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Three different aspects of plant adaptations to ecological interactions are examined. The first one is in the area of plant breeding systems. The adaptations investigated in this study involved interactions between male gametes competing for fertilizations on the plant Phormium tenax, an agavoid of New Zealand. In this system I discovered a new type of self incompatibility that depends on the levels of competition among self- and cross-pollinated fruits. This mechanism is parallel to cryptic self-incompatibility in which individual self-pollen grains are not as successful as cross-pollen when competing in the same pistil. The competition-dependent abscission of self-pollinations considered here, however, operates at the level of whole flowers. This form of self-incompatibility may allow a high level of outcrossing to be achieved while it assures seed set when pollinations are scarce. The second case focuses on the interaction between a plant of the genus Bursera and its herbivorous crysomelid beetle Blepharida. This Bursera produces terpenes that are stored in networks of canals that run throughout the leaves and the cortex of the stem. When damaged, there is often an abundant release of resins. Blepharida larvae have developed the behavioral adaptations to overcome the secretive canals of Bursera. Before feeding on the leaves they cut the leaf-veins, interrupting the flow of terpenes. By documenting the growth and survival costs of being on plants of different response strength I was able to show that canals can effectively decrease herbivory even against this specialized vein-cutting insect. The handling time involved in blocking the canals slows down larval growth, delays pupation and increases the risk of predation. Chapter III examines a more complex interaction among plants that produce extrafloral nectaries, ants, and homopterans. An alternative model to explain the function of extrafloral nectaries is proposed. According to this hypothesis, the function of these glands is not to attract ants for defense, but to distract them from tending homopterans by giving them a free source of sugar. Different sources of evidence that support this model are discussed.

The desert offers windows of opportunity to annual plants, but they must deal with temporal variation in environmental conditions. This dissertation explores the idea that temporal variation plays a role in species coexistence, enhancing the diversity of desert annuals. Theory suggests that for temporal variability to promote coexistence among annuals: (1) species must differ in their years of highest fitness, (2) species must have long-lived seed banks, (3) the success of an abundant species must be limited in its otherwise good years by competition. Chapter 1 reviews mechanisms of coexistence applicable to terrestrial plants. Chapter 2 reviews the population biology of Sonoran Desert annuals. Chapter 3 investigates whether temporal variability might allow three different species (Pectocarya recurvata, Plantago patagonica, Schismus barbatus) on a creosote flat to out-perform each other in different years. In a two year experiment, I simulated additional year-types by manipulating factors that vary across years (water and seedling density), and incorporated shrub-covered and open microhabitats. I mapped seedlings, observed rates of herbivory and reproductive success. To compare species, I calculated the average value that seeds have for population growth from 10 years of data. I found shifts in competitive hierarchies for two species pairs, depending on year-type factors. Furthermore: (1) herbivory may contribute to shifts in competitive hierarchies and (2) habitat partitioning was not evident. Chapter 4 quantifies dormancy and germination fractions in the field for a guild of winter annuals. Dormant seeds were removed from soil samples collected after germination, but prior to new seed set. Seedling densities and reproductive success were followed in nearby plots. The species with the largest dormant seed bank had higher temporal variation in reproductive success (over the last 10 years) and tended to have smaller seeds, consistent with the theory that seed dormancy and large seeds are partially substitutable bet-hedging strategies. Plants germinated more in years of higher reproductive success, suggesting that germination could be "predictive". In addition, species responded differently to years. I discuss how these experiments demonstrate that this system possesses the traits required for temporal variation to promote coexistence.