Data from: Local and regional drivers of taxonomic homogenization in stream communities along a land use gradient

Aim: The interaction of land use with local vs. regional processes driving
biological homogenization (β-diversity loss) is poorly understood. We
explored: i) stream β-diversity responses to land cover (forest vs.
agriculture) in terms of physicochemistry and physicochemical
heterogeneity, ii) whether these responses were constrained by the
regional species pool, i.e. γ-diversity, or local assembly processes
through local (α) diversity, iii) if local assembly operated through the
regional species abundance distribution (SAD) or intraspecific spatial
aggregation, and iv) the dependency on body size, dispersal capacity, and
trophic level (producer vs. consumer). Location: United States of America,
Canada, and France Time Period: 1993-2012 Major Taxa Studied: Stream
diatoms, insects, and fish Methods: We analyzed six datasets totaling
1,225 stream samples. We compared diversity responses to eutrophication
and physicochemical heterogeneity in forested vs. agricultural streams
with regression methods. Null models quantified contribution of local
assembly to β-diversity (β-deviance, βDEV) for both land covers and
partitioned it into fractions explained by the regional SAD (βSAD) vs.
aggregation (βAGG). Results: Eutrophication explained homogenization and
more uneven regional SADs across groups, but local and regional
biodiversity responses differed across taxa. βDEV was insensitive to land
use. βSAD largely exceeded βAGG and was higher in agriculture. Main
Conclusion: Eutrophication but not physicochemical heterogeneity of
agricultural streams underlay β-diversity loss in diatoms, insects and
fish. Agriculture did not constrain the magnitude of local vs. regional
effects on β-diversity, but controlled the local assembly mechanisms.
While the SAD fraction dominated in both land covers, it further increased
in agriculture at the expense of aggregation. Notably, the regional SADs
were more uneven in agriculture, exhibiting excess common species or
stronger dominance. Diatoms and insects diverged from fish in terms of
biodiversity, SAD shape, and βDEV patterns, suggesting an overriding role
of body size and/or dispersal capacity compared to trophic position.