Evolution of three seed dispersal mechanisms in North American Ephedra

Seed dispersal has a profound effect on the distribution of a plant taxon. Seed dispersal occurs in two ways: abiotically, by wind or water (Howe and Smallwood 1982); and biotically, by birds, ants, mammals, or even self-dispersal (Howe and Smallwood 1982, Price and Jenkins 1986, Vander Wall 1993). Seed morphology often indicates the general means of dispersal, but dispersal syndromes must be clarified with field evidence. Many plant genera contain species that employ different mechanisms of dispersal. Members of the genus Pinus may be dispersed by wind, birds, and/or rodents (Vander Wall 1993, 1994; Lanner 1998, Hollander and Vander Wall 2004). Ephedra may also be a genus in which taxa have evolved different dispersal mechanisms due to different selective pressures, in one geographical location (North American arid lands), which makes Ephedra an ideal candidate for studying the evolution of dispersal adaptations.;The genus Ephedra is comprised of ∼50 species worldwide (Price 1996), and has members native to arid and semiarid regions of Asia, Europe, northern Africa, western North America and South America. Members of the genus Ephedra are gymnosperms, and are one of three extant groups of the order Gnetales. The genus receives a tremendous amount of attention because of its chemistry and evolutionary history, however details about the natural history, including dispersal ecology, of members of the genus remains unknown. The 12 North American members of the genus Ephedra appear, based on morphological structures, to be dispersed by one of three mechanisms: wind, frugivorous birds, and/or seed-caching rodents.;I investigated the possible dispersal mechanisms of 5 North American species of Ephedra (Chapter 1). I chose the five species based on the availability of large amounts of seeds, and made sure to have at least one species with traits representing each of the three dispersal types. I was able to demonstrate, using field studies that included seed removal transects, wind displacement transects, bird harvest observations, and dispersal of seeds from source shrubs, that at least three dispersal mechanisms (wind, frugivorous birds, and/or seed-caching rodents) are indeed present in the genus. I also conducted a common greenhouse experiment to demonstrate that Ephedra seeds planted in a manner mimicking their dispersal mechanism had higher rates of germination and seedling emergence than those of the same species planted in a manner mimicking a different dispersal mechanism.;I examined the interaction between harvester ants (primarily Pogonomyrmex spp.) and Ephedra seeds (Chapter 2). Pogonomyrmex spp. forage on Ephedra seeds, collecting them and taking them into their nest (pers. observation), however, the fates of the seeds are unknown. The western harvester ant (Pogonomyrmex occidentalis ) was the primary ant species involved in this study, although several other Pogonomyrmex species (for example: P. salinus, P. rugosus and P. barbatus) likely also interact with Ephedra seeds. By utilizing seed removal transects, and experiments around the ant nest, I was able to demonstrate that harvester ants do, in fact, harvest Ephedra seeds if they are within at least 7 to 10 meters from the nest entrance. However, I was unable to find any evidence to indicate that the ants were dispersing Ephedra seeds. All of my results indicated harvester ants acted strictly as seed predators.;I also examined the morphological characteristics and the degree of differentiation of the seeds and bracts (cones) of the North American Ephedra species (Chapter 3). I used the results of the morphometric measurements to conduct a principal components analysis (PCA) from which I was able to reduce the data to three new variables (PC's) which explained most (91.5%) of the variation in the data set. Based on the three PC's, I was able infer the dispersal mechanisms for each of the North American Ephedra species based on their morphological characteristics, and similarity to five species for which I previously determined the dispersal mechanism (chapter 1). I was further able to clarify the evolutionary history of these differing dispersal mechanisms in the genus by mapping the mechanism of dispersal onto two recent phylogenetic analyses (Ickert-Bond and Wojciechowski 2004, Huang et al. 2005). Within the North American clade, bird-dispersal via frugivory appears to be the ancestral dispersal mechanism, with shifts to wind dispersal occurring twice, and dispersal by seed-caching rodents occurring multiple times.