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Effects of top-down and bottom-up forces in an ant-plant mutualism.
SMITH, BROOKE RAY*,1,2, MORALES, MANUEL*,1,2, GARDENER, MARK3, INOUYE, DAVID2, 1 Williams College, Williamstown, MA2 Rocky Mountain Biological Laboratory, Gothic, CO3 The Open University, Milton Keynes, United Kingdom
ABSTRACT- By mediating the attractiveness of hosts to visitors, bottom-up effects may significantly regulate host-visitor mutualisms. We examined the trophic interactions among nutrients, plants, ants, and seed predators for an ant-plant mutualism located in the southwestern Colorado Rocky Mountains. The aspen sunflower Helianthella quinquenervis secretes amino-acid-rich extrafloral nectar (EFN) that attracts ants, especially Formica obscuripes at our study site. In return, ants prevent oviposition of three main dipteran seed predators (Neotephritis finalis, Trupanea nigricornis, and Melanagromyza sp.). During the summer of 2001, ant excluded and fertilized plots were established in a factorial design (fertilizer treatments were initiated during the summer of 2000). At the end of the flowering season in 2001, we collected flower heads for analysis. Results indicate that ant tending significantly decreases the number of seed-predator puparia per flower head. Fertilizer had no significant effect on oviposition rates, ant tending level or EFN composition (at the level of plots). In contrast, fertilizer decreased the weight of both adult flies and viable seeds. Future analyses are planned to examine the EFN composition of plants in fertilized and unfertilized plots at the level of individuals to correlate with ant tending. These preliminary results suggest that bottom-up effects act largely independently of the mutualism in this system.
KEY WORDS: mutualism, ant-plant, trophic cascade