Indirect effects of mycorrhizal fungi on herbivores and mutualists of plants

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Arbuscular mycorrhizal fungi are a group of soil-dwelling fungi that form symbioses with most plant species. The symbiosis, which is frequently mutualistic, is based on the trade of resources: the fungi provide their host plants with nutrients collected from the soil while the plants provide their fungi with carbohydrates. Arbuscular mycorrhizal fungi may also help their host plants by reducing drought stress and by reducing root pathogens. As a result, arbuscular mycorrhizal fungi may affect traits of their host plants that are relevant to organisms at other trophic levels, such as herbivorous or mutualistic insects. My goal was to use experiments to investigate this type of 'trait-mediated indirect effect'. I begin with an introductory chapter (Chapter I) in which I review indirect effects, with emphasis on those between mycorrhizal fungi and insects. The subsequent five chapters each describe an experiment. In Chapter 2, I show that an assumption of the commonly used 'fungicide application method' for creating non-mycorrhizal control plants is problematic. Specifically, root-applied fungicides can negatively affect leaffeeding herbivores. In Chapter 3, I demonstrate neutral indirect effects of mycorrhizal fungi on specialist sunflower beetle larvae, in terms of survival, growth, and feeding efficiency. This runs counter to emerging theory and empirical evidence that suggests that specialists are typically positively affected by their host plants' mycorrhizal fungi. In Chapter 4, I investigate whether the stable isotope signatures of herbivores can be used to infer their host plants' mycorrhizal status, and surmise that they probably cannot. In Chapter 5, I show that mycorrhizal fungi can negatively affect the expression of extrafloral nectaries in broad bean plants, likely as a result of carbon limitation. However, in Chapter 6, I demonstrate that these effects can also be positive, at least under environmental conditions in which there is a positive effect of mycorrhizal fungi on plant size. Interestingly, the difference in extrafloral nectar production was too small to elicit a behavioural response in nectar-foraging ants. In the final chapter of the thesis (Chapter 7), I summarize and synthesize my main research findings, and consider future directions for this line of research.
Bibliography: p. 151-181
Laird, R. A. (2007). Indirect effects of mycorrhizal fungi on herbivores and mutualists of plants (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/1408