Correspondence of biological condition models of California streams at statewide and regional scales

Abstract We used boosted regression trees (BRT) to model stream biological condition as measured by ben-thic macroinvertebrate taxonomic completeness, the ra-tio of observed to expected (O/E) taxa. Models were developed with and without exclusion of rare taxa at a site. BRT models are robust, requiring few assumptions compared with traditional modeling techniques such as multiple linear regression. The BRT models were con-structed to provide baseline support to stressor delinea-tion by identifying natural physiographic and human land use gradients affecting stream biological condition statewide and for eight ecological regions within the state, as part of the development of numerical biological objectives for California’s wadeable streams. Regions were defined on the basis of ecological, hydrologic, an