Multi-decadal improvements in the ecological quality of European rivers are not consistently reflected in biodiversity metrics
Résumé
Humans impact terrestrial, marine and freshwater ecosystems, yet many broad-scale studies have found no systematic, negative biodiversity changes (for example, decreasing abundance or taxon richness). Here we show that mixed biodiversity responses may arise because community metrics show variable responses to anthropogenic impacts across broad spatial scales. We first quantified temporal trends in anthropogenic impacts for 1,365 riverine invertebrate communities from 23 European countries, based on similarity to least-impacted reference communities. Reference comparisons provide necessary, but often missing, baselines for evaluating whether communities are negatively impacted or have improved (less or more similar, respectively). We then determined whether changing impacts were consistently reflected in metrics of community abundance, taxon richness, evenness and composition. Invertebrate communities improved, that is, became more similar to reference conditions, from 1992 until the 2010s, after which improvements plateaued. Improvements were generally reflected by higher taxon richness, providing evidence that certain community metrics can broadly indicate anthropogenic impacts. However, richness responses were highly variable among sites, and we found no consistent responses in community abundance, evenness or composition. These findings suggest that, without sufficient data and careful metric selection, many common community metrics cannot reliably reflect anthropogenic impacts, helping explain the prevalence of mixed biodiversity trends.
Reports of human-driven species extinctions 1,2 and environmental change 3-5 indicate widespread degradation of Earth's ecosystems, particularly freshwaters 6 . However, a growing number of continental-and global-scale temporal studies of freshwater, terrestrial and marine communities have found no evidence of systematic, negative biodiversity changes 7-15 , instead reporting a mixture of negative, positive and neutral changes. Such studies typically infer that negative biodiversity changes (often defined as declining abundance or taxon richness) indicate anthropogenic degradation 8,16,17 , whereas positive changes indicate improving environmental quality 13,18,19 . Studies finding mixed biodiversity changes therefore suggest a balance of degradation, improvement and no change 7,9,11,12 . These studies have spurred debate about whether anthropogenic impacts are truly mixed 20 and the role methodological issues play in producing mixed biodiversity trends, including issues of poor data quality, quantity and representativeness 21-23 . One unaddressed explanation for the prevalence of mixed biodiversity trends is that common metrics used to summarize community change, such as abundance or taxon richness, cannot reliably indicate
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