Spiders and beetles as biological indicators for the assessment of ecological quality of rivers in their dry phase: A multi-metric index approach

• Dry-phase indicators are needed to achieve complete assessment of non-perennial rivers. • Spider and beetle communities inhabiting dry riverbeds and fringing riparian zones are sensitive to anthropogenic stressors. • Spiders and beetles in dry riverbeds are more sensitive to disturbances than those in riparian habitats. • The dry riverbed spider index performed the best assessment of ecological classification. Non-perennial streams and rivers, characterized by the presence of a dry phase, are widespread and abundant ecosystems worldwide, however, several research gaps for an effective assessment of their ecological status have been identified. Therefore, new tools that consider metrics for the dry riverbed phase to be applied to intermittent and ephemeral rivers are urgently needed. Here, we investigated the role of both terrestrial spider (Araneae) and beetle (Coleoptera) communities, at different taxonomic scales, as biological indicators of river health during the dry-phase and validate their use by the development of multi-metric indices in both dry riverbed and fringing riparian habitats. To this end, we sampled 24 non-perennial rivers located in a Mediterranean river basin (SE Spain) during the dry phase using pitfall traps. Our findings showed indicative taxa of anthropogenic disturbances identified for both invertebrate communities, yet spiders had a higher sensitivity than beetles. Both taxa groups exhibited habitat-specific responses to human disturbances, with dry riverbed communities markedly responded to disturbances compared to riparian communities. The multi-metrics index using the dry riverbed spiders performed the best classification of the ecological status of the study rivers, followed by riparian spider index. Conversely, the beetle index developed in dry riverbeds did not properly classify the study sites. Our findings support the incorporation of terrestrial invertebrate communities in general, and spiders of dry riverbeds in particular, for a comprehensive ecosystem monitoring and assessment in non-perennial fluvial ecosystems world-wide.