Intellectual merit: Nearly all biological organisms form partnerships with other species in which they trade resources or services: such partnerships are called mutualisms. Mutualisms incur costs as well as provide benefits to the organisms involved and the net outcome of the partnerships can vary from positive (mutualistic) to negative (parasitic). Quantifying the costs and benefits of resource exchange within mutualisms is required to predict shifts in the outcome of these interactions under a range of environmental conditions. One of the most common mutualisms on earth occurs between the roots of plants and certain fungi that live in soil. The fungi provide essential nutrients to plants, such as phosphorus and nitrogen, while plants provide the fungi with sugars in return. As these resources are traded, it can influence other organisms in the environment, such as the insects that feed on plants. At present, scientists are unable to predict how the outcome of plant-fungal partnerships will vary with the identity and abundance of the fungi in soil and the genetics of the plants. The proposed work will manipulate the identity and abundance of the partners in plant-fungus mutualisms and measure how resources are traded between them. By studying how well the partners perform, the project will help scientists understand how partnerships form and dissolve in nature.
Broader impacts: Scientists have yet to develop realistic theories of how mutualisms work, despite the fact that they are extremely common in nature. Results from this work will be broadly applied by ecologists and evolutionary biologists who work on many different kinds of organisms throughout the world. Moreover, results gained from studying mutualisms between plants and fungi can be used in a wide variety of human endeavors including agricultural and forest production, the restoration of degraded environments, the prediction and management of environmental change, and the management of invasive species. Additionally, mutualistic partnerships in nature are mimics of economic partnerships among human institutions in which goods and services are traded, and the results will apply across human and natural systems. During the work described here, scientists from a range of social and ethnic backgrounds will be trained in the methods of science. Finally, results from this work will be transmitted to local, national and international audiences through publications and presentations.
The plants, animals and microbes that occur around our parks, gardens, forests and prairies engage in a broad range of interactions with one another. Understanding variation in the outcome of species interactions is a central goal of ecology, and is key to predicting how species interactions may change with ongoing environmental change. For example, plants commonly associate with soil fungi that provide nutrients for plants in exchange for sugar, in a relationship called mycorrhizae. We already know that the identity of fungi can influence their interactions with plants, but it is unclear if the abundance of partners changes the outcome of mycorrhizal interactions. Furthermore, we don’t understand how mycorrhizal fungi affect herbivores that rely on plants for food. These questions are important because plant-fungus-herbivore interactions may affect how we grow and manage crops for food, fiber and horticulture, and how we manage feed for livestock. Using a series of experiments, we manipulated the species identity and abundance of mycorrhizal fungi that associate with milkweed plants that are common members of prairie habitats and popular garden plants. We examined which factors influence plant defenses against insect attack, and herbivore performance, both on plant shoots and roots. We found that both the identity and abundance of mycorrhizal fungi can have striking effects on plant growth, expression of defensives, and herbivore performance. For example, inoculation with one species of mycorrhizal fungus increased the concentration of phosphorus in leaves, while also decreasing leaf toughness. As a result, an herbivore that specializes on milkweeds grew larger on plants that hosted more fungi in their roots. However, when we examined the growth of a generalist root-feeding insect, we found that inoculation with mycorrhizal fungi actually decreased its performance. In addition, we found that the effects of mycorrhizal fungi on plant traits varied among different milkweed species. Overall, we found that variation in the abundance of fungal mutualists that associate with plants can influence the outcome of plant-herbivore interactions, and linked these changes in insect growth to plant traits. Our results support current thinking that mycorrhizal fungi positively affect the performance of specialist herbivores and negatively affect generalist herbivores. However, we also demonstrate that related plants may not necessarily respond similarly to mycorrhizal inoculation. The results of our work are directly relevant to how we manage plants and soils in order to grow food and fiber while minimizing attack from plant pests. Additionally, our results contribute to how we might best manage plant and fungal diversity to minimize negative impacts of environmental change. Our project helped to train one graduate student and three undergraduate students in how to perform scientific research. The students acquired new skills, including the analysis of plant nutrients, chemical analysis of plant tissue, statistical analysis, and plant genetics. These skills will be useful as the students enter professions in which they contribute to pest management of crops, management of livestock, human health and nutrition, and conservation of our natural world. This project also facilitated collaboration between students at the University of Michigan and a postdoctoral fellow at Cornell University. In addition, the project contributed to a larger Milkweed Research Project in Michigan. This project is used as a teaching tool in the Frontiers Masters Program in Ecology and Evolutionary Biology, which recruits underrepresented minorities into graduate study in ecology and evolution. Over the past six years, the Milkweed Project has engaged Frontiers students of Latin American, Native American, African America, and Pacific Island descent.
