Data from: When mutualisms matter: Rhizobia effects on plant communities depend on host plant population and soil nitrogen availability

1.Mutualistic interactions, such as the relationship between legumes and
rhizobia, can affect community properties, yet there needs to be greater
understanding of when these interactions may be most important to
communities. Resource mutualism theory provides predictions based on
variation in abiotic and biotic factors. First, there is substantial
intraspecific genetic variation in how legumes interact with rhizobia,
including variation in legume growth response to rhizobia, number of
rhizobia-housing nodules, and nitrogen fixation. Rhizobia likely have
greater community impacts when associating with hosts that benefit greatly
from rhizobia, produce many nodules, and fix a lot of nitrogen. Second,
increased soil nutrient availability typically reduces the partner
benefits of legume-rhizobium interactions. As a result, the
legume-rhizobium resource mutualism may have greater effects on
communities in low nitrogen environments. 2.We first conducted a common
garden study of the annual legume Chamaecrista fasciculata to explore
intraspecific variation in focal traits related to competition and
mutualistic interactions with rhizobia. Then, we manipulated the presence
of rhizobia, nitrogen fertilization, and population identity of C.
fasciculata in mesocosms simulating native plant communities. We measured
plant diversity, assessed changes in community composition, and measured
soil nutrient availability. 3.We detected significant genetically-based
intraspecific variation in growth and mutualism related traits across C.
fasciculata populations. From the experimental mesocosms, we found that
although rhizobia consistently reduced diversity, rhizobia most strongly
reduced diversity when associating with C. fasciculata populations that
were highly dependent on rhizobia and when nitrogen was most limiting. We
also detected greater variation among populations in their effects on
communities in the absence of rhizobia; the presence of rhizobia
dramatically minimized effects of population identity on the surrounding
community. 4.Synthesis. These findings show that abiotic environmental
factors and intraspecific variation in a dominant host plant can influence
the magnitude of mutualism effects on communities and, reciprocally, how
the presence of key mutualists can influence the importance of
genetically-based variation among populations to community outcomes. While
prior studies have detected both strong and weak effects of mutualism on
plant communities, our study illustrates that mutualism theory may predict
when strong mutualism effects are most likely to occur and also shows how
theories developed for particular species interactions may make studies
investigating the ecological impacts of genetic variation a more
predictive science.