Disentangling responses to natural stressor and human impact gradients in river ecosystems across Europe

1. Rivers are dynamic ecosystems in which both human impacts and
climate-driven drying events are increasingly common. These anthropogenic
and natural stressors interact to influence the biodiversity and
functioning of river ecosystems. Disentangling ecological responses to
these interacting stressors is necessary to guide management actions that
support ecosystems adapting to global change. 2. We analysed the
independent and interactive effects of human impacts and natural drying on
aquatic invertebrate communities—a key biotic group used to assess the
health of European freshwaters. We calculated biological response metrics
representing communities from 406 rivers in eight European countries:
taxonomic richness, functional richness and redundancy, and two
biomonitoring indices that indicate ecological status. We analysed metrics
based on the whole community and a group of taxa with traits promoting
resistance and/or resilience (‘high RR’) to drying. We also examined how
responses vary across Europe in relation to climatic aridity. 3. Most
community metrics decreased independently in response to impacts and
drying. A richness-independent biomonitoring index (the average score per
taxon; ASPT) showed particular potential for use in biomonitoring, and
should be considered alongside new metrics representing high RR diversity,
to promote accurate assessment of ecological status. 4. High RR taxonomic
richness responded only to impacts, not drying. However, these predictors
explained little variance in richness and other high RR metrics,
potentially due to low taxonomic richness. Metric responsiveness could
thus be enhanced by developing region-specific high RR groups comprising
sufficient taxa with sufficiently variable impact sensitivities to
indicate ecological status. 5. Synthesis and applications. Our results
inform recommendations guiding the development of metrics to assess the
ecological status of dynamic river ecosystems—including those that
sometimes dry—thus identifying priority sites requiring further
investigation to identify the stressors responsible for environmental
degradation. We recommend concurrent consideration of richness-independent
biomonitoring indices (such as an ASPT) and new high RR richness metrics
that characterize groups of resistant and resilient taxa for
region-specific river types. Interactions observed between aridity,
impacts and drying evidence that these new metrics should be adaptable,
promoting their ability to inform management actions that protect river
ecosystems responding to climate change.