I studied the East Pacific green turtle, Chelonia mydas agassizii , at the Bahia de los Angeles foraging grounds of the Gulf of California from July 1995 to October 1999. Entanglement nets were used to capture 153 green turtles among 16 capture sites. Pooled straight carapace length (SCL; mean ± SE) was 75.1 ± 0.79 cm. Though there was no significant variation in mean SCL among years, there was evidence of a difference in mean SCL among capture sites. Most of the population (58%) consisted of immature turtles but did not differ significantly from a 1:1 immature:adult ratio. Pooled adult sex ratio (F:M) was biased toward probable females (2.70:1.00) and differed substantially from a 1:1 ratio. Approximately 5% of the population showed signs of anthropogenic-derived injuries ranging from missing flippers to boat impact scars. I collected gastric lavage samples from 101 green turtles and fecal samples from a subset of 45 turtles. Digestive tract contents were analyzed from seven stranded carcasses. The primary alga recovered in diet samples was Gracilariopsis lemaneiformis. Other major diet items included the green algae Codium sp. and Ova lactuca. Animal matter was found in all sample types. Tube worms (Sabellidae), sponges (Porifera), sea hares (Aplysia vaccaria), and sea pens ( Ptilosarcus undulatus) were the most commonly ingested invertebrates. I studied home range sizes and movements of 23 C. m. agassizii . Mean minimum convex polygon home range area was 1,801 ha. Kernel density estimates (95%) of home range area had a mean of 1,545 ha. Home range length averaged 7,548 m. Mean 24-h vagility was 9,535 m. Diurnal and nocturnal movements were evident. Turtles exhibited a bimodal pattern in which they foraged in nearshore algae pastures during evening and night hours and moved to offshore and/or insular resting sites by midmorning. This study indicates that Bahia de los Angeles is an important foraging and developmental habitat for C. m. agassizii. Management efforts should focus on reducing human disturbance and the use of entanglement nets in the entire bay. In such efforts it is essential that local inhabitants are provided with alternatives that encourage sustainable marine resource use and economic progress.

Croton fantzianus Seymour (Euphorbiaceae), is a structurally important tree in the tropical deciduous forest of the Municipality of Alamos in southern Sonora. It has become important as a non-timber forest product within the last thirty-five years, harvested for stakes (vara blanca ) for the agricultural fields in Sonora, Sinaloa and Baja California. A study was carried out on ecology of the species, macroeconomics and household economics to determine sustainability of current patterns of use. During a study on the ecology of the tree the species was identified from specimens collected and found to be a disjunct population known previously from Nicaragua. It was determined that vegetative and reproductive phenologies are driven by water availability with responses along a water gradient varying among study sites. Rapid development of silver mining precipitated woodcutting for timber and fuel at unprecedented rates in the late 17th century. The original tropical deciduous forest was transformed to secondary forest by this extraction. Closure of the mines in the early 20th century began economic isolation of the Municipality that persists today. The economy of Alamos is depressed in response to national and state economy, people are under- or un-employed. Many cut vara blanca. Economic strategies in the Municipality include exploitation of non-timber forest products, the most important is vara blanca. There are three levels of organization for vara extraction: cutters, middlemen truckers, and large truckers. Income generated by each level is significantly larger than the daily minimum wage. Financial records indicate that nine of eleven respondents rely on the resource for all or part of their incomes. A study of the population ecology of the species indicates it is the most important component of the forest community at all study sites. Population structure and composition are variable and may be related to prior harvest history. Loss of reproductive capacity was recorded on a recently harvested area. The tree may be a successional species and does well in disturbed areas. Government records and informant interviews indicate intensification of extraction has depleted the population. It is questionable if extraction at recent rates can be sustained.

Attempts to establish native grasses in revegetation projects in semidesert grasslands in the Southwestern United States often fail whereas revegetation of non-native lovegrasses (Eragrostis spp.) is frequently more successful. The reasons for differential establishment are unclear. Species may be differentially vulnerable to variable patterns of soil moisture availability during germination and seedling growth. Field experiments described within this dissertation investigated the effects of planting date and species on germination, emergence, mortality, survival, and seedling growth of native and non-native warm-season perennial grasses seeded in southeastern Arizona. Native species [Sideoats grama (Bouteloua curtipendula (Michx.) Torr.), cane beardgrass (Bothriochloa barbinodis (Lag.) Herter), green sprangletop (Leptochloa dubia (H.B.K.) Nees), and Arizona cottontop (Digitaria californica (Benth.) Chase)] germinated rapidly, produced a few, large cohorts of seedlings, and retained limited residual germinability following initial rain events. In contrast, Lehmann lovegrass (Eragrostis lehmanniana Nees) germinated more slowly, produced more, smaller cohorts throughout the growing season, and retained more residual germinable seeds following initial rains. Mortality of all species was highest in the first week following emergence. Early development of adventitious roots and relatively high rates of biomass accumulation exhibited by Lehmann lovegrass are potentially advantageous under variable environmental conditions. Establishment of fast-germinating native species is favored by rainfall patterns that support early seedling growth subsequent to initial rains; the risk of seeding failure for these species increases when lengthy dry periods follow initial rain events. Years in which summer soil moisture conditions are highly variable would tend to favor Lehmann lovegrass establishment. Gradual depletion of the seedbank, early seedling growth characteristics, and rapid production of seed in response to drought increase the probability that at least one cohort will establish or will survive long enough to replenish the seedbank for subsequent years. In southeastern Arizona, the probability of intermittent dry periods decreases as rainstorm frequency increases near the end of July, yet the recommended time to reseed is early summer. Adjusting the planting date to late July or early August may improve the potential for successful revegetation of native species that germinate rapidly and produce few cohorts following initial rains.

Phenotypic plasticity has been proposed as an adaptive mechanism by which organisms can maximize their fitness in response to short-term environmental variability. In this dissertation, I test one prediction that comes out of this idea: that populations from more variable environments should have higher levels of phenotypic plasticity than populations from less variable environments. I first analyzed precipitation variability and predictability across nine biomes in the Southwestern U.S. to determine a gradient of environmental variability. There was a non-linear negative relationship between precipitation variability and precipitation mean. In general, contrary to common belief, desert biomes were no more variable nor less predictable than nondesert biomes. I tested the relationship between environmental variability and phenotypic plasticity in seven populations of the pallid-winged grasshopper (Trimerotropis pallidipennis). Contrary to prediction, populations from more variable environments had lower, not higher, levels of phenotypic plasticity in development time. There was a significant convex quadratic relationship between plasticity for size at maturity and precipitation variability. In general, females in populations with more plasticity in development time had lower fitness. Plasticity in size at maturity generally did not affect fecundity, but increased survivorship. Plasticities in both traits conferred no significant costs or benefits in males. I tested the hypothesis that these results were due to constraints on the evolution of plasticity: either to a lack of genetic variation for plasticity or to antagonistic pleiotropy between size at maturity and development time. I found sufficient genetic variation for plasticity to evolve in all study populations and little evidence for antagonistic pleiotropy. I further tested whether selection for developmental stability or directional selection for short development time could explain the pattern of plasticity responses across the gradient. Low plasticity responses were apparently due to selection for developmental stability in deserts. I found weak evidence that antagonistic and synergistic selection could also explain the plasticity responses. I found no evidence that directional selection for short development time in all environments could explain the lower levels of phenotypic plasticity in the desert populations.

Vegetation mapping in and regions facilitates ecological studies, land management, and provides a record to which future land changes can be compared. Accurate and representative mapping of desert vegetation requires a sound field sampling program and a methodology to transform the data collected into a representative classification system. Time and cost constraints require that a remote sensing approach be used if such a classification system is to be applied on a regional scale. However, desert vegetation may be sparse and thus difficult to sense at typical satellite resolutions, especially given the problem of soil reflectance. This study was designed to address these concerns by conducting vegetation mapping research using field and satellite data from the US Army Yuma Proving Ground (USYPG) in Southwest Arizona. Line and belt transect data from the Army's Land Condition Trend Analysis (LCTA) Program were transformed into relative cover and relative density classification schemes using cluster analysis. Ordination analysis of the same data produced two and three-dimensional graphs on which the homogeneity of each vegetation class could be examined. It was found that the use of correspondence analysis (CA), detrended correspondence analysis (DCA), and non-metric multidimensional scaling (NMS) ordination methods was superior to the use of any single ordination method for helping to clarify between-class and within-class relationships in vegetation composition. Analysis of these between-class and within-class relationships were of key importance in examining how well relative cover and relative density schemes characterize the USYPG vegetation. Using these two classification schemes as reference data, maximum likelihood and artificial neural net classifications were then performed on a coregistered dataset consisting of a summer Landsat Thematic Mapper (TM) image, one spring and one summer ERS-1 microwave image, and elevation, slope, and aspect layers. Classifications using a combination of ERS-1 imagery and elevation, slope, and aspect data were superior to classifications carried out using Landsat TM data alone. In all classification iterations it was consistently found that the highest classification accuracy was obtained by using a combination of Landsat TM, ERS-1, and elevation, slope, and aspect data. Maximum likelihood classification accuracy was found to be higher than artificial neural net classification in all cases.

The Tachinidae is a taxonomically and ecologically diverse clade of parasitoids for which evolutionary and ecological relationships with hosts are largely unknown. Here, I employed a multidisciplinary approach to evaluate the determinants of patterns of host use in the Tachinidae. First, I examined spatio-temporal variation in the tachinid-dominated parasitoid assemblage of one lepidopteran species Grammia geneura . The parasitoid assemblage and parasitism rates varied dramatically among and within sampling sites, seasons, and years. I show that this variability may be a function of habitat-specific parasitism and indirect interactions between this host and other Macrolepidoptera through shared tachinid parasitoids. I then experimentally examined the host selection process in the tachinid Exorista mella. Host movement was an important elicitor of attack behavior. Flies also responded to odors associated with food plants of their host. Experienced flies attacked hosts more readily than did inexperienced flies. Based on these results, I proposed a host selection scenario for this tachinid species. E. mella also teamed to associate colors with hosts and avoided deterrent models that they had experienced. However, I failed to find evidence for odor learning. Learning of host-associated cues by E. mella may allow this parasitoid to take advantage of abundant host populations and maintain host-searching efficiency in an unpredictable environment. To examine how host-associated characteristics evolved in the Tachinidae, I reconstructed the evolutionary relationships within the subfamily Exoristinae using molecular data. Phylogenetic analyses generally supported recent classifications. Analyses of host-related characters indicated that tachinids show great evolutionary lability in behavior, morphology, and host range. Finally, I sampled host species to assess the determinants of tachinid community structure and host range. Several host characteristics were found to affect tachinid species richness. These patterns may be due to the opportunistic use of abundant hosts by polyphagous tachinids, enemy-free space provided by well-defended hosts, and the process of host location. Patterns of tachinid host use varied significantly with sample size, host diet breadth, host gregariousness, plant form, and host morphology. Taken together, these studies indicate high levels of plasticity in tachinid-host associations. This may be responsible for their ecological and evolutionary success.

Lake Tanganyika, East Africa, is one of the world's unique freshwater ecosystems. This lake is approximately 10 million years old and contains an extremely diverse fauna with high levels of endemicity. Most of the biodiversity in the lake is concentrated along the rocky shoreline, and this dissertation focused on the land-water interactions that may affect the littoral ecosystem as human population densities increase. First, the potential of carbon and nitrogen stable isotopes as indicators of human impact was examined. Sedimentary organic matter from four river deltas with contrasting patterns of land use was used to develop a predictive relationship between terrestrial patterns and geochemical signals. Stable isotope analyses of cores taken at two disturbed and two undisturbed watersheds were consistent with patterns seen in surface sediments. These results suggest that nitrogen isotopes are a better indicator of land use while carbon isotopes are useful in determining changes in vegetation patterns. To evaluate the effect of deforestation on ecosystem dynamics, epilithic metabolism was measured at several sites over a period of one year. Epilithic net productivity did not differ significantly between the disturbed and undisturbed watersheds, but epilithic respiration and algal biomass were significantly higher at the disturbed site. Further evidence of disturbance was given by greater amounts of inorganic material on the rocks and lower ambient oxygen concentrations at the impacted site. Finally, in order to identify species more susceptible to changes in benthic productivity, the food web structure of the pelagic and littoral food webs was elucidated using stable isotopic analyses. Diet specificity is not apparent in the littoral food web, suggesting that broad dietary preferences may help stabilize this persistent food web during environmental perturbations. The isotopic structure of the pelagic food web suggests that upwelling of deeper waters is an important nutrient source for the food web, which is consistent with current nutrient and hydrodynamic models.

The "signals approach" is an articulation of signal detection theory (SDT) as a model of decision-making in behavioral ecology. Though previous models of decision-making have taken into account variation in the quality of resources among which choices are made, variation in cues that signal quality has remained unaddressed. Treating stimuli as signals, accounting for stimulus variation as a source of uncertainty, reveals that such variation can have significant consequences on choice behavior. The signals approach functions alongside traditional models to produce a more full understanding of decision making. Here, I apply SDT in novel ways to predator response to aposematic prey, mimicry, discrimination learning, and sexual selection. Using data from existing literature, I show that the signals approach offers an account of predator response to aposematic prey alternative to traditional explanations based on associative learning. The mistakes that predators make may be better characterized as "false alarm" attacks rather than due to poor associative learning. Under SDT, the number of false alarms is expected to rise as aposematic prey abundance rises from rare to moderate levels. This increase in attacks is contrary to expectations based on associative learning, wherein the mistakes are expected to decrease or remain constant. SDT explains otherwise enigmatic empirical data. I develop a novel expression of SDT by questioning the "integrated signals" assumption. Changing this assumption extends the applicability of signal detection theory, providing a model of generalization and discrimination learning. This model is contrasted to associative learning and yields a novel explanation of the "peak shift" phenomenon. Peak shift can be characterized as a directional preference for novel stimuli under conditions of signal uncertainty. In flower discrimination learning experiments designed within a signal detection framework, bumblebees (Bombus impatiens) demonstrated peak shift. Peak shift has the potential to act as an agent of selection; pollinator selection of flower morphology and sexual selection of exaggerated traits provide examples. As a model of decision-making, signal detection theory can yield insight into receiver (e.g., predator) choice behavior and the consequences of that choice behavior on the subsequent evolution of the signals (e.g., prey appearance) upon which decisions are made.

The encroachment of deeply rooted woody plants into grasslands throughout the world has the potential to alter local, regional, and global water balance. The consequence of encroachment by woody plants on ecosystem water balance is, in part, related to the sensitivity of these plants to summer and winter precipitation pulses. This dissertation addresses the primary question: does pulse sensitivity of a dominant warm-desert woody plant, velvet mesquite (Prosopis velutina Woot.) vary across soil texture and water availability gradients? To address this question, sap flow and xylem anatomy and function were evaluated in mature velvet mesquite trees at two upland sites varying in soil texture at the Santa Rita Experimental Range (SRER), and one floodplain site along the San Pedro River National Conservation Area (SPRNCA). Experimental irrigation was used to assess the sensitivity of mesquite plants to small and large precipitation pulses. There was a moderate response to both small (10 mm) and large (35 mm) irrigation inputs by trees on sandy-loam soil, while trees on loamy-clay soil were only responsive to the large pulse. The differential response between sites was associated with differences in infiltration of the experimental pulses between the two soil types. Model predictions of the critical transpiration rate (Ecrit )--above which hydraulic conductivity through the soil-plant continuum falls to zero--showed that trees at the sandy-loam site operated well below their maximum transpiration rate before the onset of the monsoon. Conversely, plants on loamy-clay soils likely operate closer to their maximum permissible transpiration rates throughout the growing season. Hydraulic redistribution was observed and rates were tightly coupled to growing season and dormant season precipitation inputs. Hydraulic redistribution could enhance pulse sensitivity by transferring soil water to regions of the root zone that are otherwise dry, thereby allowing a greater proportion of the root system to participate in the extraction of pulse water during transpiration. Results from this research suggest that patterns of mesquite water relations are strongly mediated by soil texture. Nevertheless, once established, mesquite plants substantially modify ecosystem water balance, due to their responsiveness to growing season precipitation pulses, and their ability to withstand severe water deficits between precipitation pulses.

How will the intrusion of other species that remove rewards without providing reciprocal services affect the interaction between mutualists? How do costs and benefits from these "cheaters" compare to costs and benefits from potentially mutualistic visitors? Finally, did nectar-robbing, one kind of cheating, promote the evolution of complex levels of sociality by allowing bees access to a wider range of resources? I investigated these questions in the research described below. I found that pollinators visiting Chilopsis linearis (Bignoniaceae) spent less time visiting robbed flowers than visiting unrobbed flowers, and did not visit them as often as expected. Thus, robbing appeared to have a negative effect on pollinators visiting Chilopsis linearis . I compared costs and benefits of floral visitors to Chilopsis linearis (desert willow). Chilopsis had sensitive stigmas that closed immediately upon touch and may have reopened later. I found that the probability of stigma reopening depended on the source and number of pollen grains deposited. I compared visitors by number of pollen grains deposited, viability of pollen that they deposited, and their effect on stigmas. Nectar-robbers did not benefit plants by pollen deposition, but they also did not cost plants by causing stigmas to close without adequate pollen having been deposited. I investigated the effects of robbing on pollinator behavior and plant reproductive success. Nectar volumes were lower in robbed flowers than in unrobbed flowers. However, the most effective pollinators, bumblebees, did not avoid robbed flowers. In investigating male reproductive success, I found that on some days, dye mimicking pollen traveled farther from robbed flowers, indicating that robbing may sometimes be beneficial to plants. In investigating female reproductive success, I found that there was no difference in pollen tube number between robbed and unrobbed flowers. Thus, a negative effect on one mutualist may not affect the other mutualist. I hypothesized that the evolution of robbing in bees was associated with a broad diet breadth and the evolution of complex sociality. Using phylogenetically independent contrasts for taxa within three geographical regions, I found that in some cases, a broad diet breadth was associated with sociality and robbing.