eDNA from archived leaves reveals no losses of α-diversity, but widespread community turnover and biotic homogenization as drivers of forest insect decline

A major limitation of current reports on insect declines is the lack of
standardized, long-term, and taxonomically broad time series. Here, we
demonstrate the utility of environmental DNA from archived leaf material
to characterize plant-associated arthropod communities. We base our work
on several multi-decadal leaf time series from tree canopies in four land
use types, which were sampled as part of a long-term environmental
monitoring program across Germany. Using these highly standardized and
well-preserved samples, we analyze temporal changes of diversity and
relative biomass in communities of several thousand arthropod species
belonging to 23 orders using multilocus metabarcoding and quantitative
PCR. Our data do not support widespread declines of α-diversity or genetic
diversity within species. Instead, we find a gradual community turnover,
which results in temporal and spatial biotic homogenization. Colonization
by novel widespread taxa and loss of range-restricted taxa cause spatial
diversity loss. At the same time, increasingly similar species occurrences
between consecutive sampling years cause a temporal decline of diversity.
These effects are independent of land use and observed for all arthropod
orders. Our results suggest that insect decline is more complex than mere
α-diversity loss, but can be driven by β-diversity decay across space and
time.