Plant species abundance is an essential component of ecosystem biodiversity and function, but understanding what factors regulate rarity and commonness has been elusive. Numerous studies have compared traits that might distinguish between rare and common species (e.g. seed size) and tested factors potentially contributing to rarity (e.g. competition), but these efforts have not substantially explained observed variation in plant abundances. The proposed research will test the novel hypothesis that soil microorganisms, often overlooked by community ecologists, may explain rarity in plants through soil community feedbacks. In the feedback process, a plant species alters the composition of the soil microbial community, and the altered soil community then affects plant performance relative to other plants in the community. Negative feedback should drive plants toward rarity, whereas positive feedback should promote plant dominance. If feedback regulates plant rarity, then rare and common plant species should either support different soil microbial communities or respond differently to the same community. This prediction has not been tested. A field survey will be conducted to determine whether co-occurring rare and common grass species develop consistently different microbial communities in nature. The potential function of these microorganisms will also be compared by measuring activities for enzymes involved in carbon, nitrogen and phosphorous cycles. Then, using a reciprocal soil transfer experiment in the greenhouse (i.e. rare plants will be grown on common plants' soil and vice-versa), the second part of this study will test if rare plants experience more negative feedbacks than common ones. A fractionation experiment in which plants interact with different components of the microbial community (e.g. all microbes or bacteria only or fungi only) will help understand which microbes are more actively driving soil community feedbacks.
The data generated have high potential to transform current understanding of the factors governing plant species abundance. Results will be disseminated to policy-makers and land managers through existing collaborations with the Nature Conservancy, US Forest Service, National Parks, and private seed companies. The research will integrate teaching and research by building partnerships with K-12 educators, by funding a female post-doc, and by training graduate and undergraduate students from diverse backgrounds.
Why are there naturally rare and common species in nature? In spite of an extensive research, this question still has no satisfactory answer. For plants, numerous studies have tried to link rarity or commonness to factors such as e.g. climate, seed size or number, or interactions with other species (for example insects). Very few of these studies have taken into account soil bacteria and fungi while they have recently been shown to be essential to plant growth, survival and reproduction. Indeed, some beneficial soil microbes called mutualists, help plants obtaining the nutrients they need to thrive (for example nitrogen). Conversely, other soil microbes called pathogens, are detrimental because they trigger diseases. Ultimately, depending on which soil microorganisms a plant species will encounter in its habitat, it may be put to a disadvantage compared to other species. Our hypothesis was therefore that soil microbes could be a key factor that would help explaining plant rarity. In this project, we sought to answer the following questions: (1) Do microbes differ between rare and common plant species?, (2) Do rare plant species grow better in a common plant species’ soil?, (3) which group(s) of microbes is (are) most responsible for the differences between rare and common plant species?, and (4) Do microbes have the same impact on plant depending on its stage of development (i.e. adult or seedling)? To answer these questions, we collected soil and root samples from three rare and three common grass species growing at four different sites in Indiana: Lilly-Dickey Woods, Yellowwood State Forest, Morgan Monroe State Forest and Brown County State Park. These samples were then used in varied lab and greenhouse experiments. For the first time we have demonstrated that soil microbes differ between rare and common plant species. According to our results, both rare and common plant species grew better in rare species’ soil than in common species’ soil. This might indicate that rare plant species are associated with beneficial microbes that promote their survival while common plant species are associated with detrimental microbes that prevent them from taking over. Among the different groups of microbes we studied, AMF (some fungi inhabiting plant roots) could potentially be the drivers of the observed results. Finally, our results showed that soil microorganisms have different effects at different plant life stages (i.e. adult or seedling). All together, these results are highly novel and yield information that will be useful for conservation and management strategies for rare plants.
