Ecological effects of extreme drought on Californian herbaceous plant communities

Understanding the consequences of extreme climatic events is a growing challenge in ecology. Climatic extremes may differentially affect varying elements of biodiversity, and may not always produce ecological effects exceeding those of "normal" climatic variation in space and time. We asked how the extreme drought years of 2013- 2014 affected the cover, species richness, functional trait means, functional diversity, and phylogenetic diversity of herbaceous plant communities across the California Floristic Province. We compared the directions and magnitudes of these drought effects with expectations from four "pre-drought" studies of variation in water availability: (1) a watering experiment, (2) a long- Term (15-yr) monitoring of interannual variability, (3) a resampling of historic (57-yr- old) plots within a warming and drying region, and (4) natural variation in communities over a broad geographic gradient in precipitation. We found that the drought was associated with consistent reductions in species richness and cover, especially for annual forbs and exotic annual grasses, but not with changes in functional or phylogenetic diversity. Except for total cover and cover of exotic annual grasses, most drought effects did not exceed quantitative expectations based on the four pre-drought studies. Qualitatively, plant community responses to the drought were most concordant with responses to pre-drought interannual rainfall variability in the 15-yr monitoring study, and least concordant with responses to the geographic gradient in precipitation. Our results suggest that, at least in the short term, extreme drought may cause only a subset of community metrics to respond in ways that exceed normal background variability

Change in dominance determines herbivore effects on plant biodiversity

Herbivores alter plant biodiversity (species richness) in many of the world’s ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis—that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally

Appendix E. Principal-component eigenvectors for community traits with and without annual grasses.

Principal-component eigenvectors for community traits with and without annual grasses

Appendix A. Description of precipitation by year.

Description of precipitation by year

Appendix B. Means and standard deviations of non-serpentine and serpentine soil properties.

Means and standard deviations of non-serpentine and serpentine soil properties

Appendix D. Means, standard deviations, and correlation coefficients of community weighted mean (CWM) trait values in relation to Bray-Curtis dissimilarity.

Means, standard deviations, and correlation coefficients of community weighted mean (CWM) trait values in relation to Bray-Curtis dissimilarity

Appendix C. Means and standard deviations of community properties by functional group.

Means and standard deviations of community properties by functional group

Appendix B. Supplementary tables, including correlations, means, and standard deviations of soil variables and plant functional traits.

Supplementary tables, including correlations, means, and standard deviations of soil variables and plant functional traits

Appendix A. Supplementary figures, including path diagrams and correlations between soil variables and precipitation.

Supplementary figures, including path diagrams and correlations between soil variables and precipitation

Ecological effects of extreme drought on Californian herbaceous plant communities

Understanding the consequences of extreme climatic events is a growing challenge in ecology. Climatic extremes may differentially affect varying elements of biodiversity, and may not always produce ecological effects exceeding those of "normal" climatic variation in space and time. We asked how the extreme drought years of 2013- 2014 affected the cover, species richness, functional trait means, functional diversity, and phylogenetic diversity of herbaceous plant communities across the California Floristic Province. We compared the directions and magnitudes of these drought effects with expectations from four "pre-drought" studies of variation in water availability: (1) a watering experiment, (2) a long- Term (15-yr) monitoring of interannual variability, (3) a resampling of historic (57-yr- old) plots within a warming and drying region, and (4) natural variation in communities over a broad geographic gradient in precipitation. We found that the drought was associated with consistent reductions in species richness and cover, especially for annual forbs and exotic annual grasses, but not with changes in functional or phylogenetic diversity. Except for total cover and cover of exotic annual grasses, most drought effects did not exceed quantitative expectations based on the four pre-drought studies. Qualitatively, plant community responses to the drought were most concordant with responses to pre-drought interannual rainfall variability in the 15-yr monitoring study, and least concordant with responses to the geographic gradient in precipitation. Our results suggest that, at least in the short term, extreme drought may cause only a subset of community metrics to respond in ways that exceed normal background variability