and evaluation by the user and change or interchange of simulation procedures. AESOP generates time-series data of the components and processes of the ERUs and indices of subsystem conditions. A separate analysis program summarizes the voluminous output into statistical tables and graphs. The model is currently being tested by evaluating ecosystem response to multiple-use resource management of the sagebrush-grass areas in northwestern Colorado. This region is unique in its richness of energy-producing resources, its abundance of wild herbivores, and its livestock and agricultural practices. Through analysis of the results from various management-policy scenario simulations, “hindsight” is available to the resource manager at the time of decision-making.
