Theses and Dissertations

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Open Access
Moving beyond the aggregated models: woody plant size influences on savanna function and dynamics
(Colorado State University. Libraries, 2008) Sea, William Brian, author; Hanan, Niall P., advisor
Historically, models have played important roles in studying aspects of savannas, including tree-grass competition, fire, and plant-herbivore interactions. The models can be categorized as either (1) "aggregated" ones that neglect size structure but have the advantage of mathematical tractability or (2) complicated process-oriented ecosystem models incorporating mechanistic ecophysiology capturing greater ecological realism but constrained to simulation modeling. The aggregated class of models can be further separated into those focusing on resource utilization and tree-grass competition ("resource-based models") and those focusing on demographic impacts of disturbances by fire and herbivory ("demographic bottleneck models"). The resource and demographic models separately consider important aspects of savanna ecology, yet the two approaches have rarely been integrated, resulting in a significant gap in our understanding of savannas. For this study, I investigated the role of woody plant size in savanna ecology. Using extensive datasets along broad resource gradients of annual precipitation in southern Africa, I examined patterns of size-abundance for woody plants in relatively undisturbed savannas to see if relationships for savannas showed similar patterns to theoretical predictions for tropical forests. Contrary to assumptions and predictions made by aggregate savanna models, I found that the percentage of wood biomass subject to fire loss actually decreases in wetter savannas. Since resource limitation and "thinning" have been mentioned as potential factors in savannas, I investigated the suitability of self-thinning in savannas. I developed a simple theoretical model hypothesizing three potential impacts of tree-grass interactions on the self-thinning relationship. Results from the analyses, testing with field data, suggest that tree-grass competition is asymmetric with respect to tree size. For the formal modeling component of my dissertation, I developed a simple savanna model that integrates demographic bottleneck and resource-based approaches. The model is unique in that the woody carrying capacity has both resource and demographic constraints. Model simulations showed that modest amounts of variation in adult mortality during fires and size-asymmetric tree-grass competition lead to very different model outcomes. The work opens up an entirely new class of ecological models for savanna ecology: analytically tractable with enough size structure to capture realistic savanna vegetation-disturbance interactions.
Open Access
Recovering spatially and temporally dynamic regional scale carbon flux estimates
(Colorado State University. Libraries, 2009) Schuh, Andrew, author; Denning, Scott, advisor
This dissertation presents two review type chapters and three new research chapters that contribute to our theoretical and practical knowledge about terrestrial carbon fluxes on the regional scale. This research expands on previous carbon dioxide inversion work by providing estimates of ecosystem respiration and gross primary productivity, as opposed to only net ecosystem exchange, and provides estimates on scales in time and space not previously available. The first two chapters provide an introduction and review material. This is necessary to provide the reader with an understanding of the relatively complex geostatistical atmospheric inversion process which uses carbon dioxide concentration data to provide terrestrial carbon flux estimates. Issues of scale are discussed as well previous work which was fundamental to the research presented here. The third and fourth chapters use simulated data to present an analysis of the methodology to a case study of North America in 2004. In particular, simulated data is used to investigate the sensitivity of the inversion to theoretical components of the inversion process and it is concluded that reasonably robust estimates of ecosystem respiration and gross primary productivity can be achieved by using a limited network of eight carbon dioxide observing towers. Chapter 4 specifically looks at the issue of small scale variability in carbon fluxes and the impact it has on obtaining larger scale regional estimates. Chapter five contains an analysis of real collected CO2 observation data from 2004 at the aforementioned eight observing sites. Results show significant seasonal and annual corrections to the a priori carbon flux estimates, in particular to the individual components of net ecosystem exchange, ecosystem respiration and gross primary productivity. Furthermore, the annual net ecosystem exchange, when presented spatially, provides clues to annual sources and sinks in 2004. Sensitivity is investigated with respect to numerous components of the inversion. Although large confidence bounds on estimates indicate statistical uncertainty in the mean estimate of net ecosystem exchange, estimates match reasonably well with previously conducted research as well as observational data. The research provides the estimates within a spatial context (and resolution) that was not previously available, allowing for the construction, and support, of much more descriptive hypotheses about carbon fluxes than was previously possible. Chapter six contains a summary of the results of the dissertation.
Open Access
Western Serengeti people shall not die: the relationship between Serengeti National Park and rural household economies in Tanzania
(Colorado State University. Libraries, 2009) Knapp, Eli J., author; Galvin, Kathleen, advisor
This research examined the relationship between Serengeti National Park and rural household economies living near (within 18 kilometers) its western borders in Tanzania. The study was based upon semi-structured household interviews with a general sample (N = 722), acknowledged poachers (N = 104), households with park-related employment (N = 50) and key informants (N = 15) in three two administrative regions and three districts. Interviews generated information about four primary social-ecological interactions which included crop destruction by wildlife, illegal hunting, park-related employment, and wildlife depredation on livestock. A cost-benefit analysis revealed that the average household generates a net profit of USD $13 from these interactions. Despite this, 84 percent of households were found to be food insecure for maize, the region's primary food crop. Moreover, 78 percent of households were found to be significantly over-budget over the preceding 12 months. These findings suggests that most households next to Serengeti National Park are generally impoverished and are lacking adaptive capacity to deal with severe environmental or socio-ecological changes. The first component of the research provided the context for western Serengeti. Significant findings included the importance of secondary education for increasing income to household economy and showed the level of dependence that households have on local natural resources. Households draw more heavily (often illegally) from the National Park with the advent of severe crop failures which were found to occur with a ten year periodicity. The second component revealed that neither crop damage nor wildlife depredation on livestock is distributed evenly. Rather, they are heavily localized with few effects on some households and severe effects on others. Although the effects of wildlife on crops and livestock generally decreased with distance from the Park, losses were particularly large for households within three kilometers of a boundary. The third component examined illegal bushmeat hunting and sales. Findings from respondents and extensive court documents suggested that fines and imprisonment had little effect on curbing illegal hunting behavior. The fourth component consisted of a synthesis of the cost-benefit analysis with a focus on food security and its effects on adaptive capacity. Implications of these findings are made for the resilience of the coupled socio-ecological system in western Serengeti.
Open Access
Characterize southwestern United States pinon-juniper woodlands: seeing the "old" trees for the "young" forest
(Colorado State University. Libraries, 2008) Jacobs, Brian Francis, author; Romme, William H., advisor
Southwestern U.S. piñon pine and juniper woodlands are often represented as an expanding and even invasive vegetation type, a legacy of historic grazing and culpable in the degradation of western rangelands. Yet the extent and dynamics of piñon-juniper communities pre-dating intensive Euro-American settlement activities are poorly known or understood, while the intrinsic ecological, aesthetic, and economic values of old-growth woodlands are often overlooked. Historical changes in piñon juniper include two related, but poorly differentiated, processes: recent tree expansion into grass or shrub dominated (i.e., non-woodland) vegetation and thickening or infilling of savanna or mosaic woodlands pre-dating settlement. My work addresses the expansion pattern, modeling the occurrence of "older" savanna and woodland stands extant prior to 1850, in contrast to "younger" piñon juniper growth of more recent, post-settlement origin. I present criteria in the form of a diagnostic key for distinguishing "older", pre-Euro-American settlement woodlands from "younger" (post-1850) stands, and report results of predictive modeling and mapping efforts within the Four Corners states (i.e., Arizona, Colorado, New Mexico, and Utah) of the American southwest in piñon juniper types characterized by Pinus edulis and three associated junipers (Juniperus osteosperma, J. monosperma, J. scopulorum). Selected models suggest a primary role for soil moisture in the current distribution of "old" versus "young" piñon juniper stands. Pre-settlement era woodlands are shown to occupy a discrete ecological space, defined by the interaction of effective (seasonal) moisture with landform setting and fine-scale (soil-water) depositional patterns. "Older" stands are generally found at higher elevations or on skeletal soils in upland settings, while "younger" stands (often dominated by one-seed juniper, Juniperus monosperma) are most common at lower elevations or in productive, depositional settings. Areas of the southwestern U.S. with strong monsoonal (summer moisture) patterns appear to have been the most susceptible to historical woodland expansion, but even here the great majority of extant piñon juniper has pre-settlement origins (although widely thickened and infilled historically) and old-growth structure is not uncommon in appropriate upland settings. Modeling at broad regional scales can enhance a general understanding of piñon juniper ecology, while predictive mapping of local areas has potential to provide products useful for land management.
Open Access
Early detection and rapid assessment of invasive organisms under global climate change
(Colorado State University. Libraries, 2009) Holcombe, Tracy R., author; Laituri, Melinda J., advisor; Stohlgren, Thomas J., advisor
Invasive species alter native species assemblages, effect ecosystem processes, and threaten biodiversity worldwide. Early detection and rapid assessment will help stem the problem, focusing managers attention on newly established invasive species before they spread. This is a big task requiring a coordinated effort and a centralized data sharing effort. One tool that can be used in this effort is Geographic Information Systems (GIS). GIS can be used to create potential distribution maps for all manner of taxa, including plants, animals, and diseases, and may perform well in early detection and rapid assessment of invasive species. As an example application, I created maps of potential spread of the cane toad (Bufo marinus) in the southeastern United States at an 8-digit Hydrologic Unit Code (HUC) level using regression and environmental envelope techniques. Equipped with this potential map, resource managers can target field surveys to areas most vulnerable to invasion. However, there is a general need in invasive species research to quantify the potential habitat of many invasive plant species. I was interested in modeling the shifts in suitable habitat over time, environmental space, and climate change. I used 4-km2 climate scenarios projected to the years 2020 and 2035 for the continental United States, to examine potential invasive species habitat distributions. I used maximum entropy modeling (Maxent) to create three models for 12 invasive plant species: (1) current potential habitat suitability; (2) potential habitat suitability in 2020; and (3) potential habitat suitability in 2035. These models showed areas where habitat suitability remains stable, increases, or decreases with climate change. Area under the receiver operating characteristic curve (AUC) values for the models ranged from 0.92 for Pennisetum ciliare to 0.70 for Lonicera japonica, with 10 of the 12 being above 0.83 suggesting strong and predictable species-environment matching. Change in area between the current potential habitat and the year 2035 ranged from a potential habitat loss of about 217,000 km2 for Cirsium arvense, to a potential habitat gain of about 133,000 km2 for Microstegium vimineum. These results have important implications for developing a triage approach to invasive species management under varying rates of climate change.
Open Access
Sensitivity of grassland ecosystems across the Great Plains to present and future variability in precipitation
(Colorado State University. Libraries, 2008) Heisler, Jana Lynn, author; Knapp, Alan K., advisor
Patterns and controls of aboveground net primary productivity (ANPP) have been of long-standing interest to ecologists because ANPP integrates key aspects of ecosystem structure and function through time. In many terrestrial biomes, water availability is a primary constraint to ANPP, and it is an ecosystem driver that will be affected by future climate change. To understand the sensitivity of temperate grasslands to inter- and intra-annual variability in precipitation, I analyzed long-term ANPP data, conducted a multi-site experimental manipulation in which the number of growing season rainfall events was varied, and simulated the effects of altered rainfall regimes using a terrestrial ecosystem model (DAYCENT). I conducted this research within the Great Plains of North America-a region characterized by a strong west-east precipitation-productivity gradient and three distinct grassland types-the semi-arid shortgrass, the mixed-grass prairie, and the mesic tallgrass prairie. My results demonstrate that temperate grasslands are indeed sensitive to both inter- and intra-variability in precipitation, but the ANPP response is contingent upon ecosystem structure and typical soil water levels. Additionally, both management strategies and topographic location may interact with precipitation to enhance or diminish coherence in the ANPP response. At the dry end of the gradient (semi-arid steppe), fewer, but larger rain events led to increased periods of above-average soil water content, reduced plant water stress and increased ANPP. The opposite response was observed at the mesic end of the gradient (tallgrass prairie), where longer dry intervals between large events led to extended periods of below-average soil water content, increased plant water stress, and reduced ANPP. Mixed grass prairie was intermediate along the gradient, characterized by the greatest plant species richness, and the most sensitive to within-season variability in rainfall. Comparison of these experimental data to model simulations revealed key differences in soil water dynamics and ANPP patterns, suggesting that more experimental data is needed to parameterize biological and physical processes that drive model simulations. In conclusion, these results highlight the difficulties in extending inference from single site experiments to whole ecosystems or biomes and demonstrate the complexity inherent in predicting how terrestrial ecosystems will respond to novel climate conditions.
Open Access
Trace gas biogeochemistry in response to wildfire and forest management in ponderosa pine ecosystems of Colorado
(Colorado State University. Libraries, 2008) Gathany, Mark A., author; Burke, Ingrid C., advisor
Fire exclusion practices during the last century increased fuel and fire hazard in the western U.S., where conditions have also become drier and warmer in recent decades. As a result, fire frequency and extent have increased significantly. Wildfires and forest management alter soil carbon and nitrogen availability and the physical environment. These factors are primary controls on greenhouse gas (carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) flux rates. The two-way interaction between forest wildfires/management and flux rates may be significant considering the positive feedback loop that could lead to further climate warming. I explored these relationships in a series of field studies in which I measured soil trace gas exchange rates in ponderosa pine forests of the Colorado Front Range that had recently experienced a wildfire or forest thinning. I also used the ecological simulation model, Daycent, to simulate the effects of long term climate variability, varied fire frequency and fire suppression in order to estimate the changes in CH4, N2O, NO (nitric oxide) fluxes and gross nitrification rates at four sites in the Colorado Front Range. My findings suggest that soil CO2 fluxes increase in the years after a wildfire, and that local scale variables such as soil moisture, temperature, and fire severity are important controlling factors for these trace gas fluxes. Forest thinning practices increased substrate availability in some cases such that CO2 and N2O fluxes increased, but only when soil moisture was high, during the sampling season. Using Daycent, I found CH 4 uptake was consistent among sites with different landscape characteristics, and showed minimal changes in response to fire. Daycent simulations estimate a 13-37 % decrease in N2O and NO fluxes, and gross nitrification rates during the fire suppression era relative to before the suppression era. Overall, my research revealed that wildfire and forest management do alter the exchange rates of CO2 and N2O primarily by increasing substrate availability and environmental variability. Therefore, as wildfire activity and forest management are anticipated to increase in both frequency and extent, my research suggests that CO2 and N 2O source strength may increase from Colorado ponderosa pine ecosystems. Keywords: carbon dioxide, methane, nitrous oxide, trace gas, greenhouse gases, fire, soil, ponderosa pine, Colorado Front Range, wildfire, Daycent, forest management.
Open Access
Disturbance impacts on understory plant communities of the Colorado Front Range
(Colorado State University. Libraries, 2009) Fornwalt, Paula J., author; Romme, William H., advisor
Pinus ponderosa - Pseudotsuga menziesii (ponderosa pine - Douglas-fir) forests of the Colorado Front Range have experienced a range of disturbances since they were settled by European-Americans approximately 150 years ago, including settlement-era logging and domestic grazing, and more recently, wildfire. In this dissertation, I explored the impacts of these disturbances on understory plant communities. I investigated the long-term effects of settlement-era logging and grazing on forest understories by comparing understory composition at a historically logged and grazed site to that of a site that was protected from past use. I found little to no evidence of long-term logging and grazing impacts on understory richness, cover, and composition in upland forests. Long-term changes in richness, cover, and composition due to past logging and grazing were somewhat apparent in riparian forests, however, where these activities were likely the most intense. I analyzed data collected before (1997) and after (2003-2007) the 2002 Hayman Fire to examine wildfire effects on understory communities. Some declines in species richness and cover were observed immediately following fire, but by 2007, richness and cover often exceeded prefire conditions, even in severely burned areas. Fire-induced changes in community composition were apparent in all postfire years; regardless of fire severity, these changes were primarily due to new species recruitment, particularly short-lived native forbs, rather than due to a loss of prefire species. While exotic richness and cover generally increased as fire severity and time since fire increased, they remained low at the end of the study, and have not yet interfered with the recovery of the native understory community. I conducted a literature review to examine the mechanisms through which Front Range understory species establish after fire (i.e., by sprouting, establishing from soil-stored seed, and/or establishing from offsite seed). I found that postfire establishment mechanisms for many species are poorly understood, although some broad patterns did emerge. Short-lived forbs appear to establish postfire primarily through soil-stored seed, while sprouting is the most common postfire establishment mechanism for long-lived forbs, graminoids, and woody plants. Many species have multiple postfire establishment mechanisms, which helps to ensure their continued presence after fire.
Open Access
"Staying together": people-wildlife relationships in a pastoral society in transition, Amboseli ecosystem, southern Kenya
(Colorado State University. Libraries, 2009) Ferreira de Lima Roque de Pinho, Maria Joana, author; Galvin, Kathleen A., advisor
This study looks at three dimensions of the relationship between Maasai and wildlife: attitudes towards wildlife; cultural models of human-wildlife relationships; the aesthetic value of wildlife and its relation to support for wildlife conservation. First, I found that attitudes varied with land tenure, formal education, religion and gender. I used a regression analysis to identify predictors of positive attitudes towards wildlife. Being a Christian is the strongest predictor, followed by being male and residing on communal land. Second, I followed a cognitive anthropology approach to analyze how Maasai relate to wildlife. "Cultural models" are implicit, shared cognitive representation of a conceptual domain that mediate our understanding of the world and are differentially distributed, socially transmitted and correlated with behavior. I investigated content and distribution of Maasai models of their relationship with wildlife. With discourse analysis, I identified two contrasting models of human-wildlife relationships. In the "traditional" model, wildlife are seen as different from cows in everything but as having the right to be on the land since God meant for humans, cows and wildlife to "stay together". In contrast, in the "modern" model, wild animals are useful and income-generating like cows, but people wish to be separated from them. I used cultural consensus analysis to determine the distribution of agreement with each model. It shows that there is one consensual model that is close to the "modern" model. This study shows a shift towards models of human-wildlife relationships that are informed by western culture, the market economy and conservation. The consensual model contrasts with the vision that conservationists have for the ecosystem. Investigating stakeholders' cultural models is a step towards addressing such conflicts. Lastly, I examine the role of aesthetic value in human-wildlife relationships. I show that Maasai appreciate visual beauty in wild animals and enjoy the sight of wild animals. Then, I determine that there is an association between how Maasai aesthetically value species, preferences thereof and support for their conservation. The community-based conservation approach emphasizes the economic value of wildlife to local communities. This study suggests that these strategies would benefit from considering non-economic dimensions of human-wildlife relationships.
Open Access
Ecosystem respiration and foliar morphology of a primary tropical rain forest: the effects of canopy structure and environmental gradients
(Colorado State University. Libraries, 2007) Cavaleri, Molly A., author; Binkley, Daniel, advisor; Ryan, Michael, advisor
Wood and foliage are major components of ecosystem respiration, but estimates of large-scale rates for tropical rain forests are uncertain because of poor sampling in the upper canopy and across landscapes. Carbon balance models often rely on leaf mass per area (LMA) because it correlates with many plant physiological parameters. Researchers have long assumed variation in LMA to be a response to light (sun/shade leaf dichotomy), but LMA also reflects increases in leaf density that result from decreasing water potential with height. We used a portable scaffolding tower to measure plant respiration, LMA, and light from ground level to the canopy top across 55 sites in a primary tropical rain forest in Costa Rica. The first objective of this study was to extrapolate woody CO2 efflux to the forest by characterizing its variation with canopy structure and landscape gradients. The second objective was to extrapolate foliar and total respiration to the forest by investigating the variation in foliar respiration with foliar parameters, canopy structure, and landscape gradients. The third objective was to determine whether LMA varied primarily because of light or water potential. Wood and foliage respiration rates increased with height and showed differences between plant functional groups. Wood respiration per unit ground area was 1.3 μmo1 CO2 M-2 s-1 and foliar respiration was 3.5 μmol CO2 m-2 s-1, representing 14% and 37% of total ecosystem respiration, respectively. Total ecosystem respiration (9.38 ± 1.43 μmol CO2 m-2 s-1) was 33% greater than eddy flux nighttime net ecosystem exchange for the same forest, suggesting that eddy flux studies reporting a large sink for tropical rain forests may be in error. We found LMA to be better related to height than light environment, supporting the hypothesis that the LMA gradient within forest canopies is primarily driven by a linear decrease in turgor pressure with height, caused by an increase in hydraulic resistance with gravity and longer path length. While light does affect LMA slightly, especially in the light-limited understory, the sun/shade leaf model taught in every plant physiology textbook is too simplistic to describe the large variation of LMA with vertical structure.
Open Access
Indirect effects in plant-pollinator interactions: the role of exotic plants and herbivores
(Colorado State University. Libraries, 2008) Cariveau, Daniel Paul, author; Norton, Andrew P., advisor
Flowering plants interact with a variety of other species. While numerous studies have demonstrated that pair-wise interactions between species are important factors affecting plant ecology and evolution, interaction with one species may affect the outcome of the interaction with another. I examined how pollination is influenced by both competition and herbivory. In Chapter One, I tested whether the presence of an exotic plant, musk thistle (Carduus nutans L. (Asteraceae)) influenced flower visitor behavior in relation to the native plant, bee balm (Monarda fistulosa L. (Lamiaceae)). I found that visitation rate to the native was not affected by the presence of the exotic. However, flower visitors commonly switched between the native and exotic and transferred exotic pollen to native plant stigmas. Conspecific pollen on the native plant stigmas was also reduced in the presence of the exotic. Seed set of the native plant was not affected. In a separate experiment, I examined how distance from the exotic plant influenced visitation rate to the native plant. I found that visitation rate to the native plant was reduced when the native plant was 1 and 5 meters from the exotic. However, visitation rate remained unchanged at 0 and 15 meters. This suggests that magnitude of interactions between plants through flower visitors may depend on spatial scale. In Chapter Two, I examined how the exotic plant, musk thistle, influenced visitation rate to the native plant, common harebell (Campanula rotundifolia L. (Campanulaceae)). I found that visitation rate to the native plant was reduced in the presence of the exotic plant. However, only solitary bees exhibited a reduction in visitation rate while Bombus species did not. Flower visitors did not switch between the exotic and native plants, and there were no exotic pollen grains on the native plant stigmas. Conspecific pollen deposition and seed set were not affected by the presence of the exotic plant. In Chapter Three, I explored whether the biological control Mecinus janthinus (Coleoptera) affected floral display size and visitation rate to the exotic plant Dalmatian toadflax (Linaria dalmatica (Scrophulariaceae)). In addition, I examined whether M. janthinus feeding affected female reproductive success directly or indirectly through flower visitors. I found that herbivory decreased the number of flowers and visitation rate to Dalmatian toadflax. However, I found no effect of herbivory on seed set when conducting hand-pollinations, suggesting no indirect effects of M. janthinus through flower visitors.
Open Access
Pathways of continuity and change: diversification, intensification and mobility in Maasailand, Kenya
(Colorado State University. Libraries, 2007) BurnSilver, Shauna B., author; Galvin, Kathleen A., advisor
In recent decades multiple drivers have been acting on Maasai pastoralists in the Greater Amboseli Ecosystem (GAE). The Kenyan government and international policymakers have made a systematic effort to modernize the Kenyan pastoral sector based on prescriptions of livestock intensification and a process of land tenure change that moves herders from communal land use to privatized land ownership. Additional drivers including population growth, competition for territory, greater engagement in the Kenyan economy, and frequent drought, are also pushing and pulling pastoralists to adjust their livelihood strategies. These drivers have created an atmosphere of unprecedented change in Maasailand, Kenya -- a situation with negative implications for pastoral well-being and resilience. How pastoral households cope with these challenges is the central question of this PhD study. Three key responses are identified and analyzed: economic diversification, livestock intensification and livestock mobility.
Open Access
Hybridization between spotted knapweed (Centaurea stoebe) and diffuse knapweed (Centaurea diffusa): patterns and implications for invasion
(Colorado State University. Libraries, 2008) Blair, Amy C., author; Hufbauer, Ruth, advisor
Hybridization is an evolutionary force that has the potential to alter invasion dynamics. Centaurea diffusa Lam. (diffuse knapweed) and C. stoebe L. (spotted knapweed) are two problematic invasive weeds in western North America. Anecdotal information suggested these two plants were hybridizing in the introduced range. The overall goal of my dissertation was to examine the patterns of hybridization in the introduced and native ranges in the field and at the molecular level, and determine if hybridization was altering the invasion of either species.
Open Access
Microbial responses to plant functional types and historical resources additions in the shortgrass steppe
(Colorado State University. Libraries, 2009) Bontti, Eliana E., author; Burke, Ingrid, advisor
Nutrient addition in rangelands is an appealing way to increase plant biomass and quality, but little is known about the long-term effects of these additions on soil microbial activity and nutrient cycling. In addition, microbial activity may be affected by plant functional types (PFT) through influence on the levels of inorganic nitrogen (N) and labile carbon in the rhizosphere. This is particularly important in the shortgrass steppe (SGS), where plants with the C3 or C4 photosynthetic pathway differ in phenology, which affects the timing of maximum N uptake and root exudate production.
Open Access
Nonlinear responses to food availability shape effects of habitat fragmentation on consumers
(Colorado State University. Libraries, 2009) Blackburn, Heather B., author; Detling, James K., advisor; Hobbs, N. Thompson, advisor
Fragmentation of landscapes is a pervasive source of environmental change. Although understanding the effects of fragmentation has occupied ecologists for decades, there remain important gaps in our understanding of the way that fragmentation influences populations of mobile organisms. In particular, there is little tested theory explaining the way that fragmentation shapes interactions between consumers and resources. I propose a simple model that explains why fragmentation may harm consumers even when the total amount of resources on the landscape remains unchanged. In the model, I show that nonlinearity in the relationship between resource availability and benefit acquired from resources can cause a decrease in benefits to consumers when landscapes are subdivided into isolated parts. This decrease is the result of simple mathematical properties of the form of the relationship between resource availability and benefit, and is more severe with greater nonlinearity, with increasing fragmentation, or with greater unevenness of resource availability between fragments. I tested the predictions of the model using a laboratory system of cabbage looper (Trichoplusia ni) larvae on artificial landscapes. Consistent with the model's predictions, survivorship of larvae decreased with a combination of fragmentation and heterogeneity in resource availability. However, average mass of surviving larvae did not change in response to fragmentation alone. With basic knowledge of consumer resource use patterns and landscape structure, these observations can aid in making both generalized and quantitative predictions about the resource-mediated effects of fragmentation on consumers.
Open Access
Invasive mosquitoes, larval competition, and potential implications for vector competence
(Colorado State University. Libraries, 2007) Bevins, Sarah, author; Moore, Janice, advisor
Aedes albopictus is an invasive mosquito species which established rapidly in the US. It is characterized as an aggressive biter, prolific breeder, and dominant larval resource competitor. Declines in resident mosquito species have followed its arrival, and species that persist often emerge from shared larval habitats as small, nutritionally deprived adults. Aedes albopictus is a competent disease vector in both its native range and in areas of recent establishment. It therefore has the potential to alter vector-borne disease transmission either directly, by creating a new pathogen association, or indirectly, via interactions with native mosquito vectors.
Open Access
Spatiotemporal agent-based model explorations of white-tailed deer management in New England
(Colorado State University. Libraries, 2023) Kohler, Allison M., author; Boone, Randall B., advisor; Childers, Michael, committee member; Van Deelen, Timothy, committee member; Lynn, Stacy, committee member
This dissertation research addresses the intricate challenge of managing white-tailed deer (Odocoileus virginianus, henceforth "WTD") populations in the New England region while considering evolving ecological dynamics, changing interests of various stakeholders, and the role of management coordination among municipalities across scales. With a mixed-methods approach, I integrate qualitative and quantitative techniques such as agent-based modeling and case study analysis, helping to contribute multifaceted insights into the realm of WTD management in the region. In Chapter 2, I focus on investigating the role of hunter recruitment and land access in shaping local WTD populations across 11 focal towns in New England. The purpose of this chapter is to explore how these factors influence WTD populations, specifically by identifying the thresholds at which they become significant drivers in controlling these populations. To achieve this, I employ a mixed-methods approach that combines ecological modeling and the analysis of empirical data. The study's results emphasize the specific thresholds of hunting land access required to trigger a decline in WTD populations for different hunter density scenarios. I estimate that in most towns, the existing combination of hunter density and land access is effective in managing local WTD populations. However, under conditions of declining hunter recruitment, towns may require higher amounts of hunting access to achieve similar levels of population control. These findings underscore the significance of addressing issues related to declining hunter numbers and enhancing opportunities for hunting to sustain effective deer population management the region. This chapter's implications stress the importance of adaptive strategies and community engagement in the realm of WTD management in New England. In Chapter 3, I assess the role of sharpshooting as a potential urban WTD management strategy across various contexts in New England. The chapter's primary purpose is to examine the feasibility of sharpshooting when factors like declining hunter numbers and limited hunting land access impact the efficacy of current deer management approaches. I conduct this investigation using a mixed-methods approach, combining ecological modeling with social science surveys and assessments. The results from this chapter offer insights into the role of sharpshooting as a strategy if the effectiveness of hunting diminishes. The findings suggest that most of the towns studied can effectively manage WTD populations without the need for sharpshooting. However, as hunter recruitment declines and hunting access becomes more limited, sharpshooting may become a reasonable solution when it aligns with community preferences. This chapter concludes that sharpshooting can serve as a management tool in certain scenarios, emphasizing the significance of prioritizing stakeholder education, engagement, and acceptance. The implications drawn from this research underscore the need for community involvement in shaping management decisions, particularly in relation to adopting sharpshooting for local WTD population control. In Chapter 4, I focus on the broader theme of WTD management coordination in New England, examining its implications in controlling WTD populations across spatial scales. The main purpose of this chapter is to assess the impacts of stakeholder cooperation and coordination among municipalities on the effectiveness of WTD population management. My methodology involves the development and application of theoretical agent-based models to simulate different coordination scenarios. The chapter's results consistently demonstrate the significant role of coordination in shaping management outcomes, both within individual towns and across multiple municipalities. Based on model outcomes, effective cooperation between neighboring towns consistently leads to lower WTD densities both in towns with and without active management, exemplifying the role of collaborative efforts at larger scales. Additionally, the findings highlight the need for flexible strategies that consider the unique circumstances of each municipality. When aligned with community interests, the results demonstrate the potential for significant reductions in WTD densities with coordinated lethal management efforts, offering a path for more successful WTD population management in New England and similar regions. The implications of this chapter emphasize the role of regional cooperation and the importance of tailoring management strategies to specific contexts and community dynamics. This research not only contributes insights into the complexity of WTD management in New England, but it also serves as a broader blueprint for wildlife management worldwide. I encourage other researchers to build agent-based models to inform management of other situations and species across geographical locations and contexts. In this research, I reveal that the challenge of managing WTD populations largely arises from the spatially heterogeneous distribution of WTD and resulting conflicts, in addition to the complexity of achieving stakeholder consensus across spatial scales. This demonstrates the tradeoff between accommodating diverse stakeholders and achieving regional WTD population control. With adaptive, science-based, and community-based approaches, I explore the roles of adaptability, collaboration, and innovation in contributing to sustainable WTD management efforts. In the end, I hope this research contributes to informing management practices, promoting a balanced and harmonious coexistence between humans and resident wildlife in a changing world.
Open Access
Phenotype to genotype and back in emerging and established crop species
(Colorado State University. Libraries, 2023) Woods, Patrick O'Neal David, author; McKay, John, advisor; Hufbauer, Ruth, committee member; Funk, Chris, committee member; Sloan, Dan, committee member
Understanding the relationship between the phenotype and genotype is a fundamental goal of genetics. Through the years, two primary approaches have been developed for studying the phenotype-genotype relationship: forward genetic and reverse genetics. Forward genetics enables the potential discovery of numerous candidate genes controlling a phenotype while reverse genetics allows for the mechanistic validation of a single gene's role in controlling a phenotype. Applying these two approaches to crops enables the discovery of genetic targets that can be used for crop improvement through breeding. In this dissertation, I focused on understanding the phenotype-genotype relationship in both the emerging crop Cannabis sativa and the established crop Maize. In Chapter 1, I used both a forward a reverse genetics approach to identify and validate candidate genes controlling agriculturally important traits (agronomic and biochemical) in Cannabis sativa. In Chapter 2, I used a reverse population genetics approach to identify the genetics underlying local adaptation in feral and domesticated populations of Cannabis sativa. In Chapter 3, I used a forward genetics approach to identify candidate genes controlling variation in root system architecture in Maize. Collectively, this work demonstrates how modern genomic techniques can be applied to both new and old crop systems to identify genetic targets for use in crop innovation through breeding.
Open Access
Mapping the landscape of wildfire risk mitigation: understanding the links between equity, community assets, capacities, and collaboration
(Colorado State University. Libraries, 2023) Courtney, Karissa, author; Salerno, Jonathan, advisor; Cheng, Tony, committee member; Walker, Sarah, committee member
Wildfire is a natural process that has shaped landscapes for millennia, but its exclusion since Euro-American settlement has had negative effects on forest health and composition, increasing the risk of large, high-severity wildfires. Many communities are grappling with this increased risk, as more people move into the Wildland Urban Interface (WUI) and climate change exacerbates hot, dry conditions that may help wildfires spread. Actions such as prescribed burning, hand or mechanical thinning, creating defensible space around structures, and home hardening can reduce communities' risk of wildfire but often require coordination with state, local, and federal partners, as well as time and money. Here, we explore community capacity to reduce impacts of wildfire. Collaboration, time and money are just a few indicators that make up a community's capacity reduce wildfire impacts, which can affect the strategies they are able to implement (i.e., communities with low capacity may be unable to carry out many programs). Previous work has identified that these indicators along with others that constitute community capacity, while additional studies have highlighted the connection between social vulnerability and wildfire risk. However, little work has been done to understand the link between community capacity and wildfire mitigation in specific areas and contexts. To address this gap in this two-part thesis, our objectives were: 1) to access and develop various spatial datasets for indicators of community capacity to visualize capacity for wildfire mitigation across Colorado, and 2) to understand the processes that link capacity to mitigation outcomes. For Chapter 1, we conducted a first-of-its-kind study in Colorado mapping indicators of community capacity to represent variation across the state and to identify communities where high wildfire probability co-occurred with low capacity. Our findings highlight the eastern plains, the northwestern part of the state, and San Luis Valley as areas with lower potential capacity. Further, areas within Weld, Las Animas, and Archuleta counties are places with lowest potential capacity but the greatest potential for wildfire. These findings also suggest that different communities have a variety of ways they build capacity and highlight the important role that funding plays in helping communities increase their capacity. In Chapter 2, we explored local nuances and processes that link capacity to mitigation negative outcomes from wildfire by conducting 11 group interviews across Colorado. Our findings suggest that several key pieces of the process can drive outcomes: inter-organizational collaboration, leveraging funds, prioritization and planning, having dedicated staff for wildfire mitigation, building community buy-in, and engaging with various stakeholders. Creative workarounds emerged as a unique way across communities to overcome common barriers to wildfire mitigation, suggesting policy and institutional processes to streamline mitigation work may have outsized benefits, particularly in communities with limited resources. Our results highlight a need to point back to the systems that make communities vulnerable in the first place.
Embargo
The impact of control on national-scale livestock disease outbreaks in the United States
(Colorado State University. Libraries, 2023) Smith, Samuel M., author; Webb, Colleen T., advisor; Beck-Johnson, Lindsay M., committee member; Keller, Kayleigh, committee member
Outbreaks of livestock diseases, like foot-and-mouth disease (FMD) and bovine tuberculosis (bTB), pose a significant economic threat to the United States livestock industry. Significant interest then lies in developing strategies to mitigate the impact of an outbreak should they occur. This thesis explores the effect of control interventions on outbreaks of FMD and bTB in the U.S. In chapter one, I weigh trade-offs associated with delaying the implementation of control on the economic impact of controlling an FMD outbreak in the U.S. This study aimed to understand whether control policies that adopt a conservative initial approach, but may be updated as an outbreak progresses, can reduce socioeconomic harm while achieving desired outbreak outcomes. I find that delaying the implementation of all available control interventions early on in an outbreak does not reduce the cost of small outbreaks and exacerbates the largest outbreaks, suggesting that the potential benefits of this type of adaptive response may be out weighted by the risk of allowing a large outbreak to become worse. Next, I investigate how the culling of infected cattle premises, diagnostic testing, and traceback investigations impact the size of bTB outbreaks. Results from this study show improvements to traceback investigations result in the largest decreases in bTB outbreak size, which suggests that improving the identification of premises via traceback investigations is more important than increasing antemortem diagnostic sensitivity. Although this thesis focuses on the control of livestock disease, we can abstract several broader principles that contribute to ecology and epidemiology's understanding of disease dynamics. Both chapters demonstrate the importance of a population's underlying demography to determining an outbreak's overall trajectory as well as minimizing the time until detection of an infection and the time until control is implemented.