An unprecedented era of climatic volatility is altering ecosystems across our planet 1. The potential scale, pace and consequences of this global change have been modelled extensively 2 , yet little empirical research has quantified the impacts of extreme climate events on the composition of contemporary ecological communities. Here, we quantified the responses of 423 sympatric species of plants, arthropods, birds, reptiles and mammals to California's drought of 2012-2015-the driest period in the past 1,200 years 3 for this global biodiversity hotspot. Plants were most responsive to one-year water deficits, whereas vertebrates responded to longer-term deficits, and extended drought had the greatest impact on carnivorous animals. Locally rare species were more likely to increase in numbers and abundant species were more likely to decline in response to drought, and this negative density dependence was remarkably consistent across taxa and drought durations. Our system-wide analysis reveals that droughts indirectly promote the long-term persistence of rare species by stressing dominant species throughout the food web. These findings highlight processes that shape community structure in highly variable environments and provide insights into whole-community responses to modern climate volatility. The frequency, severity and duration of droughts is increasing due to global warming 4-6. High socioeconomic costs of severe droughts are among the most worrisome of climate change impacts, and effects on natural ecosystems may likewise be substantial 7. Predicting the ecological impacts of drought is complicated by the fact that species can be impacted through multiple pathways. Drought affects communities directly through physiological impacts on species' survival and growth rates, and indirectly by altering species interactions such as competition 8,9. Some theoretical models show that droughts can increase coexistence probabilities through selective mortality on dominant species 8,10 , while others predict that drought can increase dominance through increased competitive intensity 9. Here, we tested these conflicting predictions regarding the pathways through which drought impacts ecosystems, while broadly characterizing the response of a community to a once-in-amillennium climate-induced disturbance. Theoretical predictions of drought effects are rooted in plant ecology 11 , and it is not known whether they apply to animal populations that may primarily be indirectly affected by soil moisture deficits. Drought may affect all trophic levels similarly via generalized processes of disturbance or competition. Alternatively, drought effects may move up the food web with time lags or opposing effects. For example, droughts in sub-Saharan east Africa have led to ungulate die-offs 12 , resulting in a short-term resource pulse for scavengers 13. Thus, the effects of drought on resource availability differ among trophic levels and over time. Emerging studies indicate that droughts can strongly affect the dynamics of animal popula...

Summary1. In ecological webs, net indirect interactions between species are composed of interactions that vary in sign and magnitude. Most studies have focused on negative component interactions (e.g. predation, herbivory) without considering the relative importance of positive interactions (e.g. mutualism, facilitation) for determining net indirect effects. 2. In plant ⁄ arthropod communities, ants have multiple top-down effects via mutualisms with honeydew-producing herbivores and harassment of and predation on other herbivores; these ant effects provide opportunities for testing the relative importance of positive and negative interspecific interactions. We manipulated the presence of ants, honeydew-producing membracids and leaf-chewing beetles on perennial host plants in field experiments in Colorado to quantify the relative strength of these different types of interactions and their impact on the ant's net indirect effect on plants.3. In 2007, we demonstrated that ants simultaneously had a positive effect on membracids and a negative effect on beetles, resulting in less beetle damage on plants hosting the mutualism. 4. In 2008, we used structural equation modelling to describe interaction strengths through the entire insect herbivore community on plants with and without ants. The ant's mutualism with membracids was the sole strong interaction contributing to the net indirect effect of ants on plants. Predation, herbivory and facilitation were weak, and the net effect of ants reduced plant reproduction. This net indirect effect was also partially because of behavioural changes of herbivores in the presence of ants. An additional membracid manipulation showed that the membracid's effect on ant activity was largely responsible for the ant's net effect on plants; ant workers were nearly ten times as abundant on plants with mutualists, and effects on other herbivores were similar to those in the ant manipulation experiment. 5. These results demonstrate that mutualisms can be strong relative to negative direct interspecific interactions and that positive interactions deserve attention as important components of ecological webs.