Broad-scale evaluations of biodiversity change need reference comparisons.

Biodiversity trends in 4,062 European river invertebrate time series (1971–2024) depend on impact trajectory, with stronger, more consistent trends in initially more impacted communities.

• Why it matters: Broad-scale assessments can mix communities with different initial impacts and total change, potentially producing misleading trends even when all communities degrade or recover.
• What they did: The study linked biodiversity trends across 23 European countries to impact trajectories quantified by comparison to least-impacted references.
• The result: Trends were weaker and highly inconsistent when initial impacts were lower and total change was less extreme, implying comparisons should use communities with similar impacts.