Ecology and evolution of the mycorrhizal mutualism in a changing environment

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Biologists seeks to explain why organisms have evolved so many different forms and functions, and to explain how this diversity influences the assembly and functioning of ecological communities. Climate and antagonistic interactions such as competition and predation have long been studied as causes of evolutionary and ecological change, but less well appreciated, though just as common, are mutualisms, in which partners exchange resources and services for mutual benefit. Nearly all organisms engage in one or more mutualisms, and it is predicted that without these interactions, the earth would support far fewer species. The interaction between plants and root inhabiting mycorrhizal fungi is one of the most ancient and widespread mutualisms in nature, evolving more than 400 million years ago. Nearly all plants are colonized by these fungi, who increase plant access to soil nutrients, particularly phosphorus, in exchange for sugars from photosynthesis. Though characterized as a mutualism, plant responses to fungal inoculation are quite variable, ranging from neutrality to a 100-fold stimulation of plant growth. The proposed research will experimentally test why the mutualism between plants and mycorrhizal fungi varies in nature, and how this variability affects the diversity and function of plant communities. First, we will identify the environmental factors that cause evolutionary change in the mutualism using a controlled natural selection experiment. Second, we will test whether divergence in the strength of the mutualism has consequences for population dynamics and species co-existence. Third, we will test whether widespread deposition of nitrogen from human activities, which creates acute phosphorus limitation, causes plant populations to evolve a stronger mutualism with mycorrhizal fungi, and then test whether this evolution influences nutrient cycling.The proposed research will be the first to experimentally identify the environmental causes of evolution in the mycorrhizal mutualism. Identifying these causes will enable us to predict how mutualisms will evolve in response to anthropogenic effects on climate and resource availability. By testing how the strength of a mutualism influences species co-existence, we will causally link mutualism with processes that maintain biodiversity. The research will contribute key insights into how dramatic increases in nitrogen deposition influence the evolution and ecological effects of a dominant mutualism. Knowledge of these effects is important because they are not presently considered in forecasts of how anthropogenic change will affect ecosystem function. The proposed research will provide undergraduate and graduate students with opportunities to develop strong analytical, project management and communication skills required for both scientific research and problem solving in the public and private sector.