Current Research

How common are mutualistic symbioses in natural populations? What ecological factors affect the frequency of symbiosis?

Virtually all plants host endophytic fungi in above-ground tissues. However, we are just beginning to understand their diversity and roles in biological systems, and we know little about the factors that influence the distribution and abundance of these symbioses. I am currently focused on endophyte symbiosis in the grass family (Poaceae). Grasses dominate more than one-fifth of land area on earth and include ~10,000 species, many of which are economically important. Endophytic fungi occur commonly in grasses. For a very small fraction of grass species, endophytes have been shown to increase resistance to herbivores, pathogens, and drought, often via the provision of chemical compounds that are novel to the host. For most grass endophytes, however, ecological consequences remain uninvestigated. I am surveying fungal endophyte distribution in grasses that span diverse host species and habitats to identify biotic (e.g., herbivory) and abiotic (e.g., drought) factors that may affect endophyte frequency in nature.

What are the ecological consequences of mutualism for populations?

Species vary fundamentally in abundance, spanning a range from invasive to rare. However, currently proposed mechanisms do not sufficiently account for this observed variation. Mutualistic symbionts can enhance host resistance to biotic and abiotic stress and may thereby increase host abundance. However, prior research on the population dynamics of plant-microbe interactions has focused almost exclusively on pathogenic symbionts. My ongoing work evaluates whether and how fungal endophyte symbioses explain the ecological dominance of plant species across several native, forest understory grass species.

This work includes collaborative research with Sarah Emery that examines how endophytic and mycorrhizal fungi affect the success of a key ecosystem engineer, Ammophila breviligulata, in Great Lakes dune ecosystems

What evolutionary processes maintain variation in mutualistic symbioses?

Vertically transmitted symbionts, in which symbionts are inherited by host offspring, are associated with some of the most ecologically dominant species on earth, from humans to aphids and grasses. Fixation of vertically transmitted symbionts has led to major evolutionary innovations, such as mitochondria and chloroplasts. I use endophytes in plants to provide a window into the evolution of symbiont fixation. For example, rates of vertical transmission are key elements in mathematical models of s10-oct-08 and the stability of host-symbiont dynamics. Despite this importance, vertical transmission rates have rarely been documented in mutualistic symbioses. Along with former MS student, Michelle Afkhami, we have shown that vertical transmission of endophytes is commonly imperfect (not all offspring become symbiotic), suggesting an important mechanism to maintain variation in symbiont frequency.

What are the ecological consequences of mutualism for communities?

Past work in collaboration with Keith Clay revealed that the mutualism between a dominant grass (Lolium arundinaceum) and a fungal endophyte functions as a keystone interaction in grassland communities. In large-scale field experiments, the presence of this mutualism increased host dominance, but reduced plant diversity, arthropod diversity, herbivory by mammals and insects, rates of decomposition, and the progression of plant succession from grassland to forest. My results challenge the fundamental paradigm that antagonistic species interactions are the most important forces shaping community structure. (publications)

I have also investigated whether microbial mutualists, such as endophytic fungi, can enhance the ability of their hosts to invade diverse plant communities. The biodiversity of a community can affect its functional properties, such as productivity or resistance to invasion. I developed a conceptual model that predicts plants with mutualists will have greater success invading diverse plant communities than plants without mutualists. A long-term field experiment and a manipulative greenhouse experiment confirmed this model. Altogether, this work will determine how microbial mutualists contribute to the diversity of natural ecosystems as well as to the success of exotic, invasive grasses.