The influence of biogeography and mating system on the ecology of desert annual plants

A major challenge in plant ecology is in understanding how species strategies mediate interactions between the environment and fitness. Variation in niche strategies that affect phenological, physiological, and reproductive traits will allow species to partition resources differently in space and time, allowing for coexistence of many species and strategies within a community. How species differentially respond to variable environments will ultimately influence their population dynamics and geographic distribution. This dissertation approaches this topic from two perspectives: (a) examining the interaction between biogeography and variable demographic strategies in desert annual plants, and (b) examining the costs and benefits of contrasting reproductive strategies in co-occurring selfing and outcrossing desert annuals. Firstly, I tested the abundant center model to determine the role of range position on plant population dynamics. I examined how the geographic and climatic position of 13 desert annuals found at a common location, the Desert Laboratory at Tumamoc Hill in Tucson, Arizona, related to their demography over a 25-year time span. I found that species for which the Desert Laboratory was close to the center of their geographic range have less variable long-term survival and fecundity compared to species for which the Desert Laboratory was further from the center of their range. Secondly, I studied how related species with contrasting mating systems respond to variable environments to affect plant performance. In a three-year field study I investigated how inter-annual variation in plant reproductive phenology affects synchrony with pollinators and herbivores. Since selfing species are guaranteed to reproduce in the absence of pollinators, seasonal and annual variation in phenology resulted in less variable plant reproductive success compared to outcrossing species. Greater variation in reproduction in outcrossing species resulted from asynchrony in some years between plants and pollinators. In a greenhouse study examining the interaction between mating system and drought, I found that the physiological functioning and survival of outcrossing species was more strongly negatively affected by drought conditions, suggesting that selfing species have an advantage in more arid environments. These studies demonstrate how plant reproductive and physiological strategies can play a critical role in influencing fitness, population dynamics and geographic distribution.