Is the River Environment Classification an improved landscape-scale classification of rivers?

Abstract Landscape classifications, such as ecoregions, are widely promoted as spatial frameworks for aquatic conservation and environmental management. The River Environment Classification (REC) is an alternative approach to landscape-scale river classification. The REC classifies individual sections of river networks according to the climate, topography, geology, and land cover of the unique upstream watershed of each section. The classification is hierarchical and is based on the assumption that climate, topography, geology, and land cover are dominant causes of spatial variation in physical and biological characteristics of rivers at 4 spatial scales. The ability of the REC to explain variation in invertebrate samples collected from 237 sites in New Zealand was compared to that of region-based classifications using classification strength tests. Classifications with differing numbers of classes were compared using relative classification strengths calculated by expressing absolute classification strength as a proportion of the classification strength reached by an a posteriori classification with the same number of classes. The a posteriori classifications were derived by clustering sites by their invertebrate assemblage, so the classifications were close to the maximum possible strength attainable for a set number of classes. The relative classification strength of the REC was compared with an existing ecoregion classification and with classes that were defined by clustering sites based on intersite distances. The relative classification strength of the REC was higher (>55% of maximum attainable) than the strengths of classifications based on ecoregions (45%) or distance-based classes (<47%) when presence/absence data were used. The relative classification strength of the REC also was higher (>28% of maximum atainable) than the strengths of classifications based on ecoregions (22%) or distance-basedt classes (<24%) when relative abundance data were used. The higher classification strength of the REC was attributed to its explicit consideration of the causes of spatial variation in ecological characteristics of rivers. In addition, the REC was more likely than ecoregion classification to differentiate among geographically close sites that had contrasting watershed conditions because the REC represented the network spatial structure of rivers. However, the classification strength of the REC was low, indicating that it provided a general representation of ecological patterns, but could not be used to predict assemblage characteristics reliably for a specific site.