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

University Of New Mexico, Albuquerque NM

Investigators

Abstract

Large columnar cacti, such as the saguaro (Carnegiea gigantea), are prominent features of many ecosystems. Because of their extensive distribution and abundant production of large succulent fruit they play an important role in structuring food webs and ecosystems. Up until now, however, researchers studying the ecological and evolutionary aspects of plant-frugivore mutualisms have largely ignored this important group of plants. These succulent fruit may be particularly important in hot deserts where high air temperatures and water scarcity may constrain animal behavior as well as the timing and investment in reproduction. The saguaro's fruit are available during the driest and hottest period (June-July) of the summer in the Sonoran Desert, before the onset of monsoon rains. In this research, I use a two-faceted approach to assessing the importance of a unique resource, the saguaro, to a community of avian consumers. The proposed research has several features that set it apart from earlier studies examining plant -animal interactions: 1) it combines observational data of resource use by individual consumers with direct measurements nutrient transfer between the plant resource and a community of consumers using stable isotopes as tracers; 2) it quantifies both temporal changes in fruit availability and changes in avian abundance, diversity, and productivity and 3) it directly manipulates resource availability, through fruit removal, while quantifying changes in consumer abundance, diversity, and productivity. Taken in concert, these studies provide a unique approach to assessing the importance of presumed irkeystonely or irfoundationlr resources by: 1) directly quantifying the flux of nutrients between a primary producer and individual consumers and 2) through a fruit removal 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 and energy balance of desert birds; 2) the role of cacti as consumer resources in arid and semi-arid ecosystems; 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 how different consumer groups (seed predators versus seed dispersers) interact with the plant to affect the plant's reproductive potential. It will provide the first comparative analysis of traditional methods 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 a presumed irKeystone Plant Resourcele on the abundance, diversity and reproductive output of an entire animal community. This research will also be the first to quantify the importance of a single 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 important new empirical evidence on how home range size and foraging mode affect individual exploitation of a resource system. Finally, this research will bring all facets of the scientific process to a large number of graduate and undergraduate students. Twenty undergraduate students and 1 graduate student will obtain extensive research experience working on this project and it is likely that many more will be trained through the REU program. This research is bound to have a major inpact with the public as well, because of my association with the Arizona-Sonora Desert Museum. Research results from this project will be the basis of a centerpiece display at the museum that explores the scientific process and the interactions of saguaros with the other inhabitants of the Sonoran Desert.