Mountain Scholar :: Browsing by Author "Poff, N. LeRoy, committee member"

Browsing by Author "Poff, N. LeRoy, committee member"

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Open Access
Algal blooms in the alpine: investigating the coupled effects of warming and nutrient deposition on mountain lakes
(Colorado State University. Libraries, 2019) Oleksy, Isabella Anna, author; Baron, Jill S., advisor; Spaulding, Sarah A., committee member; Poff, N. LeRoy, committee member; Covino, Timothy, committee member
While 20th century atmospheric nitrogen (N) deposition has been strongly linked to changes in diatom assemblages in high-elevation lakes, contemporaneous changes in other algae suggest additional causes. Using proxies preserved in lake sediments, we explored the origin and magnitude of changes in an alpine and subalpine lake from the end of the Little Ice Age in the 19th century to ca. 2010. We found dramatic changes in algal community structure. Diatom analyses revealed a pronounced shift from majority benthic to planktonic diatoms ca. 1950, coincident with the rise of atmospheric N deposition. Pigments representing benthic green algae have increased 200-300% since ca. 1950; diatom pigments suggest stable or slightly declining populations. Cyanophytes and cryptophytes are not abundant in the sediment record, but there has been a slight increase in some taxa since ca. 1950. While some changes began ca. 1900, the shifts in nearly all indicators of change accelerate ca. 1950 commensurate with many human-caused changes to the Earth system. In addition to N deposition, there have been marked recent increases in aeolian deposition to western mountains that contributes phosphorus. Strong increases in summer air (0.7 °C per decade) and surface water (0.2-0.5°C per decade) temperatures since 1983 have direct and indirect consequences for high elevation ecosystems. While our links between the causes of changes and the responses of mountain lake primary producers are inferred, the drivers and their responses are indicators of changes in the Earth system that have been used to define the Anthropocene. Algal communities (or assemblages) in historically unproductive mountain lakes are shifting, and these changes are taking place commensurate with increasing water temperatures and nutrient availability. However, the mechanisms promoting chlorophytes over bacillariophytes and the implications for ecosystem function are not well understood. We tested the effect of nutrient enrichment on the relative abundance of algal taxonomic groups in a field experiment. We also tested the interactive effects of nutrients and temperature on ecological function of chlorophyte-dominated benthic communities in a laboratory experiment. Nutrient enrichment of both nitrogen and phosphorus favored chlorophytes and led to the highest overall algal biomass. In the absence of nutrient enrichment, the relative abundance of bacillariophytes was significantly greater than chlorophytes and cyanobacteria. Nitrogen assimilation increased significantly, but net ecosystem production decreased, with warming temperatures. Collectively, our results show how chronic N deposition, permafrost thaw, P deposition, and a warming climate interact to alter both the structure and function of mountain lake algal communities. Climate change is altering biogeochemical, metabolic, and ecological functions in lakes across the globe. Historically, high-elevation lakes in temperate regions have been unproductive due to brief ice-free seasons, a snowmelt-driven hydrograph, cold temperatures, and steep topography with low vegetation and soil cover. Observed increases in high elevation lake productivity in the Southern Rocky Mountains over the past decade led us to ask: what are the drivers behind increasing primary productivity? We tested the relative importance of winter and summer weather, watershed characteristics, and water chemistry as drivers of phytoplankton dynamics. Boosted regression tree models were applied using data from 28 high-elevation lakes in Colorado to examine spatial, intra-seasonal, and inter-annual drivers of variability in lake phytoplankton, using chlorophyll a as a proxy. Similar to previous studies, we found that phytoplankton biomass was inversely related to the maximum snow water equivalent (SWE) of the previous winter. However, even in years with average SWE, summer precipitation extremes and warming enhanced phytoplankton biomass. Peak phytoplankton biomass consistently coincided with the warmest water temperatures and lowest nitrogen to phosphorus ratios. While links between declining snowpack, lake temperature, nutrients, and organic matter dynamics are increasingly recognized as critical drivers of change in high elevation lakes, this study identifies additional processes that will influence lake productivity as the climate continues to change. Continued changes in the timing, type, and magnitude of precipitation in combination with other global change drivers (e.g., nutrient deposition) may have consequences for production in high elevation lakes, potentially shifting these historically oligotrophic lakes toward new ecosystem states.
Open Access
Diversity, distributions, and evolution of Rocky Mountain and Andean stream insects
(Colorado State University. Libraries, 2017) Gill, Brian Andrew, author; Funk, W. Chris, advisor; Kondratieff, Boris C., advisor; Poff, N. LeRoy, committee member; Clements, Will H., committee member
Concordant with latitudinal increases in seasonality, the "climate variability hypothesis" (CVH) posits that the breadth of species' thermal tolerances should increase with latitude. Across elevations, the "mountain passes are higher in the tropics hypothesis" (MPHT) postulates that the narrow thermal tolerances of tropical species should limit their dispersal across elevations more than the broad thermal tolerances of temperate species. We consequently expect tropical species to have more limited elevational ranges and higher rates of population isolation, divergence, and speciation than temperate species, which could lead to higher tropical than temperate species richness. Moreover, many tropical species might be cryptic, as they have diverged primarily in physiological and dispersal traits, rather than traits with distinct morphological phenotypes. In this dissertation, I investigate how the CVH might provide a mechanistic explanation for global trends in species richness, cryptic diversity, and elevational distributions. In chapter one, I recapitulate the CVH and MPHT hypothesis and summarize related key literature. In chapter two, I characterize the diversity and distributions of stream insects in the Colorado Rocky Mountains. In chapter three, I compare the species richness and elevational ranges of species from the Colorado Rocky Mountains and the Andes of Ecuador. Lastly in chapter four, I integrate data from physiological, landscape genetic, and biogeographic investigations to evaluate the support for the CVH as a key mechanism determining global trends in species diversity, distributions, and vulnerability to climate change.
Open Access
Ecological and evolutionary consequences of Allee effects in small founder populations of invasive species
(Colorado State University. Libraries, 2011) Kanarek, Andrew R., author; Webb, Colleen T., advisor; Ghalambor, Cameron K., committee member; Hufbauer, Ruth A., committee member; Poff, N. LeRoy, committee member
Despite the obvious threats invasive species pose to ecosystem health, studying the characteristics that influence their colonization can provide valuable insight on fundamental issues in ecology, evolution, and biogeography. The aim of this research has been focused on the implications of mechanisms likely to affect persistence of small founder populations. Individuals can suffer a reduction in one or more components of fitness when population growth and spread are constrained at low density. This dynamical relationship between fitness and population size (i.e., positive density dependence) can be driven by a myriad of mechanisms, broadly termed Allee effects. In this dissertation, I have theoretically explored how small founder populations faced with Allee effects can overcome the demographic challenges that heighten the risk of extinction. I have developed models of increasing complexity to better understand the ecological and evolutionary consequences of Allee effects. I begin by exploring ways in which intraspecific interactions influence population dynamics and invasiveness through a review of the literature. The mechanisms that impact individual fitness at low density suggest that there are benefits to being in a large population; however, there are abundant examples of adaptations that might have evolved in small or sparse populations in response to Allee effects. Using a reaction-diffusion framework with a quantitative genetics approach, I have derived conditions and explored the dynamics for rapid adaptive evolution rescuing the population from extinction. This deterministic modeling approach broadly describes population dynamics through diffusive dispersal and density dependent growth, where the response to population density can evolve through a genetic subsystem that incorporates the intensity of selection and genetic variance. For both the spatial and non-spatial cases, invasion criteria were determined across the range of parameter space. The results emphasized that a sufficient amount of genetic variance is a crucial component for evolutionary rescue to occur. I developed a spatially explicit, individual-based stochastic simulation in order to more realistically capture the complexity of intraspecific interactions. I found that with limited dispersal and local perception, the emergence of spatial structure impacted individual fitness and could enable population persistence. Departures from the population-level model predictions demonstrate the importance of considering individual variation in assessing the consequences of Allee effects. I further incorporated immigration and genetic variation into the simulation in order to explore the relative importance of evolutionary, demographic, and genetic rescue for establishment. Additional immigration was more effective than adaptive evolution in contributing to successful invasions due to the intensity of ecological constraints on population growth and time to extinction. Without multiple introductions, evolutionary processes can contribute to recovery through genetic variation maintained and enhanced by mutation and recombination. Overall, I have demonstrated that it is possible for a small founder population to overcome a suite of ecological, evolutionary, and genetic obstacles upon introduction into a novel environment despite the paradox of invasion.
Open Access
Ecologically-focused calibration of hydrological models for environmental flow applications
(Colorado State University. Libraries, 2015) Adams, Stephen K., author; Bledsoe, Brian P., advisor; Poff, N. LeRoy, committee member; Stein, Eric D., committee member
Hydrologic alteration resulting from watershed urbanization is a common cause of aquatic ecosystem degradation. Developing environmental flow criteria for managing the effects of urbanization and other human influences requires quantitative flow-ecology relationships that link biological responses to streamflow alteration. To the extent possible, gaged flow data are used; however, bioassessment sites are frequently ungaged and hydrological models must be used to characterize flow alteration. Physically-based rainfall-runoff models typically utilize a "best overall fit" calibration criterion, such as the Nash-Sutcliffe Efficiency (NSE), that does not focus on specific aspects of the flow regime relevant to biotic endpoints. This study aims to identify how accurately coastal southern California rainfall-runoff models can be calibrated using specific elements of the flow regime known a priori to be critical to benthic macroinvertebrates (ecologically-focused) versus a traditional best overall fit criterion. Additionally, this study seeks to assess the utility of ecologically-focused calibrated models by comparing flow metric accuracy and the strength of flow-ecology relationships among different calibration approaches versus gage data. For this study, continuous HEC-HMS 4.0 models were created for 19 coastal southern California watersheds and calibrated to USGS streamflow gages with nearby bioassessment sites using one best overall fit and three ecologically-focused criteria: NSE, Richards-Baker Flashiness Index (RBI), percent of time when the flow is < 28 L/s (< 1 cfs), and a Combined Calibration (RBI and < 1 cfs), respectively. Ecologically-focused criteria were selected based on preliminary statistical flow-ecology relationships at gaged bioassessment sites. Calibrated models were compared using flow metric accuracy relative to gage data and the strength of flow-ecology relationships. Models were highly accurately calibrated to ecologically-focused criteria, with calibration median percent errors less than 1.5% and only a single model with a percent error greater than 10%, and NSE criteria, with a median value of 0.634. Regardless of high calibration accuracy for ecologically-focused models, additional flow metrics not explicitly calibrated, especially those describing magnitude or rise and fall rates at aggregated daily time scales, were not consistently reproduced by models. Despite inaccuracies across a full suite of 71 flow metrics, low flow and flashiness metrics relevant to biotic endpoints were modeled accurately (< 20% error) and often provided stronger flow-ecology relationships than best overall fit criteria in terms of adjusted R2 in multiple regression analyses and variance explained in random forest modeling. This was especially true when two ecologically-focused criteria were combined, suggesting the importance of multiple calibration criteria. Flow metrics from the Combined Calibration provided the strongest flow-ecology models in correlation and regression analyses compared to the other three calibration approaches, and perform similarly in random forest models. This study demonstrates that if ecologically relevant flow metrics can be identified using published literature or preliminary statistical analyses of gaged bioassessment sites prior to developing a hydrologic foundation, they can be incorporated as calibration criteria and provide stronger modeled flow-ecology relationships than exclusive use of a best overall fit criterion.
Open Access
Evaluating the success of Arkansas darter translocations in Colorado: an occupancy sampling approach
(Colorado State University. Libraries, 2011) Groce, Matthew Christopher, author; Fausch, Kurt D., advisor; Bailey, Larissa L., committee member; Poff, N. LeRoy, committee member
Like many fishes native to western Great Plains streams, the Arkansas darter Etheostoma cragini has declined, apparently in response to changes in flow regimes and habitat fragmentation. I investigated the effectiveness of translocation as a management strategy to conserve this threatened species in the Arkansas River basin of southeastern Colorado. I used a multiscale design to sample darters and several attributes of their habitat at the local 10-m site scale, the 3.25-km translocation segment scale, and the 10-km riverscape scale, in 19 streams where darters were previously translocated. I used multistate occupancy estimation, based on two consecutive dipnetting surveys, to determine habitat characteristics correlated with site occupancy and detectability of Arkansas darters. Darters were present in 11 of 19 streams, although 5 were completely dry when visited. Darters had reproduced in 10 of the 11 streams (one criterion in the state recovery plan), and 6 streams also met a second criterion for abundance (>500 individuals). However, populations in only two streams unequivocally met the third criterion of being self-sustaining, because the other four streams had been stocked annually with hatchery-reared darters. Detectability of darters at sites where water was present was high for both age groups, 91% for age-0 darters and 76% for age-1 darters, and was a function of Julian date (age-0) and habitat depth (age-1). Residual stream temperature (a site-scale variable) and the total length of available habitat (a riverscape-scale variable) were the strongest predictors of site occupancy for both age groups. The models were useful in identifying fragmentation by a road culvert as a potential impediment to success in another stream where conservation biologists have proposed translocating darters. These models can be used to guide habitat conservation and land management practices that seek to conserve, protect, and restore current and future critical habitat for Arkansas darters.
Open Access
Hydrologic alteration under hydropower dam operations and climate change: a case study in the Sesan River Basin, Lower Mekong Region
(Colorado State University. Libraries, 2023) Ghalley, Wangmo, author; Niemann, Jeffrey D., advisor; Shrestha, Sangam, advisor; Ettema, Robert, committee member; Poff, N. LeRoy, committee member
Hydropower dam developments exacerbated by climate change can significantly disrupt the natural flow regimes, leading to adverse effects on river ecosystems. The Sesan River, a major tributary of the Lower Mekong Basin, is renowned for its diverse biomes and is an important resource for nearby inhabitants. Rapid expansion of hydropower dams has occurred in recent years, but the hydrologic impacts remain poorly understood, particularly when combined with the effects of climate change. This study assessed the hydrologic alterations in Sesan River streamflow due to hydropower dams and potential climate change. Daily streamflow in the Sesan River was simulated using the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS), which was calibrated and evaluated based on streamflow observations. Climate change projections were based on daily precipitation and temperature, which were estimated using an ensemble of three Earth system models from the Coupled Model Intercomparison Project Phase-6 under two Socioeconomic Pathways: SSP2-4.5 (Middle of the road) and SSP5-8.5 (Fossil-fueled development). Future projections spanned 2025 to 2100, which was divided into three 25-year periods called the Near Future (NF), Mid-Future (MF), and Far Future (FF). The projections were compared to a 30-year baseline (BL) period from 1984 to 2014. Results show a consistent rise in both precipitation and temperature for the Sesan basin across all future periods and SSP scenarios. Precipitation is projected to increase by 4% to 13% for SSP2-4.5 and 7% to 29% for SSP5-8.5. Minimum temperature is projected to increase by 8% to 16% for SSP2-4.5 and 10% to 26% for SSP5-8.5, and maximum temperature is projected to increase by 3% to 7% for SSP2-4.5 and 3% to 12% for SSP5-8.5. Hydrologic alterations were assessed using the Range of Variability Approach (RVA) within the Indicators of Hydrologic Alteration (IHA). The impact of dams was assessed by comparing streamflow with dams and without dams during the BL period. The dams significantly altered the hydrograph characteristics by decreasing the high flows and increasing the low flows. The overall alteration due to dams fell within the "moderate" category. The impact of climate change was assessed by comparing streamflow without dams between the BL and the future periods. Climate change increased the high flow rates, with the impact limited to September in the NF but impacting much of the year in the MF and FF periods. Another notable change was the shift in the timing of peak flow from August in the BL to September in the future periods. The hydrologic alteration due to climate change fell within the "low" category. Finally, the combined impact of dams and climate change was assessed by comparing the BL streamflow without dams to future streamflow with dams. Dams were found to mitigate some impacts of climate change by smoothing extreme high flows, especially in the FF period. Overall, the combined impact showed greater alteration than the individual scenarios but fell within the "moderate" category.
Open Access
Improving hydrologic modeling of ungaged basins to support environmental flow management in a heterogeneous region
(Colorado State University. Libraries, 2021) Adams, Stephen K., author; Bledsoe, Brian P., advisor; Poff, N. LeRoy, committee member; Niemann, Jeffrey D., committee member; Stein, Eric D., committee member
Environmental streamflow management can sustain aquatic ecosystems and the services they provide by reestablishing elements of the natural flow regime that are necessary for ecological health. One of the more difficult challenges with developing environmental flow criteria is estimating streamflow at locations without gage data; however, this challenge is not unique to environmental flows. Streamflow prediction in ungaged basins is a very common problem in hydrology and engineering with no clear solution, but it is particularly difficult to model environmental streamflow metrics across heterogeneous regions with highly diverse land uses, geologic settings, and hydroclimatological processes. In this dissertation, I create a new regionalization framework, "Streamflow Regionalization with Hydrologic Model-based Classification" (SR-HMC), for modeling challenging flow metrics in ungaged basins across a heterogeneous region. I also test the efficacy of the new framework for developing environmental streamflow criteria. In Chapter 2, I explore different approaches for classifying streams with similar flow regimes and develop a novel classification technique for prioritizing regional accuracy of hydrologic models. As the precursor to SR-HMC, this "Hydrologic Model-based Classification" (HMC) groups hydrologically similar streams by determining the degree of reciprocity of calibrated parameters between a regional catalog of rainfall-runoff models as quantified through jackknife resampling. Results show that HMC complements traditional classifications based on streamflow metrics and watershed characteristics, and offer advantages over these traditional classifications when used to regionalize ungaged basins. Next, Chapter 3 describes implementation of ensemble modeling to optimize HMC into a regionalization framework for producing time series of streamflow at ungaged sites. For gaged locations, hydrologic model parameters that cannot be calculated directly can be calibrated using observed flows; however, these same model parameters are much more uncertain and difficult to estimate at ungaged locations. SR-HMC uses geographically-weighted model output averaging with regionally-calibrated parameter sets to reduce parameter uncertainty in models of ungaged basins. This new framework is tested at five sites across a large and diverse region. Results were improved using SR-HMC over standard nearest-neighbor regionalization approaches. Finally, I turn to management applications of these novel methods in ungaged basins by analyzing the statistical relationships between streamflow alteration and ecological integrity. In Chapter 4, I compare the explanatory power of simple flow-ecology relationships produced by different methods for regionalizing ungaged basins and different metrics of flow alteration. Results highlight robust modeling practices amenable to management. Development of environmental streamflow recommendations based on prediction in ungaged basins is an ongoing challenge; however, this research demonstrates how novel approaches to classification and model extrapolation can improve streamflow estimation at ungaged locations in heterogeneous regions, and thereby bolster the scientific basis of environmental flow management.
Open Access
Intra- and interspecific variation along environmental gradients: adaptation, plasticity, and range limits
(Colorado State University. Libraries, 2012) Torres Dowdall, Julián R., author; Ghalambor, Cameron K., advisor; Angert, Amy L., committee member; Fausch, Kurt D., committee member; Poff, N. LeRoy, committee member; Thomas, Stephen, committee member
Understanding the processes underlying patterns of intraspecific variation, and how these processes in turn shape the distributional limits of species is a fundamental goal of evolutionary ecology. The study of species distributed along environmental gradients provides a framework for testing how changing conditions lead to local adaptation, phenotypic plasticity, and ultimately shape distributional limits. Yet, environmental gradients are complex, being composed of a diversity of abiotic and biotic factors that act on individual species and shape the interactions between them. Thus, empirical studies aimed to understand patterns of intraspecific divergence and interspecific diversity need to evaluate the effects of both abiotic and biotic factors varying along gradients. Evolutionary ecologists have become increasingly interested in trying to understand the costs and limits to trait variation along environmental gradients and what factors prevent species from evolving larger geographic ranges. Theory predicts that species distributed along environmental gradients should track conditions through local adaptation or adaptive phenotypic plasticity, and that a disruptive factor along the gradient (e.g. increase in the steepness of the gradient, the presence of a competitor, etc.) could result in the formation of distribution limits as conditions become unsuitable for populations persistence. Empirical studies analyzing large-scale patterns of phenotypic variation have provided support for the formation of clines in response to environmental gradients. However, less evidence has accumulated for the formation of such patterns at local scales and clear disruptive factors leading to distributional limits remain elusive. My dissertation takes an evolutionary ecological perspective to understand how environmental gradients shape patterns of variation within and between species. Here, I attempt to understand how abiotic and biotic factors interact to drive patterns of phenotypic variation. To approach this question, I used as a study system two closely related, ecologically similar, and parapatric species of poeciliids distributed along rivers on the island of Trinidad, West Indies. In the first part of this dissertation, I focus on the patterns of intraspecific variation in the Trinidadian guppy (Poecilia reticulata) along a predation risk gradient. I used this species to explore the spatial scale at which local adaptation occurs (Chapter 1), and to investigate the role of adaptive phenotypic plasticity in allowing species to track local optima (Chapter 2). I found that local adaptation in Trinidadian guppies occurs at a smaller spatial scale than previously shown. My results also suggest that adaptive plasticity plays an important role in allowing Trinidadian guppies to track local optima along a gradient of predation risk. Furthermore, I found divergence in patterns of plasticity between Trinidadian guppy populations adapted to low- or to high-levels of predation pressure. My results suggest that this difference in adaptive phenotypic plasticity evolved as a by-product of adaptation to local environmental conditions. In the second part of my dissertation I change my focus from patterns of intraspecific variation to patterns of interspecific variation along environmental gradients. I examine how the Trinidadian guppy and its sister species, the swamp guppy (P. picta), are distributed along a complex environmental gradient in lowland rivers of Trinidad (Chapter 3), and performed a series of experiments aimed to determine what factors drive their distributions (Chapter 4). As Trinidadian rivers approach the ocean, several factors change in comparison to upstream localities, including changes in productivity, physicochemical conditions, and community composition. I found that the Trinidadian guppy and the swamp guppy show an overlapping parapatric distribution along the interface between brackish-freshwater in the lowland rivers of Trinidad. The swamp guppy is usually found in downstream sections of the rivers, both in fresh- and brackish water. On the other hand, the Trinidadian guppy is only found in freshwater, dropping off abruptly at the brackish-freshwater interface. Field and laboratory experiments suggest that brackish water environments are physiologically stressful for the two study species, as survival and growth rate in this environment were lower compared to that observed in freshwater. Also, these experiments indicate that the Trinidadian guppy is competitively dominant over the swamp guppy across all salinity conditions. Thus, I showed that asymmetric competition limits the competitively subordinate swamp guppy to the harshest end of the salinity gradient, and that stressful salinity conditions limits the dominant Trinidadian guppy to the less stressful freshwater end of the gradient.
Open Access
On the development of flow-ecology relationships for streams in coastal watersheds of southern California
(Colorado State University. Libraries, 2014) Eberhart, Sarah R., author; Bledsoe, Brian P., advisor; Poff, N. LeRoy, committee member; Stein, Eric D., committee member
Linking hydrologic alteration to the biotic responses of streams is essential for understanding and managing the effects of land use changes and other human influences on aquatic ecosystems. This study develops flow-ecology relationships for wadeable streams in coastal watersheds of southern California to understand the ecological effects of urbanization and other sources of hydromodification. Streams in this region are predominately flashy, seasonally intermittent, and fine grained; hence, the inherently harsh disturbance regime is a major determinant of biotic composition. I match biological and geomorphic data with proximate U. S. Geological Survey streamflow gages to examine flow-ecology relationships between benthic macroinvertebrates and the hydrologic and hydraulic regimes of 32 biomonitoring sites spanning a gradient of watershed urbanization. Associations between landscape, streamflow, and biotic metrics indicate that flow permanence and urbanization are overarching and interacting influences on benthic macroinvertebrate assemblages in this region. In particular, flow intermittency and flashiness are significant predictors of both taxonomic and traits-based measures of biotic composition. Urban land cover and road density are significantly correlated with higher flow flashiness and decreasing measures of biotic integrity. Hydraulic metrics describing streambed mobility are strongly positively associated with measures of biotic integrity as a result of high intercorrelation with flow permanence. Thus, it appears that benthic macroinvertebrate assemblages are fundamentally influenced by flow intermittency and urban-induced flashiness in this region. Use of daily discharge data analyzed 3 yrs prior to biological sampling events appears to result in little to no loss of resolution in flow-ecology relationships compared to sub-daily (15-min) and long-term (decadal) flow records. Results also underscore the utility of traits-based analyses and stratification of sites by flow permanence and dominant substrate in revealing mechanistic relationships between flow and biotic metrics. By using gaged sites to identify the flow metrics best describe biological variation, this study provides insight into which elements of the flow regime are most important to model accurately in future efforts to develop a regional hydrologic foundation that will allow the inclusion of ungaged biomonitoring sites in refining flow-ecology relationships.
Open Access
The ecological and evolutionary mechanisms behind the persistence of highly virulent pathogens: plague as a case study
(Colorado State University. Libraries, 2013) Buhnerkempe, Michael G., author; Webb, Colleen T., advisor; Poff, N. LeRoy, committee member; Eisen, Rebecca J., committee member; Hoeting, Jennifer A., committee member
The persistence of emerging infectious diseases is the result of eco-evolutionary feedbacks between a pathogen and its novel host. Spatial structure both within and between host populations (i.e., a metapopulation) in particular can have a large effect on the establishment and subsequent coevolution of a host and pathogen. Here, my colleagues and I explore how differing metapopulation structures in a host and pathogen affect the coevolutionary maintenance of high virulence and low resistance in an emerging infectious disease. We use the relatively recent emergence of plague, caused by the bacterium Yersinia pestis, in North America as a case study to both understand how spatial structure in the pathogen may differ from that of its host and how these differences may affect coevolutionary trajectories. Host responses to Y. pestis infection are highly variable with some species, like black-tailed prairie dogs (Cynomys ludovicianus), experiencing massive population declines upon introduction of the plague bacterium (i.e., epizootics), while others, like the California ground squirrel (Spermophilus beecheyi), exhibit enzootic maintenance of Y. pestis. These species in particular have markedly different spatial structures, but it is unclear how regional transmission of plague may structure the pathogen population. To understand transmission more fully, we developed a mechanistic model of plague infection in a single population that incorporated multiple routes of transmission and parameterized the model for the two species mentioned above. We found that transmission in the epizootic system is driven largely through on-host cycling of fleas (i.e., a booster-feed infection cycle). In contrast, enzootics are driven by an off-host, questing flea reservoir. The potential for off-host fleas to drive plague dynamics reveals the potential for non-overlapping host and pathogen metapopulation structures. The effect of such a structure on coevolution is not well-understood, particularly for quantitative traits where no theoretical methods exist to study coevolution in a metapopulation. Consequently, we also developed a novel theoretical framework for studying quantitative trait coevolution in a metapopulation. This new framework reveals that coevolutionary outcomes for resistance and virulence depend on the interaction between host and pathogen dispersal strategies with local reproduction and transmission dynamics favoring a diversity of resistance-virulence combinations. Host-pathogen coevolution is also affected by the shape of life-history trade-offs for both the host and the pathogen. We predicted coevolutionary outcomes under different host and pathogen dispersals assuming three different trade-off functions when resistance comes at the cost of reproduction and virulence increases transmission while decreasing the infectious period: accelerating, linear, and decelerating costs. We found that selection on resistance is most sensitive to concave trade-off functions, and selection on virulence was most sensitive to convex functions, although coevolutionarily stable strategies were only predicted when both resistance and virulence had accelerating cost trade-off functions. Predictions from the model also differ from those observed in well-mixed and spatially structured single populations indicating that eco-evolutionary dynamics do not scale directly with space. Implications for future models of plague coevolution are also discussed.
Open Access
The evolutionary ecology of aquatic insect range limits: a mechanistic approach using thermal tolerance
(Colorado State University. Libraries, 2018) Shah, Alisha Ajay, author; Ghalambor, Cameron K., advisor; Funk, W. Chris, advisor; Poff, N. LeRoy, committee member; Clements, William H., committee member
Understanding the effect of climate variability on species physiology and distribution is a longstanding and largely unresolved challenged in evolutionary ecology with important implications for vulnerability to climate change. My dissertation is focused on understanding the effects of temperature on physiological traits and genetic population structure of aquatic insects, to better understand the mechanisms that underlie their elevation range distributions. For my first chapter, I tested the hypothesis proposed by Dan Janzen in 1967, that temperate mountain species should have broad thermal tolerances thus allowing them to disperse easily across elevation, unhindered by the novel temperatures they encounter. On the other hand, tropical species should exhibit narrower thermal tolerances in response to the stable climate they experience. They should be physiologically challenged to disperse and be restricted to a narrow elevation range distribution. I measured critical thermal limits (CTMAX and CTMIN) and thermal breadth (difference between CTMAX and CTMIN) in several phylogenetically related temperate (Colorado) and tropical (Ecuador) aquatic insect species. I found that, as predicted, species that encounter wider stream temperature ranges, such as temperate species and high elevation tropical species, have broader thermal breadths compared to their tropical and low elevation relatives. Next, I tested how plastic the critical thermal maximum (CTMAX) response was in a subset of aquatic insects. Greater acclimation ability is thought to allow species to withstand the large temperature fluctuations associated with different seasons. Implicit in Janzen's hypothesis, is the assumption that temperate species have greater acclimation ability compared to tropical species. My experiments revealed that temperate and high elevation tropical mayfly species had greater acclimation ability compared to their relatives. However, we found no differences in acclimation capacity in stoneflies. Temperature may therefore not affect all species equally, and species acclimation ability may be a result of other factors such as body shape and evolutionary history. I then measured a third trait, metabolic rate, to investigate how it varies with temperature in temperate and tropical mayflies. Metabolic rate is arguably one of the most important traits for species because it determines the amount of energy an animal has available for its activities. I found that metabolic rates vary between temperate and tropical mayflies, and that temperatures away from a certain optimum are stressful and sometimes lethal for tropical but not temperate mayflies. Finally, I linked thermal tolerance to dispersal by correlating gene flow among populations with pairwise differences in the physiological trait CTMAX. Analyses revealed that there was lower gene flow (higher FST) among populations in Ecuador than among populations in Colorado. Within Ecuador, differences in CTMAX were highly correlated with maximum stream temperature, which was found to best explain tropical mayfly genetic structure. In Colorado, no environmental or physiological variable was found to explain population structure. Our results indicate, as Janzen predicted, that temperature can act as a significant barrier to dispersal among tropical populations but not in temperate ones. Thermal sensitivity measured as CTMAX was also correlated with FST but was not significant. As a whole, the results from my research lend support to Janzen's hypothesis and suggest that temperature plays an important role in determining range limits of aquatic insect species through its effect of thermal tolerance traits. While this research addresses long standing questions in ecology and evolution, it also has conservation implications. Most importantly, as the effects of global climate change augment, the thermally sensitive tropical species from this study system are at particular risk for extreme population declines or even extinction.
Open Access
Using species functional traits to predict community dynamics
(Colorado State University. Libraries, 2012) Ames, Gregory Michael, author; Webb, Colleen, advisor; Poff, N. LeRoy, committee member; Knapp, Alan, committee member; Noon, Barry, committee member
A major goal for community ecology has been to determine a general set of rules to explain the structure and function of communities. Traits-based methods for describing community dynamics have been touted as providing a set of general methods to describe the structure and function of communities based on measurable properties of individual organisms in the community in a changing environment. Validation of traits-based methods that describe changes in community structure as a function of the interaction between functional traits along changing environmental gradients in real systems is needed. Here we present studies of three different plant communities where we use novel applications of traits-based Bayesian hierarchical models and principal component analysis to explain the changes in community structure/function and demonstrate that the communities are primarily structured by traits and their interactions with a changing environment. In a natural tallgrass prairie we were able to explain more than 84% of the variation in community functional diversity and an average of 64% of the cover variation across the ten species in the study over a 25-year span (Chapter 1). Additionally we show that changes in community structure are primarily explained by relative growth rate and its interaction with precipitation. In an experimentally manipulated grassland, our model explains more than 75% of the variation in total plot biomass over the course of 18 years. Further, we found that this system was primarily driven by the same trait/environment interactions as the tallgrass system. Finally, we show that trait/environment interactions allow us to explain 91% of the variation in plot biomass in a restored riparian wetland. Our ability to explain large portions of the variation in community structure and performance of these three distinct types of plant communities, using similar traits and environmental drivers, provides evidence of general laws underlying the structure of plant communities. This work represents a significant step toward understanding those general laws and helping community ecology develop from a largely descriptive science to a predictive science.
Open Access
Vulnerability of cold-water and cool-water fishes to climate change within an anthropogenic context using boosted regression trees, decision scaling, and ecosystem services
(Colorado State University. Libraries, 2016) Harrison-Atlas, Dylan, author; Theobald, David, advisor; Arabi, Mazdak, committee member; Goldstein, Joshua, committee member; Poff, N. LeRoy, committee member
Across the globe, environmental changes are occurring in ways that are profoundly important for freshwater ecosystems with implications for the occurrence of species. Typically, ecologists have sought to understand the distribution of freshwater species using natural environmental gradients. However, because rivers and streams embody a wide range of conditions due to human activity, adequately characterizing modern day drivers of species occurrence requires assessing both natural and anthropogenic influences within the context of global change. In recent decades, growing concerns over climate change have further contributed to the need to assess contemporary drivers of species occurrence. Despite this urgency, forecasting ecological responses to climate change remains a key conservation challenge. The aims of my research were to: a) investigate the drivers of western US riverine fish species occurrence within the context of global change; and b) project range-wide and site-level vulnerability of cold-water fish species to climate-induced changes in stream temperature and streamflow and to alternative land use trajectories. In my assessment of contemporary drivers of cold-water and cool-water fish species distribution, I found that primary determinants of fish occurrence included human influences that accounted for a substantial portion of modeled outcomes among species. Sedimentation and nutrient enrichment were the two primary disturbance pathways by which human activities influence aspects of stream condition that drive patterns of species occurrence. I also found that species had variable responses across anthropogenic gradients, suggesting that future efforts to characterize species-environment relations consider approaches that can capture nonlinear and threshold responses that occur along continuous gradients. In a second analysis, I evaluated the range-wide vulnerability of cold-water fish species to projected climate change in the western United States and assessed site-level vulnerability to varying degrees of exposure to climate change and additional environmental stressors. I focused on rainbow trout (Oncorhynchus mykiss sp.) and cutthroat trout (Oncorhynchus clarkii sp.) -- two wide-ranging salmonids of significant conservation and economic importance. Using high resolution data on future stream temperature and mean annual flow, I projected climate-induced changes in suitable habitat across the historic native ranges of both species within the western United States. Projected declines in suitable habitat for cutthroat trout were substantial by 2080 and exceeded those of rainbow trout. A sensitivity analysis revealed that stream temperature warming was the primary driver of habitat loss for both species. Both cutthroat trout and rainbow trout exhibited regional variability in habitat loss that was consistent with the magnitude of projected warming for summer stream temperature. Cutthroat trout distributions are expected to shift upwards along an elevational gradient with warming causing fragmentation of contiguous habitat that will likely expose them to additional environmental disturbances. I conducted a complementary set of analyses using a decision-scaling approach to explore site-level vulnerability as a function of feasible climate futures and human-influenced environmental factors that have previously been implicated as key components of suitable habitat for cutthroat and rainbow trout. I uncovered important insights into species vulnerability including differential sensitivity to stream temperature warming among cutthroat trout and rainbow trout as well as predominant influences of land use on species vulnerability independent of climate. Under a hypothetical climate adaptation scenario, I found that increased riparian cover shifted the distribution of vulnerability of cutthroat trout towards less frequent extirpations and that these benefits were achieved throughout feasible climate space. My findings suggest that augmentation of riparian vegetation is likely to be a robust climate adaptation strategy in an uncertain future. I conclude by offering two complementary approaches for advancing climate adaptation for freshwater systems in the face of uncertainty. I also conducted a systematic review of hydrologic ecosystem services (HES) studies published within the past decade, finding compelling evidence that variability in methods used to quantify HES reflects an orientation towards decision making. I discuss implications of my findings on climate change vulnerability and consider ways to integrate an ecosystem services approach into the management and conservation of freshwater fish.

Browsing by Author "Poff, N. LeRoy, committee member"

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