Fungal symbiont effects on dune plant diversity depend on precipitation

Abstract

Summary Historically, mutualisms have been considered to be less important than antagonisms in affecting the composition of ecological communities. In plant communities, beneficial microbes may feature as keystone mutualists in structuring community composition. Understanding the direction and magnitude of mutualist effects at the community scale may be critical for making accurate predictions on plant responses to climate change, particularly for mutualists that ameliorate climate‐induced stressors. Such mitigation could shift outcomes between mutualist‐enhanced species diversity and mutualist‐reduced diversity, depending on whether a mutualist accelerates habitat modification or competitive exclusion by its partner species. Here, we tested the relative importance and interactive effects of altered precipitation and symbiosis between an epichloid fungal endophyte and a dominant grass species for dune plant communities along the Great Lakes, USA. In 2010, we imposed field manipulations of endophyte presence in the foundation dune grass Ammophila breviligulata in combination with rainout shelters and rainfall additions. We monitored natural rates of colonization by new plant individuals over 3 years. Under the current precipitation regime, endophyte symbiosis in A. breviligulata reduced colonizing plant diversity, species richness and evenness. This effect depended on the amount of precipitation, with the symbiosis having weaker effects on plant diversity under both augmented and reduced rainfall treatments. Despite the overall negative effect of endophyte symbiosis on plant diversity, plant responses to the endophyte were species specific. A federally threatened forb, Cirsium pitcheri, increased in abundance when the symbiosis was present, regardless of precipitation regime. Endophyte symbiosis in A. breviligulata caused minor reductions in the abundance of other grass species; however, augmented precipitation benefited other grasses. Synthesis. We show that microbial mutualisms can have strong effects on community structure in a native ecosystem and that the amount of precipitation has the potential to alter how these keystone species interactions affect community composition. Predictions on future plant community structure, for both restored and native dunes, can be improved by accounting for the presence of fungal symbionts in the foundation plant species.