Integrating network topology metrics into studies of catchment-level effects on habitat diversity

The spatial arrangement of the river network is a fundamental characteristic of the catchment, acting as a conduit between catchment-level effects and in-channel morphology and ecology. Yet river network structure is often simplified to reflect an up-to-downstream gradient of in-channel features, commonly represented by stream order. The aim of this study is to quantify network topological structure using new metrics – distance network density and elevation network density – that better account for the multi-dimensional nature of the catchment and which are functionally applicable across geomorphological, hydrological and ecological attributes of the catchment. The functional utility of the metrics in explaining patterns of physical habitat diversity is assessed in comparison to stream order. The metrics are calculated for four low-energy, anthropogenically modified catchments in the UK and compared to a physical habitat diversity score derived from England’s River Habitat Survey. The results indicate that the new metrics offer a richer, and functionally more-relevant description of network topology than stream order, highlighting differences in the density and spatial arrangement of each catchment’s internal network structure. Correlations between the new metrics and physical habitat diversity score show that distance network density is positively related to maximum habitat diversity in three of the four catchments. There is also evidence that increased distance network density may reduce minimum habitat diversity in catchments with greater anthropogenic modification. When all catchments are combined, distance network density is positively correlated with maximum, mean and minimum habitat diversity. There are no significant correlations between elevation network density and habitat diversity. In all but the largest streams, there is no significant relation between habitat diversity and stream order highlighting the limitations of stream order in accounting for network topology. Overall, the results suggest that distance network density is a more powerful metric which conceptually provides an improved method of accounting for the impacts of network topology on the fluvial system exhibiting strong relationships with habitat diversity, particularly maximum habitat diversity.