<p>The principle of competitive exclusion holds that the strongest competitor for a single resource can exclude other species. Yet in many systems, more similar species appear to stably coexist than the small number of limiting resources. Understanding how and when similar species can stably coexist has taken on new urgency in managing biological invasions and their ecological impacts. Recent theoretical advances emphasize the importance of predators in determining coexistence. The effects of predators, however, can be mediated by behavioral changes induced in their prey as well as by their lethality. In this dissertation, I ask how considering multiple trophic levels changes our understanding of how a grass invasion (Pennisetum ciliare) affects species diversity and dynamics in southeastern Arizona. In considering interactions with plant consumers, and with the predators of those consumers, this research reveals more general ecological processes that determine species diversity across biological communities. I first present evidence from a grass removal experiment in the field that shows increased emergence and short-term survival of native perennial plants without grass. This is consistent with Pennisetum ciliare causing the observed concurrent decline in native plant abundance following invasion. I then present results from greenhouse and field studies consistent with that suppression of native plants being driven primarily through resource competition rather than increased rodent granivory. Granivorous rodents do not solely function as consumers, however, because they cache their harvested seeds in shallow scatter-hoards, from which seeds can germinate. Rodents thus act also as seed dispersers in a context-dependent mutualism. They primary granivores in areas invaded by Pennisetum ciliare are pocket mice (genus Chaetodipus), which have a well-studied tendency to concentrate their activity under plant cover to avoid predation by owls. Because the dense canopy of the grass may provide safer refuge, I hypothesized the pocket mice may be directly dispersing native seeds closer to the base of the invasive grass. Such a behavior could increase the competitive effect of the grass on native plant species, further driving the impacts of the invasion. By offering experimental seeds dusted in fluorescent powder and tracking where the seeds were cached, I show that rodents do preferentially cache experimental seeds under the grass. This dispersal interaction may be more general to plant interactions with seed-caching rodents across semi-arid regions that are experiencing plant invasions. Finally, I ask how the predator avoidance behavior exhibited by these rodents affects their ability to coexist with one another. Not only could their diversity affect that of the plant community, but the effects of plant invasions can cascade through other trophic levels. Theoretical understanding of how similar predator avoidance strategy alters coexistence had not yet been developed, however. Instead of a field study, therefore, I modified a general consumer-resource model with three trophic levels to ask whether avoidance behavior by the middle trophic level alters the ability of those species to coexist. I found that more effective avoidance behavior, or greater safety for less cost, increased the importance of resource partitioning in determining overall niche overlap. Lowering niche overlap between two species promotes their coexistence in the sense that their average fitness can be more different and still permit coexistence. These results provide novel understanding of behavioral modifications to population dynamics in multi-trophic coexistence theory applicable to this invasion and more broadly.

<p> Bird migration is the regular seasonal movements between breeding and nonbreeding grounds. In general, birds that breed in the Northern Hemisphere tend to migrate northward in the spring to take advantage of increasing insect populations and lower predation pressures and fly south when food availability and weather conditions decline. Embarking on a journey that can stretch a thousand miles round trip is a dangerous and arduous undertaking. While en route migrants must stop and feed to replenish their depleted energy reserves, often in unfamiliar locations with unknown predation pressures. They also must react to weather conditions during flight and while on the ground. Additionally, areas of high quality habitats where birds can refuel efficiently and safely may be few and far between. Therefore, it's not surprising that mortality rates can be higher during migration than at any other period of the year. Behavioral decisions such as where and where to stop, how long to stay, and when to leave all involve costs and benefits with an ultimate goal to balance the costs and benefits on order to achieve a successful and efficient migration. Optimal migration theory, aims to explain how migrants balance behavioral and physiological parameters of migration that minimize total time spent on migration, total energy expended, or mortality risk. The eventual result of these optimization pressures is thought to be a gradient of behavioral strategies that optimize different combinations of the three currencies: time, energy, and risk. I investigated how migratory behaviors of North American songbirds in the autumn balance the three currencies. More specifically I 1) explored how stopover site selection varies across migratory strategies at the landscape (Chapter 2) and habitat-patch (Chapter 3) scale; 2) investigated the importance of wind for the evolution and maintenance of migratory routes (Chapter 4); and 3) explored how selection of wind conditions for migratory departure affects overall behavioral strategies (Chapter 5). With this research, I hope to further our predictive abilities of migratory behaviors under various environmental and geographic situations using an optimal migration framework.</p>

<p> The island apple snail, <i>Pomacea maculata,</i> family Ampullariidae, is a large freshwater gastropod native to South America. <i>P. maculata </i> is known as a heavy consumer of aquatic plants. <i>P. maculata </i> was introduced to Florida in the 1990s and has rapidly spread throughout natural and man-made wetlands and waterways in the southeastern United States. Negative ecosystem impacts associated with <i>P. maculata</i> invasion include destruction of macrophyte communities via overgrazing, competitive exclusion of the native Florida apple snail <i>Pomacea paludosa,</i> and the potential transmission of toxins and parasites to predators. </p><p> Populations of <i>P. maculata</i> have been documented in freshwater tributaries of estuaries such as Mobile Bay, Alabama and the Caloosahatchee Estuary, Florida, and the snails may be moving into the estuaries themselves. The objectives of this study were: 1) to evaluate <i>P. maculata</i>'s potential to harm macrophytes like tapegrass, <i>Vallisneria americana, </i> in low-salinity estuarine environments, 2) to determine how the grazer's destructiveness might by moderated by environmental context (salinity, temperature, and the presence of other macrophyte species), and 3) to identify management and restoration strategies for <i>V. americana</i> to minimize the harm done by <i>P. maculata</i>. We accomplished these objectives with feeding trials and mesocosm experiments conducted under varying conditions of salinity, temperature, and macrophyte community composition. </p><p> We found that increasing salinity lowered <i>P. maculata</i> grazing pressure on <i>V. americana</i> but increasing temperature increased grazing pressure. Herbivory on <i>V. americana</i> was not reduced and was sometimes intensified when other aquatic plant species were present. The results of two mesocosm experiments suggested that salinity and snail presence have a nonadditive, antagonistic, effect on <i>V. americana.</i> I.e., in the absence of snails the plant performed best at 0 psu, whereas when snails were present the plant did best at 5-10 psu due to reduced snail grazing. Due to the significant sub-lethal impacts of salinity on <i>P. maculata</i>'s grazing and health it is unlikely that the snail's invasion will proceed beyond the lowest salinity portions of estuaries. These estuarine regions can therefore serve as a valuable refuge for <i>V. americana</i> populations, providing that effective water management keeps salinity below the approximately 10 psu threshold where significant direct harm occurs to the plants.</p>

<p> Climate change is arguably the biggest challenge facing humanity. Successful mitigation and adaption planning vitally requires more science in regard to its impacts on ecological systems. To address knowledge gaps regarding climate change impacts within the regional level, I performed a series of analyses on an "early responder" species in the Greater Yellowstone Ecosystem and examine how its distribution may be impacted by biotic and abiotic factors. My research aids in decision making processes for regional land managers that must address climate change in their policy decisions and increases ecological understanding at a landscape level. </p><p> This manuscript includes a detailed analysis of past, present, and projected climate in the Greater Yellowstone Ecosystem. I addressed the expected impacts of present and future climate shifts on the distribution of the sub-alpine tree species, whitebark pine (<i>Pinus albicaulis</i>) and its main disturbance agent, mountain pine beetle (<i>Dendroctonus ponderosae</i>). This research found a major reduction of suitable climate habitat for <i> P.albicaulis</i> within the Greater Yellowstone Ecosystem under multiple Global Circulation Models and Representative Concentration Pathway futures. Finally, this research determined that the recent <i>D.ponderosae</i> outbreak driven by climate effects in 2000&ndash;2010, that resulted in an unprecedented mortality event on <i>P.albicaulis</i> was more than double the risk area size of any previous outbreak since 1951. Although more studies are necessary to reduce uncertainty and make assertive recommendations for management actions, this research suggests that future sub-alpine stand structure and composition may be radically different than anything in recent history due to range shifts of suitable climate habitat and disturbance agents, and advocates for land managers to apply a multifaceted approach of competitor thinning and controlled burning to ensure the resilience and persistence of <i> P.albicaulis</i>.</p><p>

The potential for enriching the population of propane-oxidizing microorganisms was tested in two sets of subsoils which yielded a total of seven different subsoils. In a preliminary experiment propane was rapidly consumed after an 18-day lag phase in two subsoils, contaminated either by chlorinated solvents or by petroleum-derived hydrocarbons, and in one surface soil with no history of contamination. In a subsequent study with five variously contaminated subsoils, one subsoil could not be enriched for propane or methane consumption, but oxidations of these gases in the remainder of the subsoils were partially enriched. Linear regression was utilized to test for the possible enrichment of propane oxidation in these four subsoils. The regressions were adjusted to avoid any unjustified claim of greater success of the enrichment process than probably occurred in some of the treatments. Overall, three soils seemed to offer some evidence of enrichment over 5 weeks. A high degree of statistical significance $(P < 0.001)$ offered the best evidence that enrichment of propane-oxidizing activity occurred in one subsoil. Screening for propane-oxidizing bacteria in the subsoils resulted in the isolation of 51 bacterial species including several aerobic bacteria such as Variovorax paradoxus, Pseudomonas putida, and Rhodococcus rhodochrous. These species were all capable of growth on propane, 2-propanol, and n-propanol as sole sources of carbon and energy. Fatty Acid Methyl Ester (FAME) analysis of isolate JGiao22 showed no match with fatty acid profiles of different genera recorded in the Microbial Identification System (Microbial ID, Inc., Newark, DE). This isolate was further characterized utilizing partial 16S rRNA sequencing that indicated a close relation to the genus Variovorax. Isolate JGiao22 is a Gram negative, motile rod with peritrichous flagella, and contains two plasmids, a single gas vesicle, unique circular dense inclusions, and extracellular polymers. Our results justify naming this isolate as a new species, Variovorax propanotrophica sp. nov. JGiao22.

Field studies were conducted in Tha-Mai District, Chanthaburi Province. Larval habitat characteristics of 42 gem pits were examined from November 1986 through June 1988. Larvae were found in pits containing clear water under full or partial shade. Relationships between habitat characteristics and density were tested by stepwise regression analysis. High dissolved oxygen and humidity and low pH were associated with higher densities. Populations fluctuated with rainfall. Stepwise logistic regression analysis was used to predict the occurrence of An. dirus. Populations were positively correlated with humidity and collection water temperature but negatively correlated with pH and minimum water temperature. Variations in occurrence and density also were related to predators (Notonectidae and fish). High minimum water temperature and rainfall were correlated with decreases in wing length among both emerging males and females. High turbidity was associated with increased size in male but not in female. Seasonal variation in the size of both sexes was related to rainfall. Minimum water temperature, pH, protein and rainfall were correlated with the survivorship of emerging An. dirus. Adult size was positively correlated with survivorship but not as strongly as in lab-reared populations. Mean wing length of nulliparous females (3.035 mm) was not significantly smaller than parous females (3.039 mm). However, there was a significant difference when seasonal variation was considered. Since size variation was correlated with rainfall and minimum air temperature, females tended to be smaller in the rainy season and larger (with a higher parity rate) in the dry season. On average, $>$40% of dry season An. dirus live long enough to complete the extrinsic incubation period for Plasmodium falciparum. Both P. falciparum and P. vivax were detected and mosquito infection rates determined by ELISA. Malaria sporozoites were found in Anopheles dirus, vagus tessellatus and hyrcanus group so all are potential vectors in this area. Four size classes of An. dirus were reared from different larval densities. Higher densities produced smaller adults with lower survivorship. Larger females took larger blood meals and thereby were more susceptible to infection by artificial feeding with cultured P. falciparum gametocytes.

<p>Hawaiian lowland wet forest (HLWF) plant species are light-limited, yet no information exists on how the understory light varies in relation to species invasion, or if patterns of seedling regeneration and light are linked. I measured the red-to-far-red ratio (R:FR) of light to assess light quality and quantified diurnal variability in three forest types: native-dominated, partially-invaded, and fully invaded by strawberry guava (Psidium cattleianum). I asked: (1) how does understory light quality vary relative to invasion? (2) Are there differences in light quality moving vertically among forest types? (3) Are patterns of seedling regeneration and understory light related? Native-dominated forests had the greatest light quality (highest R:FR), and Psidium cattleianum-dominated forests had the lowest. While I predicted that native seedlings would prefer high-quality light sites, all seedlings preferred medium quality environments. In invaded HLWF, native seedling regeneration is hindered, and restoration efforts should focus on non-native understory removal.

<p> Empirical studies of graphs have contributed enormously to our understanding of complex systems, growing into a more scientific exploration of communities spanning the physical, biological, and social called network science. As the quantity and types of networks have grown so has their heterogeneity in quality and specificity resulting in a wealth of datasets that are not matched by existing theoretical methods. This is especially true in ecology where the majority of interactions are indirect and unobservable even in well-studied systems. As a result ecologists continue to grapple with three fundamental questions: Most basically, (i) `How do ecosystems function?' I answered this question by comparing networks to each other such that poorly-studied systems can be understood through their similarity to well-understood ones and theoretical models. To do this I created the alignment algorithm netcom which recasts ecosystem processes as statistical dynamics of diffusion kernels originating from a network's constituent nodes. Using netcom I constructed a supervised classifier which can distinguish processes in both synthetic and empirical network data. While this kind of inference works on currently available network data, I have shown how causality can serve as a more effective and unifying currency of ecological interaction. Measures of causality are even able to identify complex interactions across organizational scales of communities, answering the longstanding question (ii) `Can community structure causally determine dynamics of constituent species?' Moreover, causal inference can be readily combined with existing modeling frameworks to quantify dynamic interactions at the same scale as the underlying data. In this way we can answer the question (iii) `Which species in an ecosystem cause which other species?' These tools are part of a paradigm shift in ecology that offers the potential to make more reliable management decisions for dynamic ecosystems in real time using only observational data.</p><p>

Although the unprotected lands of northern Tanzania support large numbers of elephants, and provide critical linkages for wildlife movements across the region, there is little information on the dispersal patterns of elephants in these unprotected lands. Our home range measures (100% MCP) of 21 elephants with satellite collars in four study regions were highly variable (191 to 3,698 km2). Home range sizes (95% fixed kernel) of bulls were typically larger than those of females, and wet season ranges were typically larger than dry season ranges. There were large differences in average home range sizes reflected varying strategies for obtaining food and water and avoiding humans. All eight radio-collared elephants (3 bulls, 5 females) in the West Kilimanjaro study region crossed the Tanzania-Kenya border, but typically elephants crossed more frequently in the wet than the dry season, and bulls crossed 47% more frequently than females. These extensive transboundary movements indicate that the elephant populations of West Kilimanjaro and Amboseli NP constitute a single transboundary population. Based upon 14,287 fixes from eight collared elephants, the vast majority of time was spent in unprotected (X¯ =91.5%) versus protected (X¯ =8.5%) areas. Amboseli NP was visited by all eight elephants and was the protected area most utilized (X¯ =8%, range 2-24%). Based upon the movements of 15 GPS-collared elephants in northern Tanzania, we identified eight areas that we considered important for wildlife conservation corridors/linkages for elephants. Our conservation priorities for these corridors were based upon the levels of threats and conservation potential. Community interviews and hilltop surveys were used in two Maasai villages to determine the extent of wildlife conflict, community attitudes towards elephants, and if elephants were using a vegetation corridor to move between Tanzania and southern Kenya. Elephants were the most problematic wildlife species and were considered a nuisance. However, they believed they attracted tourists, and generally did not believe elephant numbers should be reduced. Based upon elephant conflict and use and the communities' need to maintain areas for cattle grazing and medicinal plant collection, the two communities established the first wildlife conservation corridor in Tanzania.

Thesis (M. S.)--Texas A&M University, 2005. / "December 2005." Title taken from PDF title screen (viewed October 23, 2007). Includes bibliographical references (p. 63-67) and appendix.