<p>The seasonal response patterns of net photosynthesis and respiration (and nitrogenase activity in Nostoc) are described within a multivariate framework of temperature, moisture and light for the alpine and grassland crustaceous lichens Rhizocarpon superficiale and Caloplaca trachyphylla respectively and for the grassland surface cyanophyte Nostoc commune. These physiological responses are discussed in context of each species' boundary layer environment, with particular emphasis placed on interactions between environmental constancy and adoption of acclimation strategies.</p> <p>For R. superficiale the high frequency of thermal fluctuations experienced by hydrated thalli, sometimes on an hourly basis, precluded any strategy of seasonal acclimation. Instead, photosynthetic rates exhibited a broad temperature response, remaining near 1 mg CO₂ h⁻¹g⁻¹ from 1 up to 21°C, with no changes evident between seasonal responses. In marked contrast C. trachyphylla shows a distinct winter/summer pattern of photosynthetic acclimation. In winter months rates are optimal near 7°C, while in summer the temperature optima of net photosynthesis shift to 21°C. These changes correlate well with predictable seasonal microclimate events, particularly those associated with winter Chinook snowmelt periods. A third pattern of response was seen in N. commune, where no seasonal changes in response patterns were evident and both nitrogenase activity and net photosynthesis were maximal near 35°C. This response pattern allows maximum carbon gain and nitrogen fixation during spring and summer periods following precipitation events, while its more sheltered aspect reduces the importance of winter snowmelt periods.</p> / Doctor of Philosophy (PhD)

Water is a critical resource for which plants compete in many terrestrial communities. In arid communities where water most limits plant growth, rainfall events occur in discrete, pulsed events. These pulses of water create highly variable soil moisture availabilities. Plant species respond differently to variation in soil water availabilities throughout a season and between years. How species vary in their responses to a range of water availabilities is thought to influence community and ecosystem properties. Many previously proposed hypotheses are not suitable to explain rapidly fluctuating resource availabilities or numerous input events throughout the growing season. This dissertation uses variation in water availability as a model resource to examine how species characteristics influence the process of exploitation competition within plant communities.Experiments were conducted to examine variation in growing season, exploitation competition between several pairs of co-occurring species in the Sonoran desert. Three separate studies evaluated several components of community dynamics thought to be influenced by exploitation competition. Spatial attributes of exploitation competition were assessed by measuring the performance of a deep-rooted species across the boundary of a natural expansion of a shallowly rooted species. Then, neighborhood composition was varied for species of similar growth-form to address the affects of species characteristics to shifts in abundances under field conditions. Lastly, species from the neighborhood composition study were placed under controlled, manipulated water availabilities to measure their fundamental operational conditions.Performances of plant species in all experiments were assessed using a combination of physiological and vegetative measurements, capturing the responses of the plants to both the dynamic growth conditions during the growing season, and integrated measures of plant performance post growth season. A shared preference was found for all species, where the performance of all species was greatest when water was most available in the soil profile. This work suggests the mechanism within a functional type by which plants coexist at various abundances is in part due to the variation in responses to temporal resource gradients. The variation in availability of resources and the species composition within the community should be considered in studies of competition between plant species.

The role of temperature variation in organismal performance is understudied, but is critically important for understanding the response of biodiversity to climate change. To address this issue in herbivorous insects, I studied the direct and interactive effects of thermal regime (constant vs. fluctuating temperatures) and nutrition (dietary nitrogen) on gypsy moth (Lymantria dispar) performance under laboratory conditions. Predictions for differences between constant and fluctuating thermal conditions were derived from Jensen’s inequality, and artificial diets of differing nutritional quality were made by modifying nitrogen (casein) content. Larvae were reared in the laboratory under four temperature regimes (22°C constant, 22°C fluctuating (±6°C), 28°C constant, and 28°C fluctuating (±6°C)) and two diet treatments (high N, and low N). Gravimetric analyses were also conducted to calculate nutritional indices and assess the short-term effects of temperature and diet quality on fourth instar larvae growth efficiencies. Consistent with predictions from Jensen’s inequality, fluctuating thermal conditions significantly reduced larval performance in both sexes across ontogeny. Low quality diet also reduced performance, but interactions between diet and thermal regime were only found in early instars.

Coastal fishes of the western North Atlantic, such as sciaenids and their competitors, support substantial commercial and recreational fisheries in waters that may vary widely in temperature, salinity, light intensity and spectral distrubution, and dissolved oxygen levels, yet their ecophysiological abilities to cope with such variability have received little attention. I therefore applied multidisciplinary comparative techniques to investigate aspects of the sensory and energetic ecophysiology of several sciaenid fishes and non-sciaenid competitors common in the western North Atlantic. Auditory brainstem response experiments demonstrated that sciaenid fishes have greatest auditory sensitivity at low frequencies that match their vocalizations. Based upon both anatomy and auditory bandwidths, most sciaenids appear to be hearing generalists that are likely sensitive to the particle motion components of aquatic sounds. Electroretinographic experiments revealed that the luminous sensitivities, temporal properties, and chromatic characteristics of the visual systems of phylogenetically-similar sciaenid fishes from different microhabitats, and those of phylogenetically-dissimilar piscivores from similar microhabitats, all correlated with lifestyle and ecology. The eyes of benthic and nocturnal fishes were typified by high luminous sensitivity, slow temporal resolution, and relative diel-invariance, consistent with foraging in dim photoclimates. By contrast, the eyes of pelagic diurnal piscivores had comparatively lower luminous sensitivity, higher temporal resolution, and exhibited higher diel variation, consistent with specific diurnal light niches. Accordingly, visually-foraging diurnal piscivores may be disadvantaged in eutrophied, turbid waters characteristic of many modern estuaries. Intermittent-flow respirometry experiments revealed that the majority of sciaenid fishes had resting and active metabolic rates similar to those of most teleost fishes but significantly lower than high-demand species such as tunas. However, the metabolic rates of kingfishes (Menticirrhus sp.) were significantly higher than other sciaenids, but significantly lower than those of tunalike fishes. Estimates of standard metabolic rate from power performance curves fitted to active metabolic rate data did not differ significantly from experimentally-derived measurements in static chambers, validating the experimental approach. Data from these chapters were analyzed with linear repeated measures and nonlinear mixed effects models that considered repeated measurement of subjects, modeled within-individual correlations, and the included random factors that improved the scope of inference. Although not novel approaches, these methods demonstrate quantitative advancements for future analyses of physiological data comprised of multiple measurements taken from individual experimental subjects. Collectively, the results of this dissertation underscore the potential power and utility of physiological techniques to provide a wide variety of information that may complement more traditional techniques used in fisheries science, particularly when coupled with appropriate analytical strategies. Sciaenid fishes are model organisms for investigations of the links between form, function, and the environment in coastal ecosystems.

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We applied the Floristic Quality Index (FQI) to vegetation data collected across a chronosequence of created wetland (CW) sites in Virginia ranging in age from one to 15 years post-construction. at each site, we also applied FQI to a nearby forested reference wetland (REF), for a total of 30 sites (15 created, 15 reference). We tested the performance of the index against a selection of community metrics (species richness, diversity, evenness, percent native species) and site attributes (age, soil physiochemical variables). The relationship between FQI and community and environmental variables was analyzed with Spearman's rank order correlation coefficient and Canonical Correspondence Analysis (CCA). Calculation of FQI with all species (including non-natives) did not increase the number of significant correlations (p<0.05) with community attributes and/or environmental parameters when compared with FQI based on native species alone. Further, vegetation layer-based FQI calculations improved the sensitivity of the index to differences in floristic quality between sites when compared with an "overall" index calculated across layers, and a modified, abundance-weighted FQI showed a unique correspondence with community and environmental variables in the CW herbaceous layer and REF herbaceous and shrub-sapling layers. These results suggest that a "natives only", layer-based version of the index should be used in wetland assessment in Virginia, and an abundance-weighted FQI may be a useful tool for assessing floristic quality in certain layers. An abundance-weighted format is perhaps desirable because such an index preserves the "heritage" aspect of the species conservatism concept inherent in floristic quality assessment, and also entrains the "ecology" aspect of site assessment based on relative abundances of the inhabiting species. FQI did not successfully relate CW sites to REF sites, bringing into question the applicability of the FQI concept in comparing created wetlands to reference wetlands, and the use of forested reference wetlands in general to assess vegetation development in created sites. Based on correlations with soil nutrient variables and ordination results, we propose a conceptual model of vegetation development in created wetlands described as the "Initial Conditions" model, which is expressed in terms of initial site conditions, soil chemistry, species diversity, and floristic quality.

Age and growth rates for salmon sharks (Lamna ditropis) in the eastern North Pacific (ENP) were estimated from seasonally formed bands in the vertebrae, and compared to previously published life history parameters for this species from the western North Pacific (WNP). Results of this study show that salmon sharks in the ENP achieve their maximum length at a faster rate, reach sexual maturity at an earlier age and achieve a greater weight-at-length than those in the WNP. Additionally, this dissertation shows that adult salmon sharks maintain a specific body temperature independent of changes in ambient temperature through a combination of physical and physiological means, and essentially function as homeotherms. Due to uncertainty in previous life history parameter estimates for sand tiger sharks (Carcharias taurus) in the western North Atlantic, age and growth rates were re-estimated using a larger sample size and captive individuals injected with oxytetracycline (OTC), a fluorescent skeletal marker. The results support a hypothesis that this species forms one pair of growth bands annually in the vertebral centra, whereas previous growth rate estimates were based on the formation of two bands per year. as such, the growth rate of this species is considerably slower than previously predicted and the population more susceptible to fishing mortality. Demographic analyses were conducted for salmon sharks in the ENP and WNP, and for sand tiger sharks (based on new life history parameter estimates) with uncertainty in vital rates incorporated via Monte Carlo simulations. Density-dependent compensation was included in models where fishing mortality was imposed by increasing sub-adult survivorship from output values generated by a previously published "Intrinsic Rebound Potential" model. The results indicate that both species are extremely vulnerable to fishing mortality and that no fishery should be implemented for sand tiger sharks or salmon sharks in the WNP. Salmon sharks in the ENP were the only population examined that indicated the potential to tolerate any fishing mortality. A comparison of growth completion rates and other life history parameters of ectothermic and endothermic sharks did not indicate that endothermic sharks achieve their maximum length at a faster rate than ectothermic sharks.

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Theory and small-scale experiments predict that biodiversity losses can decrease the magnitude and stability of ecosystem services such as production and nutrient cycling. Most of this research, however, has been isolated from spatial processes, such as dispersal and disturbance, which create and maintain diversity in nature. Since common anthropogenic drivers of biodiversity change, such as habitat fragmentation, species introductions, and climate change, are mediated by these understudied processes, it is unclear how environmental degradation will affect ecosystem services. This dissertation examines how diversity interacts with spatial processes to affect the magnitude and stability of ecosystem functions, using seagrass communities as a model system. Diverse communities were more resistant to colonization, but the order of species arrivals affected competition outcomes. as predicted, grazer metacommunities assembled from diverse species pools were more diverse at all scales, had larger grazer populations, and usually kept their primary food resource, epiphytic algae, at lower abundances than metacommunities assembled from smaller species pools. Counter to theory, increasing the number of mobile grazer species in these metacommunities increased spatial and temporal variability of producers and grazers. Effects of diversity on stability also differed qualitatively between patch and metacommunity scales. Moreover, allowing grazers to move among patches reduced diversity effects on production and modified relationships between grazer diversity and stability. Finally, dispersal significantly increased resistance to and recovery from a mimicked macroalgal bloom. However, diversity did not. None of the existing theories for biodiversity-ecosystem function relationships or consumer-resource metacommunity dynamics completely explained patterns observed in these experiments. Effects of diversity and dispersal on ecosystem functions were complex, but seemed to be influenced by habitat choice and synchronization of grazer and epiphyte dynamics among patches. Overall, these results emphasize the importance of incorporating spatial processes and trophic interactions into the study of biodiversity-ecosystem function relationships. This information is critical for conserving diversity and managing ecosystem services in light of the ongoing changes to regional species pools caused by anthropogenic disturbance.