Date of Award

Fall 2018

Project Type

Dissertation

Program or Major

Natural Resources

Degree Name

Doctor of Philosophy

First Advisor

Rebecca J Rowe

Second Advisor

Erik A Hobbie

Third Advisor

Mark J Ducey

Abstract

Animals are an integral component to forests, yet few studies have examined the direct and indirect effects of their resource use on ecosystem function. The mutualistic interactions between small mammals and truffles offers an excellent opportunity to investigate how animals contribute to ecosystem function. Truffles, the spore-bearing fruiting bodies of mycorrhizal fungi, are consumed by forest-dwelling small mammals. Mycorrhizal fungi are symbiotic with trees, colonizing roots and increasing the tree’s ability to take up nutrients. Because mycorrhizal fungi are extremely limited in their ability to disperse, small mammals play a critical role in forests by consuming truffles and dispersing their mycorrhizal spores. However, little is known about the factors that contribute to truffle consumption and subsequent spore dispersal by small mammals.

I investigated small mammal (rodents and shrews) community dynamics and truffle diversity in New England and explored how competition and habitat specificity interacted with resource availability to shape small mammal diets and spore dispersal capacity. In Chapter 1, to determine the impact of species population fluctuations on community dynamics, I analyzed a USFS dataset on small mammal occurrence and abundance collected over a three-year period in the White Mountain National Forest (WMNF). I found that population fluctuations were synchronized among species, creating high within year concordance in community dynamics in the region (independent of forest type), and low among year similarity in communities. In Chapter 2, to better understand truffle diversity, environmental associations, and phenology, I conducted detailed truffle surveys among forest types at Bartlett Experimental Forest (within the WMNF region). I also contrasted field survey data with those derived from the truffle spores contained in scat samples of a widespread generalist, the eastern chipmunk (Tamias striatus). I found that truffle biomass was 10 times higher in softwood than hardwood forest and that richness of fruiting truffles increased over the summer. Basal area of eastern hemlock (Tsuga canadensis; a tree species in regional decline) was the primary driver of truffle biomass and community composition.

To explore how resource availability shapes dietary niches among small mammal species and contributes to mycorrhizal spore dispersal, I trapped small mammals in hardwood, mixed, and softwood forest at Bartlett Experimental Forest over a three-year period. During this time, natural pulses of mast-fruiting of American beech (Fagus grandifolia) created variable levels of beech nut availability (a high quality food resource). In Chapter 3, I focused on two congener rodent species (white-footed mouse [Peromyscus leucopus] and deer mouse [P. maniculatus]), both of which are generalist consumers. I reconstructed their diets seasonally using stable isotope analysis (δ13C, δ15N) of hair and measured both intraspecific dietary niche breadth and interspecific niche overlap. Changes in niche breadth were generally consistent with predictions of optimal foraging theory, with both species consuming more beech nuts, less fungi, and having a narrower niche breadth during masting seasons compared to non-masting seasons. In contrast, changes in niche overlap were consistent with competition theory, with higher niche overlap during masting seasons than during non-masting seasons. In Chapter 4, I used microscopy of scat and network analyses to determine how population fluctuations, diet, and resource availability interact to shape mycorrhizal dispersal by small mammals. The southern red-backed vole (Myodes gapperi), a fungal specialist, carried a more diverse spore community than rodent generalists and was consistently the most important disperser in its favored habitat (softwood forest where truffle abundance was high). Nevertheless, during years when generalist species such as T. striatus and P. maniculatus reached high abundance (following beech masting) their relative importance in networks was equal to or greater than that of M. gapperi, particularly in hardwood and mixed forest where M. gapperi was less common. These findings suggest that although fungal specialists play key roles in rodent-mycorrhizal dispersal networks, generalists play a heretofore underappreciated role.

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