Increasing synchrony opposes stabilizing effects of species richness on terrestrial communities

Aim: Ecological theory has predicted that species richness should
stabilize communities, with mechanisms including species synchrony and
population variability determining the net impacts. While these theories
have been supported empirically, results can be sensitive to taxonomic
bias as studies are often focused on plants. Trophic differences between
consumers and primary producers can lead to varying stabilizing effects of
species richness. Here, we compared the impact of species richness on
community variability in four taxonomic groups: terrestrial birds,
mammals, invertebrates, and plants. Location: Global Method: Using data
from 6,763 time series globally (BioTIME) for four terrestrial taxa, we
quantified community and population variability and species synchrony
based on abundance fluctuations over time. Results: Species richness
destabilized communities through increasing synchrony and stabilized
communities through reducing population variability in all taxa. Such
opposing effects weakened net impacts of species richness on communities.
Population variability had higher importance relative to synchrony in
plant communities. In contrast, synchrony had more comparable (or even
higher) importance compared to population variability in animal
communities. When synchrony and population variability were not
controlled, stabilizing impacts of species richness were detected in plant
communities only. Main conclusions: Our results highlight how species
richness drives stabilizing and destabilizing mechanisms simultaneously
across all taxa, with strong taxonomic variation in the relative
importance of these mechanisms in regulating community variability. This
questions the generality of previous findings on stabilizing impacts of
species richness based on limited taxonomic coverage. Additionally, our
results indicate the need to understand how the importance of stabilizing
and destabilizing mechanisms differ in determining community variability
across organisms and environments.