Evaluation of Individual-Based Models of Different Complexity Applied to a Site-Specific Resource Management Question

Can complex models help meet the needs of resource managers to evaluate the ecological consequences of site-specific activities? We addressed the need to evaluate a specific diversion of a small stream by combining 4 years of biophysical monitoring with application of two spatially explicit individual-based trout models which simulated different levels of behavioral complexity. The diversion captured 24% of streamflow between June and October but had little effect over the remainder of the year. After calibration to the upstream control reach, both individual-based models predicted the observed difference in fish biomass between control and diversion reaches at the ends of the dry-seasons. Both models suggested the difference was attributable in part to differences in habitat structure, unrelated to streamflow, that favored the upstream reach. The two models also reproduced the small differences between reaches in individual growth, large seasonal differences in growth, and natural distributions of growth among individuals. In multi-year simulations, the model incorporating greater flexibility in fish behavior exhibited weaker population-level responses to more extreme reductions in dry-season streamflow. In this case, use of process-based models of varying complexity identified key uncertainties while offering the potential to forecast the consequences of future environmental alterations at the study site.