Data from: A new composite abundance metric detects stream fish declines and community homogenization during six decades of invasions

Aim: We developed a new technique, utilizing species-specific counts of
individuals from historical fish community samples, to examine
landscape-level, spatiotemporal trends in relative abundance
distributions. Abundance-based historical distribution analyses are often
plagued by data comparability issues, but provide critical information
about community composition trends inaccessible to those using analyses
based only on species presence-absence. We established trends in native
and non-native fish abundance and community homogenization, uniqueness,
and diversity to help local conservation managers prioritize targets and
motivate similar studies globally to support fish conservation.
Location:  Upper and middle New River (UMNR) basin, Appalachian
Mountains, USA. Methods: We compiled catch data from 61 years of fish
community surveys (1958-2019) and tested for community homogenization by
comparing data from repeatedly sampled sites (1900s versus 2000s samples)
using dispersion analyses. We measured community uniqueness (site
contributions to beta diversity) and species diversity (Shannon index) at
sampled streams to identify potential conservation hotspots. We then used
regression analyses and Wilcoxon signed-rank tests to examine
species-specific basin-wide and local abundance trends and identify
species of potential conservation concern. Results: Dispersion of sites in
species-abundance space was significantly greater in the 1900s compared to
the 2000s, indicating homogenization had occurred. Of 36 native species
analyzed, 44.4% (16) showed basin-wide declines. Non-native species
exhibited mixed patterns; site-level abundance increased in 2 of 15
species analyzed (13%). Main conclusions: Our results indicate basin-wide
community homogenization has occurred within the UMNR, but many unique and
diverse communities persist. If conserved, these could help maintain
regional fish diversity. We found basin-wide declines in four endemic
species, as well as spread patterns of non-native and native species that
were not detected by a presence-absence analysis applied within the same
study area. This finding illustrates the importance of considering both
species’ abundance and occurrence patterns as separate dimensions of
biodiversity to inform conservation planning.