Columnar Cacti; A Critical Resource For Avian Consumers? Quantifying Avian Community Responses and Nutrient Fluxes in Natural and Experimental Settings

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Large columnar cacti, such as the saguaro (Carnegiea gigantea), are prominent features ofmany ecosystems. Because of their extensive distribution and abundant production of largesucculent fruit they play an important role in structuring food webs and ecosystems. Up untilnow, however, researchers studying the ecological and evolutionary aspects of plant-frugivoremutualisms have largely ignored this important group of plants. These succulent fruit may beparticularly important in hot deserts where high air temperatures and water scarcity mayconstrain animal behavior as well as the timing and investment in reproduction. The saguaro'sfruit are available during the driest and hottest period (June-July) of the summer in the SonoranDesert, before the onset of monsoon rains.In this research, I use a two-faceted approach to assessing the importance of a uniqueresource, the saguaro, to a community of avian consumers. The proposed research has severalfeatures that set it apart from earlier studies examining plant -animal interactions: 1) it combinesobservational data of resource use by individual consumers with direct measurements nutrienttransfer between the plant resource and a community of consumers using stable isotopes astracers; 2) it quantifies both temporal changes in fruit availability and changes in avianabundance, diversity, and productivity and 3) it directly manipulates resource availability,through fruit removal, while quantifying changes in consumer abundance, diversity, andproductivity. Taken in concert, these studies provide a unique approach to assessing theimportance of presumed irkeystonely or irfoundationlr resources by: 1) directly quantifying theflux of nutrients between a primary producer and individual consumers and 2) through a fruitremoval experiment, examining the effects of the fruit resource on consumer abundance,biodiversity, and productivity.I expect this research to produce an extensive array of new insights into: 1) the water andenergy balance of desert birds; 2) the role of cacti as consumer resources in arid and semi-aridecosystems; 3) frugivory and nutrient transfer rates between plant-consumer interactions and 4)community dynamics as it relates to changes in resource levels. This work will also quantify howdifferent consumer groups (seed predators versus seed dispersers) interact with the plant to affectthe plant's reproductive potential. It will provide the first comparative analysis of traditionalmethods of studying resource use to the newer techniques using stable isotopes.At the same time, it will be among the first studies to quantify the effects of removal of apresumed irKeystone Plant Resourcele on the abundance, diversity and reproductive output of anentire animal community. This research will also be the first to quantify the importance of asingle resource to the nutrient and water balance of multiple species of different body sizes,foraging modes and dietary guilds in any ecosystem. This work should also provide importantnew empirical evidence on how home range size and foraging mode affect individualexploitation of a resource system.Finally, this research will bring all facets of the scientific process to a large number ofgraduate and undergraduate students. Twenty undergraduate students and 1 graduate student willobtain extensive research experience working on this project and it is likely that many more willbe trained through the REU program. This research is bound to have a major inpact with thepublic as well, because of my association with the Arizona-Sonora Desert Museum. Researchresults from this project will be the basis of a centerpiece display at the museum that explores thescientific process and the interactions of saguaros with the other inhabitants of the SonoranDesert.