My research uses an integrative approach to explore how ecological and evolutionary processes influence population and community patterns. I am interested in the mechanisms by which positive species interactions and intraspecific variation in traits related to species interactions can impact population and community dynamics and co-evolutionary relationships. For my dissertation, I am studying how mutualisms and intraspecific genetic variation in both partners influence plant community diversity and composition. Specifically, I am currently exploring how the mutualism between the legume Chamaecrista fasciculata and nitrogen-fixing rhizobia bacteria influence the establishment of other species, and how intraspecific genetic variation within both mutualist partners influences species interactions and community dynamics.
Effects of the legume-rhizobium mutualism on species interactions and community composition and diversity
Mutualisms can drive population dynamics and evolutionary processes, but there is still only a limited understanding of how mutualisms may be important to communities. Resource mutualisms, such as the interaction between legumes and rhizobia, not only directly influence the partners involved in the interaction, but they also have the potential to either inhibit or facilitate other species in the community by altering the abiotic and biotic environment. Yet, understanding the role of mutualisms in a community context has typically received less attention than other species interactions. In experimental mesocosms simulating early successional prairie communities, I manipulated the presence of rhizobia across an environmental gradient to explore:
- How do rhizobia influence the dominance of a legume host?
- Do rhizobia affect α- and β-diversity, and community composition, and productivity
- Are rhizobia effects on communities more pronounced in nitrogen-limited systems?
Context-dependent effects of rhizobia on community patterns: The role of intraspecific variation and nitrogen availability
There is substantial intraspecific genetic variation in how legumes such as C. fasciculata interact with rhizobia, including variation in legume growth response to rhizobia, number of rhizobia-housing nodules, and nitrogen fixation. Therefore, not only can the presence of a particular legume species and its associated rhizobia influence the surrounding community, but the genetic identity of the colonizing legume could also alter community processes when populations vary in traits associated with ecologically important interactions with rhizobia. Given this variation, the effects of rhizobia on communities may be context-dependent on the genetic identity of the legume host. I have explored:
- Do populations of C. fasciculata vary in their effects on community and ecosystem processes, such as diversity, composition, productivity, and nitrogen availability?
- Are the effects of rhizobia on communities dependent on the genetic identity of their legume host?
- Is the magnitude of these effects greater in nitrogen-limited systems?
Mechanisms by which rhizobia alter the competitive dynamics between
C. fasciculata and other species
Through a series of experiments, I have explored the mechanisms underlying the decrease in diversity, changes in community composition, and varied responses of different species in the community as a result of the legume-rhizobium mutualism. Rhizobia may shift the relative importance of aboveground and belowground competition for light and soil nutrients. Moreover, the strength of intra- and inter-specific competition may vary across habitats depending on the availability of mutualists, which may have implications on coexistence following establishment.
Resource mutualisms, such as the legume-nitrogen-fixing rhizobia mutualism, may be especially important to communities due to their ability to alter the nutrient availability in the ecosystem. N-fixing rhizobia provide the legume with a direct nitrogen benefit, which can increase the legume’s growth and alter the expression of key traits. Some of these traits may be especially important to species interactions, such as traits related to competition, nutrient acquisition, or herbivory. Therefore, because of their mutualism with legumes, rhizobia could be key drivers of multiple trophic level interactions from soil to plants to insects.
Moreover, some species may experience more than one type of mutualism at the same time. In addition to its mutualism with rhizobia, the legume C. fasciculata also has a mutualistic interaction with ants, where it provides ants nectar in exchange for defense against herbivores. While they provide the plant with very different benefits, these two interactions may be connected, with one influencing the effects of the other. For example, the increased nitrogen may allow the plant to put more resources into nectar production, resulting in more herbivore defense. Conversely, a lack of rhizobia may result in less nectar production, less defense, and reduced fitness of the plant from increased herbivory and nutrient limitation. I am currently exploring how rhizobia and ants independently and interactively influence the growth and fitness of the legume Chamaecrista fasciculata as well as how these mutualists may affect the residing arthropod community.
Local co-adaptation between legume and rhizobium
My work shows that genetic variation in the legume is important to community assembly. Yet, genetic variation in the
rhizobia and co-adaptation between the legume and rhizobia may also be important to community dynamics. I am currently conducting a greenhouse experiment that manipulates the population identity of both the legume C. fasciculata and the rhizobia (obtained from each C. fasciculata population source location) in a fully-factorial design. This study will further expand the linkages between ecology and evolution in communities by testing for coadaptation across a broad geographic scale and another current experiment will investigate the importance of this co-adaptation for other species in the community.
Effects of intraspecific variation on the establishment of an invasive species
With: Jen Lau and Casey terHorst
Role of genetic diversity in a dominant legume on population and community dynamics
With: Susan Magnoli
Role of seed addition and disturbance on the restoration of prairie grasslands
With Bryan Foster (University of Kansas), I examined how propagule availability and a disturbance regime influencing microsite availability affected diversity, community dynamics, and the feedbacks on ecosystem functions in a successional grassland.
Foster, B.L., C.A. Murphy, K.R. Keller, T.A. Aschenbach, E.J. Questad, and K. Kindscher. 2007. Restoration of prairie community structure and ecosystem function in an abandoned hayfield: a sowing experiment. Restoration Ecology 15 (4): 652-661.