Does the taxonomic completeness of headwater stream assemblages reflect the conservation status of the riparian forest?

Headwater streams are intimately linked to their riparian zones, yet in conservation planning streams have attracted much less attention than riparian forests. We assessed the relation between the conservation status of riparian forests and taxonomic completeness (ratio of observed to expected taxa richness, O/E) of stream bryophytes and benthic invertebrates in fifty northern Finnish headwater streams. The streams represent three levels of riparian modification: pristine old-growth forests (Woodland Key Habitats, WKHs), slightly modified and strongly modified (mainly drainage ditching) forests. Macroinvertebrate O/E ratios were significantly, by about 40%, higher in natural than in modified sites. Bryophyte O/E ratios in strongly modified sites were about half of those in pristine sites (mean O/Es: 0.95 vs. 0.50, respectively) whereas the pristine and slightly modified sites did not differ. O/E ratios of both groups were negatively correlated with fine sediments, indicating drainage as the primary factor impairing stream condition. Eight streams (four pristine, four slightly modified) monitored through six years showed little temporal variability in taxonomic completeness. Nonetheless, inter-annual variability in O/E was sufficient to cause frequent misclassifications of stream status, slightly modified sites being frequently classified as no different from the reference. For macroinvertebrates, sites indicative of high conservation value (CV) were rather evenly distributed among the three status classes, whereas the bryophyte CV index was lower for the strongly modified than for pristine sites. Several sites defined degraded a priori scored high values for the CV index of macroinvertebrates. Conservation efforts focusing on small-scale key habitats may therefore be insufficient and need to be complemented by sites that remain beyond protected area networks (WKHs), yet support high conservation value in terms of rare taxa occurrence. Conclusions about the performance of protected area networks should not be based on a single taxonomic group as different groups may yield different assessment outcomes. We also suggest that sites that are in inferior ecological condition (O/E<1.0) yet support unexpectedly high numbers of rare taxa (CV>1.0) should be addressed as ‘restoration priority sites’ that might benefit from in-stream or riparian (or both) restoration.