Theses and Dissertations

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Open Access
Evolution at the edge: how hibernation, heat waves, and hybridization impact a range expansion
(Colorado State University. Libraries, 2024) Clark, Eliza, author; Hufbauer, Ruth, advisor; Bitume, Ellyn, committee member; Norton, Andrew, committee member; Funk, Chris, committee member
Evolutionary processes shape the diversity of life on earth. Over millennia, species diverge from one another, radiating out into the tree of life. The same processes of evolution are also acting in much shorter periods of time, selecting for traits, mixing genes across populations, and generating new mutations each generation. These rapid evolutionary processes interact with ecological processes, which are happening on similar time scales. Range expansions, or expansions of a population's geographic distribution, were once considered strictly ecological processes of populations interacting with other populations and the environment, unaffected by evolution. However, modern theory understands range expansions to be crucibles of rapid evolution. Rapid evolution shapes the process of range expansion itself, and is also integral to determining the outcomes of range expansion. During range expansions, ecological and evolutionary processes intertwine, combining to shape the dynamics of a range expansion, like where a population can establish, and how quickly the expansion moves. The study of evolution during range expansions has only just begun to make it out of the theory to be tested in wild populations in nature, so we don't yet know how common evolution during range expansion is, or how large its effects might be. Here, I explore how evolution impacts range expansions that are current and ongoing in natural systems in the wild. I focus on the tamarisk beetle (Diorhabda spp.), deliberately introduced in the United States about two decades ago for biological control of a widespread invasive weed. Through its role as a biological control agent, the tamarisk beetle has expanded its range hundreds of kilometers along rivers, colonizing new areas of the invasive weed in environments very different from its original release habitat. The range expansion of the tamarisk beetle provides a unique opportunity to study evolution during an ongoing natural range expansion across an environmental gradient. Through the following four chapters, I document evolution of dispersal ability and life history traits (Chapter 1), evolution of seasonal dormancy and genetic variation of that trait (Chapter 2), evolution of phenotypic plasticity (Chapter 3), and the impacts of hybridization (Chapter 4). Throughout, I discuss the implications for biological control and the tamarisk beetle specifically, and more generally how these results improve our understanding of how evolution is caused by, enables, and alters natural range expansions over short time periods, even in natural range expansions.
Open Access
Plant-mediated interactions between herbivory and soil microbial communities in biocontrol programs of Russian knapweed
(Colorado State University. Libraries, 2024) Matos Franco, Giovana, author; Ode, Paul, advisor; Pearse, Ian, committee member; Smith, Melinda, committee member; Trivedi, Pankaj, committee member
Russian knapweed (Rhaponticum repens) is an invasive noxious weed present in the United States and two insect biocontrol agents have been released to assist with its management: the gall midge (Jaapiella ivannikovi) and the gall wasp (Aulacidea acroptilonica). Since their establishment, no concrete impacts of biocontrol agents onto Russian knapweed have been measured, neither their impacts on interactions between Russian knapweed and local microbiomes. To address this knowledge gap, observational and manipulative studies were conducted to investigate the effects of biocontrol agents on Russian knapweed fitness as well as its associated microbiomes. We found that Russian knapweed associates with a core microbiome that can assist with invasion in the introduced range as well as, in root samples collected from sites where gall wasp were present, lower microbiome diversity was observed, indicating potential negative effects on overall plant health. In garden conditions, water availability positively correlated with plant growth, negatively correlated with insect establishment, and shaped microbiomes in root associated tissues. Results of this dissertation highlights how introduction of biocontrol agents shifts pre-established relationships between invasive plants and microbiomes as well as how such relationships could be impacting the success of biocontrol programs.
Open Access
Dynamics of stress and mortality for grass dominated ecosystems: an interplay of water limitation, heat, and erosion
(Colorado State University. Libraries, 2024) Bradfield, Scott J., author; Ocheltree, Troy, advisor; Knapp, Alan, committee member; Augustine, David, committee member; Hoffman, Chad, committee member
Grass dominated systems account for ~40% of the earth's terrestrial surface and typically occur in semi-arid and arid regions. The plant species that grow in these systems are known for their ability to withstand disturbance, including drought, grazing, and fire. While it is understood that the plants in these systems often experience multiple forms of stress in a growing season, interactions among these stress variables are not well represented in the literature. In this research, I sought to determine how combinations of stress variables influence the shortgrass steppe, this includes: long-term grazing, drought, erosion, and temperature. Specifically, I examined (1) how the interaction of long-term grazing and drought influences the recovery of the vegetation on the SGS following single-year and multi-year droughts, (2) how the interaction of grazing and erosion influence mortality following exposure to extreme surface temperatures, and (3) performed a comparative analysis of the microclimate of grass dominated systems in the United States to determine the intensity and frequency of stressful abiotic conditions that the vegetation experiences. First, I quantified the interactive effects of single and multi-year droughts with grazing pressure, because the Bouteloua species that dominate the region have been shown to be tolerant of grazing and drought independently, but the interactive effects of the two have not been well studied. Past research has focused on heavy cattle grazing but I included a mixture of moderate cattle grazing with prairie dogs, which is more intensive grazing than heavy cattle grazing. I found that the combined stress of multiple years of drought along with high grazing pressure has the potential to increase mortality in these Bouteloua species. Next, I quantified the erosion severity by ranking the amount crown exposure of the Bouteloua species during a drought on the SGS and then determined how erosion influenced bud outgrowth (production of a tiller) during the recovery year. I combined these data with environmental data collected by the National Ecological Organization Network (NEON) to determine the environmental conditions that the meristems of the plants experienced during the drought. My results showed that the temperatures at the surface of the soil, and exposed meristems, frequently reached levels thought to be lethal to plant cells. I acknowledge that it was likely a combination of water deficit and temperature that led to mortality of Bouteloua species that experienced erosion, but the high temperatures alone had the capacity to cause mortality of the meristems. Finally, I compared several near surface micrometeorological variables of grass dominated systems across the United States. Ultimately, I wanted to determine the frequency that these systems experienced temperatures near the surface that would be damaging to plants, if conventional methods for determining heatwaves represents damaging conditions to grassland plants, and what environmental factors lead to potentially damaging surface temperatures. I found that damaging temperatures occur often at arid sites, conventional heatwaves overestimate heat stress in sites that are wet or at higher latitudes, and underestimates heat stress for arid sites.
Embargo
The genomics of habitat-linked microgeographic adaptation in an island endemic bird
(Colorado State University. Libraries, 2024) Cheek, Rebecca G., author; Ghalambor, Cameron K., advisor; Funk, W. Chris, advisor; Sillett, T. Scott, committee member; Aubry, Lise M., committee member
A fundamental goal of evolutionary biology is to understand the mechanisms that maintain adaptive diversity. This dissertation focuses on the interplay of two key evolutionary mechanisms - natural selection and gene flow. While natural selection is often portrayed as a driving force of adaptive evolution, gene flow is assumed to disrupt selection by introducing maladapted alleles into locally adapted populations. Yet this paradigm is beginning to shift as a growing appreciation for the role gene flow may play in concert with natural selection to facilitate adaptative divergence. I explore this interaction of selection and gene flow in island scrub-jays (Aphelocoma insularis), a highly mobile bird experiencing local adaptation at a microgeographic scale. First, I demonstrated that observed differences in bill morphology between pine-oak ecotones are likely genetically based despite overall limited population genetic structure. Second, I found that the genetic underpinnings of divergent bill morphologies are highly parallel at higher genetic levels, which is indicative of selection acting on shared, but highly polygenic, molecular pathways. Finally, I tested alternate dispersal mechanisms potentially impacting patterns of limited gene flow and found evidence for sex-biased natal habitat preference shaping limited dispersal. Collectively, these results show how gene flow can enhance adaptive divergence at microgeographic scales.
Embargo
A case for context in quantitative ecology: statistical techniques to increase efficiency, accuracy, and equity in biodiversity research
(Colorado State University. Libraries, 2024) McCaslin, Hanna M., author; Bombaci, Sara, advisor; Hooten, Mevin, committee member; Koons, David, committee member; Hoeting, Jennifer, committee member
The current era of ecological research is characterized by rapid technological innovation, large datasets, and numerous computational and quantitative techniques. Together, big data and advanced computing are expanding our understanding of natural systems, allowing us to capture more complexity in our models, and helping us find solutions for salient challenges facing modern ecology and conservation, including climate change and biodiversity loss. However, large datasets are often characterized by noise, complex observational processes, and other challenges that can impede our ability to apply these data to address ecological research gaps. In each chapter of this dissertation, I seek to address a data problem inherent to the 'big data' that characterizes modern ecological research. Together, they extend the strategies available for addressing a problem facing many ecologists – how to make use of the large volumes of data we are collecting given (1) current computational limitations and (2) specific sampling biases that characterize various methods for data collection. In the first chapter, I present a recursive Bayesian computing (RB) method that can be used to fit Bayesian hierarchical models in sequential MCMC stages to ease computation and streamline hierarchical inference. I also demonstrate the application of transformation-assisted RB (TARB) to a hierarchical animal movement model to create unsupervised MCMC algorithms and obtain inference about individual- and population-level migratory characteristics. This recursive procedure reduced computation time for fitting our hierarchical movement model by half compared to fitting the model with a single MCMC algorithm. Transformation-assisted RB is a relatively accessible method for reducing the computational demands of fitting complex ecological statistical models, like those for animal movement, multi-species systems, or large spatial and temporal scales. Biodiversity monitoring projects that rely on collaborative, crowdsourced data collection are characterized by huge volumes of data that represent a major facet of 'big data ecology,' and quantitative methods designed to use these data for ecological research and conservation represent a leading edge of contemporary quantitative ecology. However, because participants select where to observe biodiversity, crowdsourced data are often influenced by sampling bias, including being biased toward affluent, white neighborhoods in urban areas. Despite the growing evidence of social sampling bias, research has yet to explore how socially driven sampling bias impacts inference and prediction informed by crowdsourced data, or if existing data pre-processing or analytical methods can effectively mitigate this bias. Thus, in Chapters 2 and 3, I explored social sampling bias in data from the crowdsourced avian biodiversity platform eBird. In Chapter 2, I studied patterns of social sampling bias in the locations of eBird "hotspots" to determine whether hotspots in Fresno, California, U.S.A. are more biased by social factors than the locations of Fresno eBird observations overall. My findings support previous work showing that eBird locations are biased by demographics. Further, I found that demographic bias is most pronounced in the locations of hotspots specifically, with hotspots being more likely to occur in areas with higher proportions of non-Hispanic white residents than eBird locations overall. This relationship is reinforced because hotspots in these predominantly white areas also amass more eBird checklists overall than hotspots in areas with more demographic diversity. These findings raise concerns that the eBird hotspot system may be exacerbating spatial bias in sampling and reinforcing patterns of inequity in data availability and eBird participation, by leading to datasets and user-facing maps of birding hotspots that mostly represent predominantly white neighborhoods. Then, in Chapter 3, I investigated the impacts of not accounting for socially biased sampling when using eBird data to study patterns of urban biodiversity. The luxury effect has emerged as a prominent hypothesis in urban ecology, describing a pattern of higher biodiversity associated with greater socioeconomic status observed in many cities. Using eBird data from 2015-2019, I tested whether an avian luxury effect is observed in Raleigh-Durham, North Carolina, U.S.A. before and after accounting for social sampling bias. By jointly modeling sampling intensity and species richness, I found that sampling intensity and species richness are positively correlated and sampling bias influences the estimated relationship between species richness and income. Thus, failing to account for sampling bias can hinder our ability to accurately observe social-ecological dynamics. Additionally, I found that randomly spatially subsampling eBird data prior to analysis, as recommended by existing guidelines to mitigate sampling bias in eBird data, does not reduce biased sampling related to demographics, because there are data gaps in communities of color and low-income communities that cannot be addressed via spatial subsampling. Therefore, it is paramount that crowdsourced and contributory science projects prioritize more equitable participation in their platforms, both for more ethical, equitable practice and because current sampling inequity negatively impacts data quality and project goals. Quantitative techniques can help us understand the complex observational processes influencing ecological data, and each chapter of this dissertation highlights how tailoring statistical or computing methods to these observational contexts can advance ecological knowledge – either by extending the complexity of models we can feasibly fit, as in Chapter 1, or by acknowledging and accounting for sampling inequity, in Chapters 2 and 3. We are all participants actively shaping the ecological processes we observe, and the actions, approaches, and assumptions used in our research reflect societal systems and biases. Data are never objective, and it is dangerous and false to assume that quantitative techniques can take data out of the contexts in which they were collected. Instead, quantitative frameworks that embrace, reflect, and seek to improve the ways in which social and observational contexts inform what is observed can elevate analytical techniques to tools towards more just, inclusive, and transparent ecological research and conservation.
Open Access
Effects of disturbance on tree level resistance in ponderosa pine trees along the Colorado Front Range
(Colorado State University. Libraries, 2024) Woodard, Kelby, author; Stevens-Rumann, Camille, advisor; Negrón, José, committee member; Rocca, Monique, committee member
Forest restoration treatments are being implemented across ponderosa pine systems along the Colorado Front Range with goals of reducing risk of catastrophic wildfire, returning forest structure to historical conditions, and increasing ecosystem resilience and resistance in the face of climate change. While there are studies monitoring effects of thinning and wildfires on forest structure across the Front Range, few studies assess the effects of disturbances from wildfires and thinning treatments on tree-level resistance. Here we examined forest stand structure, growth, and defense characteristics in response to treatments and wildfires through the collection of plot level data, tree-level characteristics, and tree cores. We sampled 160 plots in areas that experienced thinning treatments between 2007-2012, were burned by low-severity wildfires (2012 Hewlett Gulch and High Park Fire, 2010 Fourmile Canyon Fire and Dome Fire, 2012 Flagstaff Fire, 2012 Waldo Canyon Fire), or that were untreated and unburned (hereafter "control"). Our findings reveal that tree growth and resin duct size significantly increased following thinning treatments. Relative resin duct area and duct density were significantly higher in trees following wildfire compared to trees that experienced thinning or to those trees within control plots. Control plots exhibited the highest mean basal area and stand density index, coupled with the lowest quadratic mean diameter, indicating high inter-tree competition, which both thinning and low-severity wildfire helped alleviate. Overall, our results highlight the beneficial impacts of both thinning and low-severity wildfire on mature ponderosa pine trees by enhancing their resistance to future disturbance, such as bark beetle outbreaks and drought.
Open Access
Wildfire effects on host-parasite interactions in freshwater streams
(Colorado State University. Libraries, 2024) Svatos, Emma C., author; Preston, Daniel, advisor; Hart, Sarah, committee member; Wells, Caitlin, committee member
Wildfires are increasing in intensity and frequency globally, accentuating the need to understand the implications of fire on community interactions. While previous research has focused on fire effects on free-living species, our knowledge of how wildfires influence parasite interactions with hosts and predators remains limited, especially in freshwater ecosystems. This thesis addresses this knowledge gap and presents results from two distinct multi-year observational field studies that explore how wildfires influence interactions among parasites, hosts, and predators in freshwater streams. In Chapter 1, I used a Before-After-Control-Impact design to compare freshwater snail (Juga plicifera) host populations and trematode parasite communities in Oregon streams before and after wildfire disturbance. In Chapter 2, I investigated host-parasite-predator interactions involving mermithid nematode parasites (Family Mermithidae), mayfly hosts (Order Ephemeroptera), and trout predators (Salvelinus fontinalis, Salmo trutta, and Oncorhynchus clarkia) in the southern Rocky Mountains after severe wildfires. Despite substantial changes to stream habitat, snail host populations and trematode infection patterns and community structure remained relatively stable following fire disturbance in Oregon streams; however, I observed subtle taxon-specific responses to fire, suggesting changes in abundance or behavior of definitive hosts. In Rocky Mountain watersheds, mermithid parasite patterns varied considerably over time in burned streams, which coincided with similar responses in mayfly host densities, suggesting that wildfire indirectly affected mermithid infection patterns through host-density changes in the stream. Host attributes also influenced parasite interactions, as intermediate-size Baetidae mayflies experienced the highest probability of infection. Furthermore, infection prevalences of mayflies consumed by trout were positively related to mayflies in the benthos; yet, infection prevalences in trout stomachs were lower on average, potentially due to parasite-induced behavioral changes in infected mayfly hosts that reduced susceptibility to predation. Wildfire did not seem to affect rates of this predator-parasite interaction, as instances of concomitant predation remained consistent in burned streams over time. Comparing two different host-parasite systems sheds light on how environmental variables and host-parasite ecologies mediate wildfire effects on parasite interactions with hosts and predators. Together these findings expand our knowledge of parasite ecology in aquatic macroinvertebrate hosts, offer insights into the role of parasites in energy flow through food webs and as bioindicators of environmental change, and help integrate parasite interactions into our understanding of disturbance ecology in freshwater streams.
Open Access
Evaluating Bouteloua gracilis cultivars' performance after drought; The role of the soil microbiome
(Colorado State University. Libraries, 2024) Donne, Carina, author; Smith, Melinda, advisor; Havrilla, Caroline, committee member; Trivedi, Pankaj, committee member; Metcalf, Jessica, committee member
Drought has affected the Great Plains throughout history, most notably during the Dust Bowl of the 1930's. While most drought events are not as severe as the Dust Bowl, they still cause significant agricultural losses every year. As research has begun to uncover the mechanisms and responses of drought, there are still unanswered questions. For instance, the mechanisms of ecosystem recovery after drought ends remain relatively unexplored. It is possible that intervention methods such as reseeding will need to be done to help restore ecosystem structure and function after drought. After the Dust Bowl, it was a common practice to reseed native grasses, such as Blue Grama (Bouteloua gracilis), in sites severely impacted by the drought. Given forecasts of droughts on par or even more severe than the Dust Bowl, reseeding may need to be employed more frequently in the future to enhance post-drought recovery. However, with reseeding efforts, it is imperative to understand the adaptability of cultivars to the environmental conditions in which they are planted. One aspect of environmental conditions that has rarely been examined the soil microbiome. Here, I used a common garden experiment that included two cultivars of B. gracilis that were planted with soil microbial inocula extracted from either previously droughted or non-droughted soils. These soils were collected from a recently ended four-year drought experiment in the shortgrass steppe of northeastern Colorado, which caused the widespread loss of B. gracilis. The goal of the greenhouse experiment I conducted was to examine whether the post-drought legacy of altered soil microbial communities affected the growth and performance of two common cultivars of B. gracilis. I assessed plant performance by measuring weekly height to estimate relative growth rate and at the end of the experiment, I measured plant above- and belowground biomass. I found no significant differences in relative growth rate or plant biomass, and minimal differences in the bacterial community composition between the two cultivars. These results suggest that the post-drought legacy of altered soil bacterial communities did not differentially affect growth and performance of the two common B. gracilis cultivars evaluated in this study, and that the growth of these cultivars did not differ in their effects on the soil bacterial communities found under ambient vs. previously droughted conditions. Overall, both cultivars may be suitable for reseeding in the shortgrass steppe grassland after extreme drought, yet further studies are needed to examine a broader range of B. gracilis cultivars and whether soil bacterial communities previously exposed to extreme drought would allow for improved growth and performance of different cultivars to future drought conditions.
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.