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Abstract The disruptive effects of tertiary species on otherwise positive pairwise species interactions (e.g. context-dependent parasitism in pollinator syndromes) is well-known. However, few—if any—studies have investigated how invasive plants affect interactions between facilitative plants and their native plant communities. Further, if tertiary invasive species can change interactions among native species from positive to negative, then a tertiary native should be capable of the same phenom for pairwise interactions between natives and invasives. Our previous research indicates invasive black mustard ( Brassica nigra ) changes interaction signs for otherwise positive species interactions between the dominant, native facilitator California buckwheat ( Eriogonum fasciculatum ) and its co-dominant beneficiary California sagebrush ( Artemisia californica ) in semi-arid California coastal sage scrub habitat. Here, E. fasciculatum and A. californica seedlings increased B. nigra shoot growth in pairwise species interactions in the greenhouse. However, in three-way species interactions, E. fasciculatum and A. californica together reduced B. nigra SLA, height, and reproductive potential while not increasing shoot DW. In three-way species interactions, B. nigra did not significantly reduce E. fasciculatum facilitation of A. californica . Also surprisingly, light competition with B. nigra resulted in an increase in A. californica height , which reduced the negative effects of A. californica light competition on shade-intolerant E. fasciculatum. In an additive field experiment, A. californica protected E. fasciculatum from facilitating germination and growth of B. nigra when water competition was minimized. Taken together, this study demonstrates the importance of studying species interactions between competitive, native perennials in the current ecological context of invaded ecosystems. 

Climate change is acting on species and modifying communities and ecosystems through changes not only with respect to mean abiotic conditions, but also through increases in the frequency and severity of extreme events. Changes in mean aridity associated with climate change can generate ecotype by environment mismatch (i.e., climatic displacement). At the same time, variability around these shifting means is predicted to increase, resulting in more extreme droughts. We characterized the effects of two axes of climate change, climatic displacement and drought, on the shrubArtemisia californicaand its arthropods. We established common gardens of plants sourced along an aridity gradient (3.5‐fold variation in mean annual precipitation) in an arid region of the species distribution, thus generating a gradient of climatic displacement (sustained increase in aridity) as predicted with climate change. We surveyed plants and arthropods over eight years where precipitation varied sixfold, including both extreme drought and relatively mesic conditions. These two axes of climate change interacted to influence plant performance, such that climatically displaced populations grew slowly regardless of drought and suffered substantial mortality during drought years. Conversely, local populations grew quickly, increased growth during wet years, and had low mortality regardless of drought. Effects on plant annual arthropod yield were negative and additive, with drought effects exceeding that of climatic displacement by 24%. However, for plant lifetime arthropod yield, incorporating effects on both plant growth and survival, climatic displacement exacerbated the negative effects of drought. Collectively these results demonstrate how climatic displacement (through increasing aridity stress) strengthens the negative effects of drought on plants and, indirectly, on arthropods, suggesting the possibility of climate‐mediated trophic collapse.