Diversity mediates the responses of invertebrate density to duration and frequency of rivers’ annual drying regime

Predicting the impacts of global change on highly dynamic ecosystems
requires a better understanding of how communities respond to disturbance
duration, frequency and timing. Intermittent rivers and ephemeral streams
are dynamic ecosystems that are recognized as the most common fluvial
ecosystem globally. The complexity of the drying process can give rise to
different annual and antecedent hydrological conditions, but their effect
on aquatic communities remains unclear. Here, using aquatic invertebrates
from 33 streams across a flow-intermittence gradient, we assessed how
annual (drying duration and frequency) and recent drying characteristics
(duration of the last dry period and flowing duration since the last
rewetting) affect the density and diversity metrics of communities and
trophic groups while controlling for other key abiotic factors (dissolved
oxygen and altitude). We characterized invertebrate communities using
taxonomy and functional traits to capture biological features that
increase vulnerability to drying. In addition, using structural equation
modelling (SEM), we evaluated pathways by which drying characteristics
directly impact invertebrate density and whether diversity indirectly
mediates such relationships. We show that drying frequency drove
reductions in diversity at the community level and within trophic groups,
whereas both the drying duration and frequency had a negative influence on
density metrics. Reductions in taxonomic richness were linked to increased
annual drying duration, whereas functional diversity declined in response
to annual drying frequency. Filterer, predator and shredder trophic groups
exhibited the strongest negative responses to drying. Recent drying
characteristics had a minor effect on density and diversity metrics. Our
SEM results demonstrated that diversity mediates the negative impacts of
annual drying duration and frequency on invertebrate density through
reductions in their taxonomic richness and functional diversity. Our
results underscore the importance of considering multiple drying
characteristics together with the interdependence of density and diversity
to better anticipate drying responses in freshwater ecosystems.