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1. Coexistence and diversity in plant communities depend upon outcomes of plant competition. Competition and coexistence can be mediated by abiotic soil nutrient differences as well as by soil microbial communities. The latter effects occur through various mechanisms including negative plant-soil feedbacks, when plants foster the build-up of specialized pathogenic microbes, which ultimately reduce conspecific, but not heterospecific, densities. Microbial mutualists can have generalized associations with host plants, and by associating with multiple species might affect coexistence by conferring different levels of benefit to hosts.2. We examined the effects of abiotic differences and soil microbial communities, including mutualistic nitrogen-fixing rhizobial bacteria, on coexistence processes and asked whether these interactions inform patterns of co-occurrence in natural communities. We measured plant-soil feedbacks in the greenhouse for three native Trifolium species that either highly co-occurred or were spatially repulsed at our field site. Using size-fractioned soil microbial inocula prepared from field-collected soils, we explored the effect of soil microbes on nodulation and the outcome of competition (relative interaction intensity). We also examined the effects of soil origin (home vs. away soil) on the outcome of competition between species. Soil microbes had strong positive effects on plant growth and nodulation.Microbes in general reduced the strength of plant competition relative to competition in sterilized soil and altered the relative strength of interactions with conspecific vs. congeneric neighbours, which often occurred in ways predicted to enhance coexistence. In one pair of highly co-occurring Trifolium, competition was strong in sterilized soils, but these species facilitated one other in the presence of the full microbial community. The net effect of these microbes in general reduced competition while also increasing performance over that in sterilized soil, suggesting a role for soil microbial mutualists in host coexistence. Synthesis.Our results demonstrate that interactions between plants and diverse soil communities can alter plant-plant interactions and plant-soil feedbacks. They may increase niche differences and equalize fitness differences in ways consistent with observed co-occurrence in the field. | 1877Journal of Ecology SIEFERT ET al.

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In light of ongoing environmental change, understanding the complex impact of interacting stressors on species, communities, and ecosystems is an important challenge. Many studies to date examine the effects of potential stressors on a single species of concern. Yet these effects often resonate throughout a community and may produce changes in ecosystem dynamics that are equally critical to species resilience. The aim of this study was to develop a mechanistic understanding of how a rapidly changing stressor, water temperature, will alter trophic interactions among ectothermic fish species. In our region, California's Sacramento–San Joaquin River Delta system, it has been speculated that the decreased survivorship of juvenile Chinook salmon (Oncorhynchus tshawytscha) in warming waters may be caused partly by increased predation. Temperature influences metabolic rate functions and the amount of energy available for fitness‐relevant parameters (i.e., swim performance and escape response). Consequently, we hypothesized that these patterns of predation emerge due to a physiological advantage of predators over prey at warmer temperatures. To explore this, our first objective was to determine the fundamental thermal physiology of juvenile Chinook salmon and their potential predators in the Delta. Three physiological performance traits were measured for each species across a spectrum of temperatures: aerobic scope, burst speed, and the ability to burst repeatedly. For our second objective, we assessed whether the effect of temperature on these performance traits predicted the outcome of predation trials conducted across the same temperature spectrum. We found that temperature effects were species or population specific. Additionally, absolute burst swimming ability and the relative burst performance between predator and prey were stronger indicators of trophic dynamics than aerobic scope. Our analyses also confirmed that a major predator in the Delta, specifically largemouth bass (Micropterus salmoides), is more thermally adapted to higher temperatures and will likely consume salmon with an increasing frequency as waters warm. Thus, we show that an improved understanding of how fundamental thermal physiology impacts predation can provide ecosystem managers with better tools to predictively model predation upon juvenile salmon based on prevailing and future water temperatures.