DOI: 10.5061/dryad.qnk98sfsw
title: Decline in plant species richness with a chronic decrease of precipitation: The mediating role of the dominant species

description: 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.
description: # Decline in plant species richness with a chronic decrease of
 precipitation: The mediating role of the dominant species
 [https://doi.org/10.5061/dryad.qnk98sfsw](https://doi.org/10.5061/dryad.qnk98sfsw) ## Description of the data and file structure The data were collected in the ExDRain ([https://kiskun.lter.hu/en/node/837](https://kiskun.lter.hu/en/node/837)) field climate change experiment between 2013 and 2021. As described in the "comments" sheet of the "SEM_*ExDRain_2013*_2021.xlsx" file, the data table includes time, treatment, blockID, species richness, precipitation, and biomass data. ### Files and variables #### File: SEM\_Appendix\_241217.Rmd **Description:** Statistical codes and explanations of the statistical methods used. This file works using SEM_ExDRain_2013_2021.xlsx as a data source. #### File: SEM\_ExDRain\_2013\_2021.xlsx **Description:**  ##### Variables | **year** | **sampling year** | | :-------- | :--------------------------------------------------------------------------------------------------------------------------------------------------------- | | block | A numeric identifier without a unit that indicates from which of the six replicates the data originates | | extreme | This identifier specifies whether the data are from an extreme treated (X) or control (C) plot | | recurrent | This identifier indicates the level of chronic precipitation change management: W - Water addition, C - Control, M - Moderate drought, S - Severe drought. | | BiomassD | Biomass of the dominant species (g\*m-2) | | SR | Species richness (number of plant species per plot) | | precip | Precipitation (mm) | ## Code/software We conducted all analyses in the R environment, codes are included in SEM_Appendix_241217.Rmd