Decline in plant species richness with a chronic decrease of precipitation: The mediating role of the dominant species

Despite the increased frequency and severity of droughts in many regions
and declining biodiversity, existing experimental evidence for changes in
plant species richness associated with altered water availability is
limited. The growing extent of drylands highlights the need to predict
precipitation-related changes in species richness, which requires a better
understanding of the mechanisms. We carried out a field experiment
applying a single extreme drought event and combined it with subsequent
chronic alteration of summer precipitation for seven years in a
water-limited temperate grassland. We assessed how altered precipitation
regimes and a previous extreme drought affect species richness. We
compared a simple analysis assuming only the net effect of precipitation
on species richness to a complex approach by structural equation modelling
that included both the direct effects of precipitation and indirect
effects through the biomass of dominant grass species. Using simple
analysis, we found significant positive and nonsignificant
precipitation–species richness relationships in the presence and absence
of extreme drought, respectively. The complex analysis disentangled direct
and indirect pathways between precipitation and species richness. The
indirect pathway acted only in the absence of drought. In this case,
increasing precipitation increased the biomass of dominant species, which,
in turn, decreased species richness, acting as a mediator variable. The
direct relationship was positive, independent of the presence of drought.
Synthesis: Consistent with the global relationship between water
availability and species richness, we experimentally showed that
decreasing precipitation decreases species richness. Furthermore, we found
that increasing precipitation may also decrease plant species richness via
an indirect pathway acting through the biomass of the dominant species.
Our results highlight that species richness can become more sensitive to
changes in precipitation after extreme drought events that eliminate or
set back dominant species.