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.

The foraging behavior of six species of insectivorous foliage-gleaning birds was studied in three different communities in Arizona to examine the role of resource partitioning in coexistence. I recorded data during winter and spring, the harshest seasons. Two to four species coexisted in each community. I recorded plant species, plant life-form, foraging height, plant portion, perch size, capture size, capture technique, and foraging rate. Foraging behavior was compared to the plant distribution profile, interspecifically among sympatric guild members within a season, intraspecifically between seasons, and intraspecifically between communities within a season. Species differed most in plant species selected, foraging height, and capture technique, and varied the least in plant portion, perch site, and capture site. All species overlapped in most behaviors, but differed from other guild members in at least two foraging variables. Permanent residents showed the greatest differences. These findings are consistent with niche complementarity and the idea that competition has led to partitioning of the resources. However, other hypotheses cannot be ruled out.

I used age structure to examine the role of fire disturbance and climate on the population dynamics of the subalpine forest in the southern Sierra Nevada. I cored trees on ten 0.1 ha plots (3300-3400 m elevation) that varied in species composition, from single-species foxtail pine (Pinus balfouriana) or lodgepole pine (Pinus contorta, var. murrayana), to mixed-species stands of both pines. Crossdating was used to produce accurate dates of tree recruitment and fire events. Age structure varied by plot species composition: lodgepole pine recruitment pattern is pulsed, sometimes forming single-cohort patches in response to fire; foxtail pine plots have a more steady pattern of recruitment; mixed-species plots show an intermediate recruitment pattern. Fire may maintain a species composition mosaic in the subalpine forest. Foxtail pine regeneration may increase in areas opened by fire, although not immediately following fire. Low-intensity fire may spread over areas larger than previously reported under certain conditions in the subalpine zone. In addition, unusually frequent, extreme, and/or extended periods of drought may severely limit subalpine tree regeneration. Growing season frost events and grazing before 1900 may also have affected trees establishing in the subalpine zone.

This study compared the effects of sampling metrics (basal area, density) on community structure analyses. Additionally, an oak woodland classification system was developed from the basal area data for the southwestern United States. Minimum-variance clustering was used to define communities within the oak woodlands. The communities defined by basal area data were more evenly distributed, showed more distinct communities, and were more homogeneous than the density-based communities. Thirteen communities were identified in the classification of oak woodlands, with mean basal area varying by an order of magnitude. Elevation had a major influence on the distribution of the communities.

I predicted the likely effects of fire on the abundance of grassland birds by determining which vegetal features influenced the distribution and abundance of grassland birds at the Buenos Aires National Wildlife Refuge, Arizona. Abundance of pyrrhuloxia (Cardinalis sinuatus) (r² = 0.363), Lucy's warbler (Vermivora luciae) (r² = 0.348), and total abundance of birds (r² = 0.358) was positively correlated with increasing density of mesquite (Prosopis velutina), whereas abundance of cactus wren (Campylorhynchus brunneicappillus) (r² = 0.452) was negatively correlated with increasing mesquite density. Abundance of loggerhead shrikes (Lanius ludovicianus) (r² = 0.693) was positively correlated with increasing environmental patchiness. Fire is likely to reduce the density of mesquite, thus I predict that those species positively correlated with mesquite density will decline following fire whereas species negatively correlated with mesquite density will benefit.

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