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Date of Award

9-2011

Document Type

Campus Access

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Organismic and Evolutionary Biology; Entomology

First Advisor

Lynn S. Adler

Second Advisor

Elizabeth Jakob

Third Advisor

Joseph Elkinton

Subject Categories

Ecology and Evolutionary Biology | Entomology | Parasitology

Abstract

Mutualisms are major structuring forces in biological communities. However, the concept of the trophic cascade has rarely been explicitly applied to mutualisms. Antagonists of one mutualist have the potential to negatively affect the second mutualist through effects on their partner, and the magnitude of such effects should vary with mutualism strength. Bumblebees represent an ecologically and economically important mutualist pollinator group. They are attacked by a range of parasites, and visit a variety of plants that vary in reliance on bumblebees for pollination service. Using bumblebees and their parasites, I investigated whether mutualisms can mediate trophic cascades.

I surveyed three parasites (Crithidia bombi , Nosema bombi and conopid flies) in Bombus spp. in Massachusetts and found that parasites are more common in wild bumblebees than previously believed (Chapter 1). To test whether infection by these parasites has top-down effects on pollination service to plants, I examined whether site-level parasitism rates correlated with pollination service to greenhouse-raised plants. I used several plant species which varied in their dependence on bumblebees for pollination. The relationship between parasitism and pollination service differed between plants and parasite species, and there was evidence that plants more reliant on bees for pollination experienced stronger negative indirect effects of parasites (Chapter 2).

Finally, I developed an ordinary differential equation model of mutualist population dynamics incorporating a producer, its mutualist, and an antagonist of the mutualist. I varied the mutualism from obligate to facultative, and investigated effects of the antagonist on mutualist coexistence. My results highlight how mutualisms differ from traditional trophic cascades, and the importance of considering obligate, facultative, and asymmetric mutualisms in attempting to understand their interactions with the community (Chapter 3).

My contributes to conservation issues and extends our theoretical understanding of basic ecology. I provided valuable data about the incidence and multitrophic effects of parasites in an important native pollinator. By combining mutualisms with the theoretical structure of the trophic cascade, two previously independent areas of ecological research that will benefit from integration, my research improves our understanding of how mutualisms structure the community as a whole.

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