Browsing by Author "Hufbauer, Ruth, advisor"
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Item Open Access Disentangling drivers of colonization success in laboratory and natural systems(Colorado State University. Libraries, 2017) Vahsen, Megan, author; Hufbauer, Ruth, advisor; Brown, Cynthia, committee member; Hobbs, N. Thompson, committee memberUnderstanding why colonizing populations successfully establish is important for predicting dynamics of invasive species. Propagule pressure, or the number of individuals in a founding group, is considered the most consistent predictor of establishment success, however, there remains considerable variance around predictions that demography alone cannot explain. The identity of individuals within a founding group (e.g. level of pre-adaptation to the recipient environment, diversity) as well as how individuals are introduced (e.g. frequency and timing of discrete introduction events) can influence establishment. The relative importance of these factors is unclear, and could vary across species and environmental contexts. To address these inconsistencies, we conducted two experiments: one with Tribolium castaneum (red flour beetle) populations maintained in controlled laboratory conditions, and one with Bromus tectorum (cheatgrass) founding populations introduced to a natural environment. For the Tribolium experiment, we varied the level of prior adaptation, diversity, and introduction frequency and timing for groups of eggs colonizing in a novel environment across three levels of propagule pressure (n = 15, 30, 60). Founding groups that were larger and more adapted to the novel environment survived the founding event better than smaller and less adapted groups. Further, we found that a high frequency of smaller introductions reduced initial survival. After a generation of mating, establishment success was driven predominantly by adaptation to the novel environment and diversity of founders. In the second experiment, we introduced groups of B. tectorum seeds at a constant propagule pressure (n = 32) to a common garden in Colorado, varying in source diversity (1, 2, 4, 8, or 16 source populations) and source region (Colorado = pre-adapted or Nevada = unadapted). We evaluated establishment success by deriving the number of seeds produced by each founding group after one generation of growth and reproduction using a hierarchical Bayesian model. We found that increasing source diversity increased the number of seeds produced per founding group, but source region did not influence establishment success. Results from these experiments particularly speak to the context-dependency of the importance of pre-adaptation and diversity in predicting establishment success. This suggests that propagule pressure alone is not enough to explain why founding populations establish.Item 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 memberEvolutionary 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.Item Open Access Examining maternal effects and genetic differentiation in P. flexilis and P. aristata to improve success of conservation actions(Colorado State University. Libraries, 2013) Borgman, Erin M., author; Hufbauer, Ruth, advisor; Schoettle, Anna, advisor; Angert, Amy, committee member; Steingraeber, David, committee memberAs the climate changes and invasive species continue to spread, proactive management may be needed to conserve native plant populations. Selecting appropriate plant material for restoration or other actions that will sustain populations is an integral part of any such plan and must take into account genetic differentiation to limit maladaptation. Common garden studies are used to determine the genetic basis of trait variation among populations from different geographic sources. However, maternal effects, the effect of environment during offspring development, can also affect performance, complicating the interpretation of these studies. Growing one generation in a common environment can help correct for maternal effects, but is often not practical with long-lived species. Using limber pine (Pinus flexilis) and Rocky Mountain bristlecone pine (Pinus aristata) as model species, I explored the contribution of maternal effects to early seedling growth among populations in a greenhouse common garden study. I grew offspring sourced over multiple years from the same mother trees, comparing growth traits between source years. Additionally, I collected five twig clippings from the upper canopy of each mother tree and measured characteristics indicative of the relative vigor of the tree during each seed source year. There were significant (p<0.05) differences in year-to-year variation in twig growth characteristics, seed size, and seedling performance. For bristlecone pine, there was a significant positive relationship between the relative inter-annual (RIA) variation in seed mass and seedling total dry mass and a negative relationship between the RIA variation in seed mass and needle growth at 210 days. For limber pine, there were significant positive relationships between RIA variation in seed mass and cotyledon length, stem height, stem diameter, and needle length at 20, 120, and 190 days. These results a) support the hypotheses that maternal effects are evident in both P. flexilis and P. aristata and that these effects translate into variation in early seedling growth and b) suggest possibilities for statistically correcting for maternal effects in genetic differentiation common garden studies involving long-lived species. Using these data I then conducted a common garden greenhouse study to determine the degree of genetic differentiation in limber pine populations in the Southern Rockies. Mid-summer precipitation varies greatly along a latitudinal gradient throughout this region, potentially selecting for local adaptation of populations to their native moisture regime. I evaluated the differential response of seed sources from northern and southern portions of the range to different moisture regimes during early seeding growth. To test whether seedling growth traits, which are often adaptive, differed between northern and southern seed sources, I measured primary needle length, stem diameter, water potential, and biomass allocation between root and shoot before, during, and after treatments. To test for differentiation in the adaptive traits associated with water use, I also measured carbon isotope ratios (δ13C) as a proxy for water use efficiency. To account for maternal effects I used cotyledon length as a covariate, which I found in the previous study to be a good indicator of maternal year-to-year variation in seedling growth. There were significant (p<0.05) effects of source region for root length, stem diameter, needle length, and total dry mass, where seedlings from southern sources were bigger than those from northern sources. Seedlings from the north had a higher probability of mortality than those from southern populations, as did seedlings in the dry treatment. The only significant interaction between growth response and source region, signifying the possibility of local adaptation of populations, was with the carbon isotope ratio (δ13C, p<0.1). All seedlings in the dry treatment regardless of origin had higher δ13C, while seedlings in the wet treatment varied between slightly higher (southern populations) and lower (northern populations) values of δ13C. These data indicate that genetic differentiation exists among populations in the Southern Rockies, potentially increasing the risk of maladaptation when moving seed far from its source.Item 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, advisorHybridization 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.Item Open Access Mullein it over: an examination of the impacts of common mullein (Verbascum thapsus) invasion and management(Colorado State University. Libraries, 2015) Leipzig-Scott, Peter, author; Hufbauer, Ruth, advisor; Blumenthal, Dana, committee member; Wallenstein, Matt, committee memberBiological invasion is one of the most important problems facing modern ecologists, and while research has shown the detrimental effects caused by many invasive species, the impact of the majority of invaders in largely unknown. I investigated the effects of the ubiquitous exotic plant species common mullein (Verbascum thapsus) on plant communities in northern Colorado. Additionally, I studied the impact of a second common invasive species, cheatgrass (Bromus tectorum), and examined its interactions with mullein. I conducted a field experiment over two years, manipulating mullein and cheatgrass presence at two sites, either removing the aboveground biomass of one or both of the exotic plants, or leaving them intact. I measured a number of plant community and abiotic characteristics as responses to these treatments. I found that mullein invasion has few negative impacts, and that the benefits associated with management are lost within a year. Removal of mullein acts as a disturbance, creating physical and ecological openings for cheatgrass and other exotics to occupy. Percent cover of exotic species was highest when mullein was removed and cheatgrass was left intact. Since cheatgrass invasion is associated with undesired changes in the community, I suggest that resources should be used for its management rather than the management of mullein, and that mullein removal in areas with cheatgrass should be accompanied by an aggressive strategy addressing cheatgrass invasion.Item Open Access Pavement, pests, & parasitoids, oh my! Elm herbivores and their natural enemies in the urban forest(Colorado State University. Libraries, 2022) Buenrostro, Jacqueline, author; Hufbauer, Ruth, advisor; Cranshaw, Whitney, committee member; Redmond, Miranda, committee member; Stewart, Jane, committee memberUrban areas are the fastest growing habitat type in the world, and an increasing proportion of the United States and global population lives in urban areas. Urban forests provide essential ecosystem services to rapidly expanding urban populations, but their health is threatened by damaging herbivory from non-native, invasive insects. To address this problem, my masters research investigated two critical questions that limit our ability to sustainably manage invasive insects in urban forest ecosystems: (1) How do urban environments impact the density of invasive herbivores in the urban forest? and (2) Are predator and parasitoid natural enemies present, and, if so, what is the level of parasitism? I investigated these questions in the context of the elm-herbivore study system, analyzing a globally distributed host tree and its complex of invasive insect pests in Colorado, USA. In my first chapter, I address the first question and explore how a variety of environmental factors that vary across urban habitats influence the density of several invasive insects. Specifically, I evaluate how vegetational complexity, distance to buildings, impervious surface, canopy temperature, host availability, and density of co-occurring herbivores impact three invasive pests of elm trees: the elm leaf beetle Xanthogaleruca luteola Müller (Coleoptera: Chrysomelidae), the European elm flea weevil Orchestes steppensis Korotyaev (Coleoptera: Curculionidae), and the elm leafminer Fenusa ulmi Sundevall (Hymenoptera: Tenthredinidae). I found that insect responses to these factors were species-specific, and all environmental factors were associated with density of at least one pest species except for distance to buildings. Elm leafminer density decreased with higher temperatures and was influenced by an interaction between vegetational complexity and impervious surface. Elm flea weevil density increased with greater host availability, and elm leaf beetle density increased with higher temperatures. Both elm leaf beetle and elm flea weevil density decreased with greater leafminer density, suggesting that insect density is mediated by species interactions. Results of this study can be used to inform future tree planting efforts through the selection of "safe sites", or locations where trees will be less likely to experience damaging outbreaks of insect pests. Additionally, these results can be used to strategize preventative management on trees that are located in outbreak "hotspots", or locations where environmental conditions make trees predisposed to insect outbreaks. Finally, results of this study contribute to our knowledge of the dynamic ways in which multiple invasive insects interact in urban environments. This information will be especially valuable as non-native insect introductions continue to increase into the future. In my second chapter, I narrow my focus to two historically important and particularly damaging pests of elm, the elm leaf beetle and elm flea weevil, to address the second question listed above and explore the complex of natural enemies attacking these two pests. In many areas of these insects' invaded range, outbreaks severely damage elm hosts. Natural enemies are thought to be important in regulating elm leaf beetle and elm flea weevil populations in other regions, but whether natural enemies are present in Colorado is largely unknown. As such, the aim of chapter 2 was to identify which predators and parasitoids of these pests are present in Colorado, a state with frequent pest outbreaks and where the natural enemy community is almost entirely undescribed. In June – August 2021, I identified predators through field observations and laboratory feeding trials, finding seven species of predators from six arthropod orders that fed on elm leaf beetle or elm flea weevil. Additionally, I reared 58 elm leaf beetle egg clusters, 539 elm leaf beetle larvae, and 435 elm flea weevil mines to detect parasitoids. Two parasitoids of elm leaf beetle, the egg parasitoid Oomyzus gallerucae and the larval-adult parasitoid Erynniopsis antennata, are present in Colorado, representing novel records of these species in the state. However, combined parasitism of elm leaf beetle eggs and larvae was low at <3% across the season, with parasitoids nearly absent early in the season and peak parasitism occurring in late summer. I found five families of parasitoid wasps that emerged from leaves containing weevil mines: Chalcididae, Encyrtidae, Eulophidae, Euplemidae, and Pteromalidae. Parasitoids emerged from <20% of leaves containing weevil mines with almost no parasitism early in the season. Given the low parasitism rates and few predators observed in our study, it seems unlikely that predator and parasitoid natural enemies exert effective control over elm leaf beetle and elm flea weevil in Colorado. This finding challenges the assumption that natural enemies are a driving force of elm leaf beetle and elm flea weevil control in Colorado. Additional research is needed to confirm species identifications for parasitoids of the elm flea weevil, disentangle elm leaf beetle and elm flea weevil population dynamics, and establish effective and sustainable control methods amidst frequent pest outbreaks. Together, these two research projects enhance our knowledge of what triggers outbreaks of the elm leaf beetle Xanthogaleruca luteola, the elm flea weevil Orchestes steppensis, and the elm leafminer Fenusa ulmi in urban areas while also laying the groundwork for a renewed interest in biological control of elm leaf beetle and elm flea weevil. It is my hope that this work can be applied to other invasive insect pests in urban forest ecosystems and make urban forests more resilient in an era when they are increasingly vulnerable to insect attack.Item Open Access Plant-insect interactions and management options for Verbascum thapsus L. in Colorado(Colorado State University. Libraries, 2011) Wilbur, Hannah Darrough, author; Hufbauer, Ruth, advisor; Norton, Andrew, committee member; Meiman, Paul, committee memberIn the field of invasion biology, understanding the ecology of a species goes hand and hand with understanding management options. Common mullein (Verbascum thapsus L.) is a non-native plant introduced to North America from Europe in the 1700s. It has naturalized and spread throughout North America, with populations establishing after disturbance. Through a series of field experiments, I tested the effect of insect herbivory on the growth of V. thapsus in Colorado and also investigated management options for the reproductive stage of this biennial plant. Verbascum thapsus, while widespread, is rarely problematic in undisturbed habitats. This may be due to interactions with new community members, such as insect herbivores, that limit population size. Starting in 2009, I followed a population of V. thapsus plants in a disturbed landscape in Loveland, Colorado. A subset of the population was protected from herbivores with insecticides, reducing overall herbivore damage during the bolting year, for the first year, and throughout the lifecycle of the plants. Reducing herbivory in the first year increased the size that V. thapsus rosettes reached by the fall. Reducing herbivory during only the bolting year of the plant increased the height of the bolting plants. Reducing herbivory over both growing seasons increased plant height, the length of the reproductive stalk, and seed set. Plants with reduced herbivory also had higher rates of survival. Thus, insect herbivory reduced performance of V. thapsus in this study and may be one of the factors that prevents this introduced plant from dominating its recipient communities. Mechanical control of V. thapsus via digging or cutting stalks is considered the best option for management. Current practice encourages the collection of reproductive stalks from the field to limit the spread of seeds. While effective, this substantially increases the labor involved in management. I tested the viability of V. thapsus seeds collected at three sites in Colorado at various times during the growing season to see if the collection and disposal of reproductive stalks is necessary. I found that harvesting before seed capsules had turned brown reduced seed viability dramatically (mean early reproductive stage = 0.08%, 95% CI = 0.06%, 0.67%; mean mid reproductive stage = 1.52%, 95% CI = 0.49%, 3.11%). I suggest focusing mechanical control efforts during the early and mid-season if possible, as at those times plants can safely be left on site. When control efforts occur later in the season after seed capsules begin to turn brown, it is important that reproductive stalks be removed from the field.Item Open Access Population genetics prior to biological control: Ceutorhynchus weevils proposed for managing garlic mustard(Colorado State University. Libraries, 2009) Rauth, Steven J., author; Hufbauer, Ruth, advisorI studied the population genetic structure of three weevil species, Ceutorhynchus alliariae, C. roberti, and C. scrobicollis, being considered for the biological control of garlic mustard, Alliaria petiolata, in North America. My first objective was to develop a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay which could be used to identify the morphologically indistinguishable larvae. This assay was developed for use in the analysis of population genetic structure of the three species and to aid scientists in evaluating host-specificity test results where larval development was incomplete or adults failed to emerge. The resulting assay provides a fast and inexpensive means of identifying otherwise indistinguishable larvae. My second objective was to study the population genetic structure of C. scrobicollis, to evaluate whether the areas where individuals were being collected for host-specificity testing consisted of one or more populations, to estimate the numbers of individuals needed during host-specificity testing and later introduction to adequately represent the diversity of the population, and to evaluate dispersal potential. Results suggest that C. scrobicollis in the area of Berlin, Germany constitute a network of subpopulations with low but significant differentiation among sites and movement of individuals between sites. I estimated that the number of individuals that would need to be sampled to capture 90% or 99% of the genetic diversity in the Berlin area was 10 and 27, respectively. The estimated average dispersal distance based on assignment tests for C. scrobicollis was 28 km. My third objective was to compare the differences in population genetic structure between C. alliariae and C. roberti to determine whether differences in genetic diversity or dispersal potential might aid in prioritizing one species over the other. These two species have similar life histories, distributions, and effects on garlic mustard. Results showed that, over a comparable region in central Europe, total gene diversity was significantly higher in C. roberti, though the difference between the two species was relatively small. Assignment tests suggest there is substantial gene flow among sites for both species. Overall, the results were similar for both species, and I recommend prioritizing based on biological or methodological attributes.Item Open Access Reproductive responses of an apex predator to changing climatic conditions in a variable forest environment(Colorado State University. Libraries, 2015) Salafsky, Susan Rebecca, author; Hufbauer, Ruth, advisor; Franklin, Alan, committee member; Reynolds, Richard, committee member; Savidge, Julie, committee memberApex predators are ideal subjects for evaluating the effects of changing climatic conditions on the productivity of forested landscapes, because the quality of their breeding habitat depends primarily on the availability of resources at lower trophic levels. Identifying the environmental factors that influence the reproductive output of apex predators can, therefore, enhance our understanding of the ecological relationships that provide the foundation for effective forest management strategies in a variable environment. To identify the determinants of breeding-habitat quality for an apex predator in a forest food web, I investigated the relationships between site-specific environmental attributes and the reproductive probabilities of northern goshawks (Accipiter gentilis) on the Kaibab Plateau, Arizona during 1999-2004. I used dynamic multistate site occupancy models to quantify annual breeding probabilities (eggs laid) and successful reproduction probabilities (≥1 young fledged) relative to temporal and spatial variation in climatic conditions (precipitation and temperature), vegetation attributes (forest composition, structure, and productivity), and prey resources (abundances of 5 mammal and bird species). Climatic conditions during the study period varied extensively, and included extreme drought in 2003 and record-high precipitation in 2004. There was also substantial variation in the amount and distribution of 4 forest cover types among 102 goshawk territories within the 1,285-km² study area. The abundance of most prey species, especially mammals, also varied considerably among years, cover types, and goshawk territories. I identified the environmental components that best explained spatiotemporal variation in goshawk reproductive parameters using an information-theoretic approach to evaluate the relative weight of evidence for each model. Out of 62 potential models representing how the distribution of territories with breeding goshawks varied among years and across the landscape, the model with the most evidence indicated that breeding attempts depended on the interaction between the current-year conditions and the site-specific attributes of territories, whereas fledgling production only varied among years. The best-supported model containing variables for climatic conditions included the effects of average annual precipitation and ambient temperature prior to egg-laying on breeding probabilities, and cumulative precipitation during the first 3 weeks post-hatching on successful reproduction probabilities. The best-supported model incorporating vegetation attributes included year-specific effects of forest cover type on breeding probabilities, and conifer cone production on successful reproduction probabilities. Of the prey models I considered, the best-supported model included the effects of the abundance of prey species with distinct ecological niches on both breeding and successful reproduction probabilities. Overall, a habitat model representing food resource availability was the most parsimonious explanation of variation in goshawk reproduction, because it incorporated the effects of temporal variation in climatic conditions and spatial variation in vegetation attributes on the abundance and distribution of prey species with different functional traits. I found that greater heterogeneity in habitat attributes mitigated the effects of changing climatic conditions on reproductive probabilities, because more diverse prey communities increased the abundance of food resources for goshawks during and following drought. Based on the environmental factors that had the greatest influence on reproductive output of an apex predator, I identified management actions designed to enhance the diversity of niches at a landscape scale and reduce the effects of climatic extremes on the productivity of forest food webs.Item Open Access The eco-evolutionary consequences of multiple introductions for colonizing individuals(Colorado State University. Libraries, 2014) Koontz, Michael, author; Hufbauer, Ruth, advisor; Hobbs, Tom, committee member; Melbourne, Brett, committee memberPredicting the fate of individuals colonizing novel habitats is an elusive but critical goal in fields as diverse as invasion biology, biological control, climate change-induced species range shifts, and reintroductions of rare species. Propagule pressure, which comprises the number of introduction events (propagule number) and the number of individuals per introduction event (propagule size), consistently correlates with a greater probability of population establishment. It is unclear which component, propagule number or propagule size, is more important for establishment, or under what environmental conditions their relative importance may shift. We used 917 independent Tribolium flour beetle populations in a microcosm experiment to disentangle the importance of the different components of propagule pressure. In a factorial design, we held the total number of introduced individuals constant (20) and varied the number of introductions used to distribute them (1, 2, 4, or 5 events) into stable or randomly fluctuating novel environments. Counter to expectations, we found no effect of environmental stability on extinction probability or time to extinction. We also found that several, small introduction events resulted in the lowest extinction probability and the longest time to extinction. We propose that continuing introductions provided low amounts of gene flow that were critical to alleviating inbreeding depression and/or reducing allelic loss by drift in the incipient populations. Our results speak to the importance of preventing future introductions of invasive species (even those that are already established), and using sustained efforts to establish biological control agents or reintroduce desirable organisms to their former range.