Graduate Degree Program in Ecology

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https://hdl.handle.net/10217/100395

These digital collections include theses, dissertations, faculty publications, photographs, and datasets from the Graduate Degree Program in Ecology.

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Open Access
The influence of individual plants on soil nutrient dynamics in the Central Grassland region of the United States
(Colorado State University. Libraries, 1994) Vinton, Mary Ann, author; Burke, Ingrid C., advisor; Coffin, Debra P., committee member; Grier, Charles, C., committee member; Detling, J. K. (James K.), committee member
The extent to which plant community structure influences ecosystem nutrient cycling is an important but poorly understood element of ecosystem ecology. I studied the effects of two aspects of vegetation structure, plant cover patterns and plant species composition, on nutrient cycling in soils of shortgrass-steppe, mid- and tallgrass prairie, and desert grassland in the Great Plains. My general objective was to identify the importance of plant cover patterns and species composition, especially in the context of other environmental variables, to soil nutrient dynamics in these grasslands. In the dry shortgrass-steppe and desert grasslands, plant cover patterns were very important in determining patterns of soil nutrient dynamics. Soils under plants had generally higher rates of carbon and nitrogen pool sizes and turnover rates than soils from adjacent bare ground areas between plants. Individual plant characteristics, such as lifespan and growth form, explained the degree of soil heterogeneity in some cases, with the most long-lived, productive species fostering the most plant-interspace soil heterogeneity. Also, abiotic environmental variables explained patterns in plant-induced soil heterogeneity. The desert grassland with the largest proportion of bare ground, and thus possibly the most soil erosion, had the largest plant-interspace soil heterogeneity. The wet grasslands, the mid- and tallgrass prairies, had more continuous plant cover; thus plant cover did not impose strong control over soil nutrient patterns in these ecosystems. Plant litter quantity and quality of tissue for decomposers differed between species and grassland ecosystems and, in some cases, affected soil nutrient cycling. Kochia scoparia, an introduced species in shortgrass steppe, had high quality tissue (low carbon:nitrogen and lignin:nitrogen) and had relatively high rates of nitrogen and carbon mineralization in its soils. Precipitation affected plant tissue quality, with a general decrease in average quality and increase in inter-species variation in quality from dry to wet grasslands. Vegetation structure, and its interaction with site-based abiotic variables such as precipitation, had important effects on soil carbon and nitrogen dynamics in these grassland ecosystems. Results indicate that information about plant community structure may be critical to large-scale estimates of ecosystem function.
Open Access
Interactions between individual plant species and soil nutrient status in Shortgrass Steppe
(Colorado State University. Libraries, 1995-06) Burke, Ingrid C., author; Vinton, Mary Ann, author; Ecological Society of America, publisher
The effect of plant community structure on nutrient cycling is fundamental to our understanding of ecosystem function. We examined the importance of plant species and plant cover (i.e., plant covered microsites vs. bare soil) on nutrient cycling in shortgrass steppe of northeastern Colorado. We tested the effects of both plant species and cover on soils in an area of undisturbed shortgrass steppe and an area that had undergone nitrogen and water additions from 1971 to 1974, resulting in significant shifts in plant species composition. Soils under plants had consistently higher C and N mineralization rates and, in some cases, higher total and microbial C and N levels than soils without plant cover. Four native grasses, one sedge, and one shrub differed from one another in the quantity and quality of above- and belowground biomass but differences among the six species in soil nutrient cycling under their canopies were slight. However, soils under bunchgrasses tended to have higher C mineralization and microbial biomass C than soil under the rhizomatous grass, Agropyron smithii. Also, the one introduced annual in the study, Kochia scoparia, had soils with less plant-induced heterogeneity and higher rates of C and N mineralization as well as higher levels of microbial biomass C than soils associated with the other species. This species was abundant only on plots that had received water and nitrogen for a 4-yr period that ended 20 year ago, where it has persisted in the absence of resource additions for 20 yr, suggesting a positive feedback between plant persistence and soil nutrient status. Plant cover patterns had larger effects on ecosystem scale estimates of soil properties than the attributes of a particular plant species. This result may be due to the semiarid nature of this grassland in which plant cover is discontinuous and decomposition and nutrient availability are primarily limited by water, not by plant species-mediated characteristics such as litter quality. That local plant-induced patterns in soil properties significantly affected ecosystem scale estimates of these properties indicates that consideration of structural attributes, particularly plant cover patterns, is critical to estimates of ecosystem function in shortgrass steppe.
Open Access
Landscape heterogeneity at multiple scales: effects on movement patterns and habitat selection of eleodid beetles
(Colorado State University. Libraries, 1998) McIntyre, Nancy E., author; Wiens, John A., advisor; Wilson, Thomas (Tom) G., committee member; Kondratieff, Boris C., committee member; Hobbs, Nicholas Thompson (Tom), committee member
I combined observational studies with field experiments to investigate how landscape heterogeneity influences habitat selection in eleodid beetles of the shortgrass prairie of Colorado. I examined correlations in spatio-temporal patterns of habitat occupancy, population density, and community structure in eleodids with spatial and abiotic features of the landscape; I then explored how variations in spatial structure could affect how animals move through a landscape, accounting for the observed patterns of habitat occupancy. I combined these observations with experimental manipulations of several landscape features that affect the movement behaviors of beetles. The results from these experiments show how interactions among animal behavior, landscape composition and configuration, and the scale of spatial structure determine where animals occur in a heterogeneous environment.
Open Access
The role of historical land-cover changes as a mechanism for global and regional climate change
(Colorado State University. Libraries, 1999) Chase, Thomas N. (Thomas Newell), 1962-, author
This paper describes the results from several modeling studies and an observational analysis as to the effect of historical land-cover change on regional and global climates. We discuss methods for determining historical vegetation change and present results from model simulations at the global and regional scale which compare climates generated using currently observed vegetation versus natural vegetation as a boundary condition. We also compare these modeling studies with recent observational data and with simulations of climate change resulting from increased greenhouse gases. We conclude from this research that vegetation change, as it has already occurred, globally and regionally, can have significant effects on both global and regional climates. These effects are not limited to the regions of direct land-cover change forcing. For example, as a result of tropical deforestation, the position and intensity of the ITCZ is affected by the change in land surface characteristics resulting in global-scale effects which are similar in nature to the climatic effects associated with El Niño Southern Oscillation (ENSO). These include changes in high-latitude circulations, the generation of low frequency waves which appear to propagate to the extratropics in well-defined teleconnection patterns, and reduced low-level easterlies over most of the topical Pacific basin under current vegetation. This implies an interaction mechanism between tropical deforestation and ENSO. The model simulations of climate change due to land-cover change compare favorably in spatial patterns and amplitude with recently observed temperature trends. Additionally, a comparison between simulations of climate changes due to land-cover disturbance and changes due to rising atmospheric CO2 concentration show that global land-cover changes as they have already occurred, are responsible for shifts in climate which are of similar amplitude and occur in the same regions as simulated climate changes resulting from increased CO2. A comparison of three independent observational datasets shows strong disagreement not only in the sign of recent globally-averaged temperature trends but also disagree as to regions where significant climate shifts are occurring. Unlike model simulations of greenhouse gas warming, warm anomalies do not occur preferentially over land during this period and do not increase with height in the tropics.
Open Access
The assimilation and elimination of cesium by freshwater invertebrates
(Colorado State University. Libraries, 2000) Tostowaryk, Tracy M., author; Whicker, F. Ward, advisor; Rowan, David, advisor; Clements, W. H. (William H.), committee member; Hinton, Tom, committee member
Freshwater invertebrates are important vectors of radioactive cesium 134Cs and 137CS) in aquatic food webs, yet little is known about their cesium uptake and loss kinetics. This study provides a detailed investigation of cesium assimilation and elimination by freshwater invertebrates. Using five common freshwater invertebrates (Gammarus lacustris, Anisoptera sp. nymphs, Claassenia sabulosa and Megarcys signata nymphs, and Orconetes sp.), a variety of food types (oligochaete worms, mayfly nymphs and algae) and six temperature treatments (3.5 to 30°C), the following hypotheses were tested: 1) cesium elimination rates are a positive function of water temperature; 2) cesium elimination rates increase with decreasing body size; 3) assimilation efficiencies range between 0.6 and 0.8 for diet items low in clay. Cesium loss exhibited first order, non-linear kinetics, best described by a two component exponential model. Cesium assimilation efficiencies were higher for invertebrates fed oligochaetes (0.77) and algae (0.80) than those fed mayfly nymphs (0.20). Cesium elimination rate constants ranged from 0.002 to 0.125 d-1 across taxa and temperatures. Within each taxon, linear regressions of the natural logarithm of cesium elimination rate constants on temperature yielded positive, significant relationships. As temperature coefficients were not significantly different across taxa, the data were combined into a general model of cesium elimination by freshwater invertebrates as a function of temperature, body size and a categorical variable for thermal optima (warmwater and cool-water adapted taxa). Cesium elimination rate constants were found to increase with temperature, decrease with body size, and be much lower for warmwater adapted invertebrates than cool-water adapted invertebrates. Both the cesium assimilation efficiencies and general model of cesium elimination rate constants for freshwater invertebrates are in excellent agreement with those for fish. Quantification of cesium assimilation efficiencies and elimination rate constants for freshwater invertebrates allows, for the first time, development of dynamic aquatic food web models for .risk assessments, and it enables the in situ quantification of invertebrate feeding rates and other bioenergetic parameters.
Open Access
Effects of irrigated and dryland cultivation on soil carbon, nitrogen and phosphorus in northeastern Colorado
(Colorado State University. Libraries, 2001) Sinton, Penelope J., author; Burke, Ingrid C., advisor; Kelly, Eugene F., committee member; Peterson, Gary A., 1940-, committee member; Lauenroth, William K., committee member
I investigated the effects of irrigated and fertilized com agriculture on soil C, N and P in northeastern Colorado as they compare to dryland wheat-fallow fields and native rangelands in the semiarid shortgrass steppe of northeastern Colorado. Three replicates each of native rangeland, dry land wheat-fallow, and irrigated corn fields located in or adjacent to the Pawnee National Grasslands were selected for this study. I measured potentially mineralizable C and N from 0-15cm in the soil profile, particulate organic matter (POM) C and Nin the upper 30cm, total and NaHC03-P to a depth of 105cm, and total soil C and N to a depth of 195cm in the soil profile. Irrigated corn fields contained significantly lower mineralizable, POM, and total C and N than rangelands in the upper 5cm of soil. Com fields also had significantly greater NaHCOrP content than rangelands or wheat-fallow fields to a 1-meter depth in the soil. Wheat-fallow fields had significantly less potentially mineralizable and POM C and N than rangelands or corn fields in the upper 5cm of soil. Cumulative losses of total C and N in wheat-fallow fields extended to depths of 75cm or more. There were no significant differences in total P among land use types. Differences in C and N between corn and wheat-fallow fields are likely due to differences in the quantity of plant residue inputs. The distribution of C, N and NaHC03-P through the soil profile in corn fields also differed from rangelands. Soil C, N and NaHC03-P in the soil profile of rangelands decreased from the surf ace down, whereas in com fields C, N and NaHC03-P increased from the surf ace to 30cm and then decreased. Distribution of C, N and P in corn fields may be due to leaching of C or N or decomposition changes in the soil profile. In wheat-fallow fields, C, N and NaHC03-P showed a more uniform distribution in the upper 30cm of soil than rangelands, likely due to tillage practices that mix the upper soil layers in wheat-fallow fields. These results indicate that irrigated and fertilized corn crops in this region of the semiarid shortgrass steppe depletes pools of C and N at the soil surf ace but does not cause a change in C or N below the 5cm layer of soil. The differences in amount and distribution of C and N observed in this study among dryland wheat-fallow and irrigated corn fields indicate that the type of crop grown in this region should be an important consideration for regional studies that evaluate C and N changes due to cultivation.
Open Access
Biogeochemical response of U.S. Great Plains grasslands to regional and interannual variability in precipitation
(Colorado State University. Libraries, 2002) McCulley, Rebecca Lynne, author; Burke, Ingrid C., advisor; Lauenroth, William K., committee member; Kelly, Eugene F., committee member
Current climate change scenarios predict increasing variability in both the amount and timing of rainfall for the Great Plains region of North America. In this region, aboveground production is tightly linked to both long-term average and interannual precipitation patterns, suggesting that future changes in climate may have significant consequences for grassland ecosystem function. However, aboveground production accounts for only ~50% of the carbon input into these ecosystems, and little is known about the belowground production response or biogeochemical consequences of interannual variability in precipitation. Biogeochemical processes, such as nitrogen mineralization, determine the amount of resources available for plant growth and have shown sensitivity to alterations in water availability. Thus, interannual variability in precipitation is likely to have direct and indirect effects on plant production by influencing water availability and by altering biogeochemical processes. In this dissertation, I address the influence of regional, seasonal, and interannual variability in precipitation on nitrogen (N) and carbon (C) cycling and microbial biomass and community composition in grassland ecosystems of the Great Plains. At 5 sites spanning a 500 mm mean annual precipitation gradient and encompassing, from west to east, shortgrass steppe, mixed grass prairie, and tallgrass prairie plant community types, I measured monthly in situ net N mineralization and soil respiration rates and annual above- and belowground net primary production and litter decomposition rates during the 1999-2001 time period. To quantify variability in the microbial biomass and community composition I analyzed the phospholipid fatty acid content of soil samples taken in October 2000 and June 2001 from these 5 sites. Carbon cycling rates and microbial biomass increased from semi-arid shortgrass steppe to sub-humid tallgrass prairie. At each site, C cycling rates were responsive to interannual variability in precipitation and this responsiveness varied across grassland community types. There were no significant regional, seasonal, or interannual trends in N cycling rates. Microbial biomass was larger during the growing season than in the fall, and microbial community composition was different for each of the 3 grassland types but was not significantly different across landscapes (uplands or lowlands) or between seasons at any of the sites.
Open Access
The influence of diet, habitat, and recreational shooting of prairie dogs on burrowing owl demography
(Colorado State University. Libraries, 2002) Woodard, Jason Daniel, author; Van Horne, Beatrice, advisor; Wiens, John A., committee member; Kennedy, Patricia (Patricia L.), committee member
The burrowing owl is a ground-nesting raptor that is in decline across much of its geographic range. Habitat loss and widespread control of fossorial rodents on which burrowing owls regionally rely for nest sites are the primary contributors to owl decline. In addition to reducing breeding and foraging opportunities, habitat loss and eradication programs may result in suboptimal habitat conditions at remaining sites. Identifying the habitat characteristics preferred by burrowing owls and the prey important to successful reproduction are top priorities. Human activity may exacerbate declines in local breeding populations. Isolating and mitigating sources of disturbance to nesting owls is a critical step in achieving conservation goals. Black-tailed prairie dog colonies are frequently occupied by burrowing owls where ranges overlap. In northeastern Colorado, prairie dog colonies provide the majority of suitable nesting habitat. I studied a population of burrowing owls nesting on the Pawnee National Grassland, Weld County, Colorado, to determine how prey use and nest placement affect demography. Specifically, my objectives were to (1) describe local prey use, compare the owl's diet across its geographic range, and relate diet measures to reproduction; (2) identify the habitat features that drive nesting patterns, and evaluate the reproductive consequences of nest placement; and (3) identify the factors influencing recreational shooting of black- tailed prairie dogs, and relate burrowing owl breeding numbers and reproductive output to the presence and intensity of shooting activity. Owls foraged opportunistically for invertebrate prey, using readily available sources to supplement intake of more profitable vertebrate species. Vertebrate use was low and decreased throughout the breeding cycle. Consumption rates may have met minimum dietary levels necessary to initiate nesting, but may not have been sufficient to benefit breeding owls through increased biomass gains. Invertebrates comprised the majority of the diet, a theme common to most diet studies. Their ready abundance and availability provided owls with an attractive alternate food source, and may have buffered owls against the reproductive consequences of short-term food shortage. Nearest-neighbor distance and satellite burrow density were poor predictors of nest placement. Habitat-based models explained little variation in reproductive performance at the nest level. Annual variation was significant. Nesting pairs in 2000 had a higher probability of success and fledged more young per breeding attempt than did their 1999 counterparts. Increased rates of starvation and flooding may have contributed to lower reproduction in 1999. Prairie dog colony size moderately influenced breeding densities and colony reproductive performance. Large colonies supported lower breeding densities, were less successful, and fledged fewer young per breeding attempt than did small colonies. Small colonies may constitute superior habitat if breeding densities are indicative of site quality. High rates of landscape fragmentation and human disturbance may have reduced reproductive performance on large, public sites. Burrow availability did not limit breeding densities, but it may have provided a source of refuge and prey to nesting pairs. No effect of prairie dog activity on measured demographic parameters was evident. Land ownership primarily governed recreational shooting patterns. Shooters did not demonstrate strong preferences for specific colony characteristics, using the majority of available public sites. Colonies subject to recreational shooting activity supported more breeding owls, but nests had lower success rates and fledged fewer young than did nests on colonies not exposed to shooting activity. Reproduction did not exhibit a linear response to recreational shooting intensity suggesting other factors contributed to reproductive variation. Adult owl mortality from recreational shooting invariably resulted in nest failure. Although gunshot trauma was infrequent, the additive effects of breeder loss and reduced reproductive output remain a cause for concern. Increasing the number of small, expanding prairie dog colonies will provide prospective breeders with potentially productive sites. As burrowing owls occupy the majority of colonies on the Pawnee National Grassland, many sites are likely to support nesting pairs. Maintaining a sizable pool of breeders will help to minimize the effects of annual reproductive variability and episodic plague on population viability. Protecting important vertebrate and invertebrate prey sources and reducing shooting activity on colonies historically productive or preferred by burrowing owls are essential to effective management. Incorporating reproductive, survival, and recruitment data into monitoring efforts will provide managers with a clearer picture of overall breeding conditions.
Open Access
Soil mite biodiversity: its relationship to grass species and influence on decomposition in the Konza tallgrass prairie
(Colorado State University. Libraries, 2005) St. John, Mark George. author; Wall, Diana H., advisor; Hunt, H. William, committee member; Kondratieff, Boris C., committee member; Seastedt, Timothy R., committee member; Stohlgren, Thomas J., committee member
Human activities are responsible for unprecedented extinction rates and global change. Species are disappearing faster than we can record their existence and before we determine their role in ecosystems. In no other system on Earth are we more uncertain about the true diversity of organisms and their roles than in soils. I have examined soil mite (Acari) species at the Konza Prairie Biological Station (KPBS), Kansas, USA, an uncultivated tallgrass prairie, to determine what mechanisms are responsible for their diversity, how alien invasive grasses may impact them, and what role their diversity plays in decomposition. The hypotheses that soil mite species richness, abundance and taxonomic diversity is greater beneath grasses in dicultures (different species) compared to monocultures (same species), beneath grasses of higher resource quality (lower C:N) compared to lower resource quality, and beneath heterogeneous mixes of grasses (C3 andC4 grasses growing together) compared to homogeneous mixes (C3 or C4 grasses) were tested using natural occurrences of grass species as treatments. Increased grass diversity supported a more species and phylogenetically rich soil mite fauna. This relationship was significant at depth but not in the upper soil horizon. Soil mite richness increased nonlinearly with grass species richness suggesting that simple extrapolations of soil faunal diversity based on plant species inventories may underestimate the richness of associated11lsoil mite communities. The proportion of mite size classes in dicultures was considerably different than those for monocultures. These data suggest that interspecific root competition results in increased mite habitat, abundance and diversity. There was no difference in soil mite richness between grass combinations of differing resource quality, or resource heterogeneity. Soil mites sampled beneath six native and one alien-invasive species of grass were similarly abundant, species rich, diverse, and taxonomically distinct. There was no evidence that the community composition of soil mites was specific to grass species or that a significant number of mite species had affinities for different grass species. The soil mite community was weakly related to soil environmental conditions. Only oribatid mites were related to, marginally, the species of grass present. The alien invasive grass species did not support a successionally younger mite fauna and had no influence on mite community structure, possibly because it had not substantially altered the soil environment. Rates of cotton strip decomposition (percent cotton strip tensile strength loss per day, CTSL), and soil mite abundance and species richness were measured at high and low fire frequency sites of the KPBS. Likelihood-based and information theoretic approaches were used to examine strength of evidence in data for models of CTSL representing the Null, Rivet and Redundant hypotheses of biodiversity and ecosystem function (BEF). The Null model including temperature, moisture and saturating effects in the total abundance of predatory mites (Mesostigmata) had more support in the data than any other models. Models representing Rivet and Redundant patterns of BEF settled on parameter values distinct from the Null models but had less support in the data regardless of which mite group was being considered. A significant trend was observed in the models' residuals from low fire frequency sites trends not observed in high fire frequency sites. I speculate that annually burned sites more closely emulate the agricultural system the models were originally designed for than low fire frequency sites, accounting for differences in model performance. Biophysical properties on low fire frequency sites such as increased litter cover, different soil carbon constituents or a different microbial community may regulate decomposition in a manner not accounted for by only soil temperature and moisture driving variables.
Open Access
Soil carbon saturation: a new model of soil organic matter stabilization and turnover
(Colorado State University. Libraries, 2006) Stewart, Catherine E., author; Paustian, Keith H., advisor; Six, Johan, committee member; Conant, Richard T., committee member; Sutton, Sally, committee member
The soil C saturation concept suggests an ultimate capacity of the soil to store C, dictating the rate and duration that soil may be effective in mitigating increasing atmospheric C02. This places a physicochemical limit on soil that is associated with textural, mineralogical and structural soil properties. This concept has been articulated in terms of four theoretical pools capable of C saturation: non-protected, physically- (micro-aggregate), chemically- (silt + clay), and biochemically-protected pools. My dissertation represents a multifaceted approach to examine C saturation in both whole soil and measurable soil fractions representing the four conceptual C pools. I evaluate the soil C saturation concept theoretically by modeling these relationships using published whole soil data, primary field data and through laboratory experiments. Analyses using published long-term soil C data from agroecosystem experiments suggested that within a given site, there was little support for models including C saturation, but when all sites were combined; there was strong support for the C saturation model. In general, published data were too sparse to adequately test individual sites. To evaluate the concept of C saturation for the four C pools, I used a three-part density, chemical, and physical fractionation scheme combined with modeling, using new data collected from eight agroecosystems in the US and Canada. I found that the chemically- and biochemically-protected pools showed strong evidence for C saturation, while the non-protected and physically-protected pools were non-saturating. In a 2.5 year laboratory experiment, I tested C stabilization rates and limits at two C addition rate to soils differing in soil C content and physicochemical characteristics. I found C saturation dynamics were most evident in the chemically-, biochemically- and some micro-aggregate protected C pools. I found greater C accumulation in the non-protected pool of the high C soil, suggesting C saturation of other pools. I conclude that SOC sequestration in many soils may be influenced by C saturation dynamics, impacting both decomposition kinetics and C stabilization. Soil C sequestration may be overestimated in models that do not account for C saturation dynamics.
Open Access
Fens of Yellowstone National Park, USA: regional and local controls over plant species distribution
(Colorado State University. Libraries, 2007) Lemly, Joanna M., author
Regional and local scale gradients controlling plant species distribution in mountain fens were studied in Yellowstone National Park, Wyoming, USA. Data on vascular and nonvascular plant cover, groundwater and soil chemistry, landforms, microtopography, and regional gradients of elevation, precipitation, and bedrock geology were collected and analyzed for 476 relevés from 166 fens. The pH of groundwater supporting fens ranged from 2.89 to 7.98. Six major bedrock types influence the chemical content of groundwater: three volcanic types, a glacial till complex containing sedimentary deposits, and two rock types altered by geothermal activity. Twenty-eight plant communities were identified through cluster analysis and table methods. Vegetation data were related to environmental gradients using DCA, CCA, and CCA with variance partitioning. The main environmental gradients affecting vascular plant species were site landform and stand topography, which separated fens formed in basins from sloping fens. Bryophytes were more strongly correlated with the acidity/alkalinity gradient of groundwater pH. For all species, the regional variables elevation, annual precipitation, and groundwater chemistry accounted for 40.7% of the total variation explained, while local variables site landform, stand topography and microtopography, and soil characteristics accounted for 43.9%.
Open Access
Hydrologic regime and herbivory stabilize an alternative state in Yellowstone National Park
(Colorado State University. Libraries, 2007) Wolf, Evan C., author; Cooper, David J., author; Hobbs, N. Thompson, author; Ecological Society of America, publisher
A decline in the stature and abundance of willows during the 20th century occurred throughout the northern range of Yellowstone National Park, where riparian woody-plant communities are key components in multiple-trophic-level interactions. The potential causes of willow decline include climate change, increased elk browsing coincident with the loss of an apex predator, the gray wolf, and an absence of habitat engineering by beavers. The goal of this study was to determine the spatial and temporal patterns of willow establishment through the 20th century and to identify causal processes. Sampled willows established from 1917 to 1999 and contained far fewer young individuals than was predicted from a modeled stable willow population, indicating reduced establishment during recent decades. Two hydrologically distinct willow establishment environments were identified: fine grained beaver pond sediments and coarse-grained alluvium. Willows established on beaver pond sediment earlier in time, higher on floodplain surfaces, and farther from the current stream channel than did willows on alluvial sediment. Significant linear declines from the 1940s to the 1990s in alluvial willow establishment elevation and lateral distance from the stream channel resulted in a much reduced area of alluvial willow establishment. Willow establishment was not well correlated with climate-driven hydrologic variables, but the trends were consistent with the effects of stream channel incision initiated in ca. 1950, 20-30 years after beaver dam abandonment. Radiocarbon dates and floodplain stratigraphy indicate that stream incision of the present magnitude may be unprecedented in the past two millennia. We propose that hydrologic changes, stemming from competitive exclusion of beaver by elk over browsing, caused the landscape to transition from a historical beaver-pond and willow mosaic state to its current alternative stable state where active beaver dams and many willow stands are absent. Because of hydrologic changes in streams, a rapid return to the historical state may not occur by reduction of elk browsing alone. Management intervention to restore the historical hydrologic regime may be necessary to recover willows and beavers across the landscape.
Open Access
Invasive mosquitoes, larval competition, and potential implications for vector competence
(Colorado State University. Libraries, 2007) Bevins, Sarah, author; Moore, Janice, advisor
Aedes albopictus is an invasive mosquito species which established rapidly in the US. It is characterized as an aggressive biter, prolific breeder, and dominant larval resource competitor. Declines in resident mosquito species have followed its arrival, and species that persist often emerge from shared larval habitats as small, nutritionally deprived adults. Aedes albopictus is a competent disease vector in both its native range and in areas of recent establishment. It therefore has the potential to alter vector-borne disease transmission either directly, by creating a new pathogen association, or indirectly, via interactions with native mosquito vectors.
Open Access
Pathways of continuity and change: diversification, intensification and mobility in Maasailand, Kenya
(Colorado State University. Libraries, 2007) BurnSilver, Shauna B., author; Galvin, Kathleen A., advisor
In recent decades multiple drivers have been acting on Maasai pastoralists in the Greater Amboseli Ecosystem (GAE). The Kenyan government and international policymakers have made a systematic effort to modernize the Kenyan pastoral sector based on prescriptions of livestock intensification and a process of land tenure change that moves herders from communal land use to privatized land ownership. Additional drivers including population growth, competition for territory, greater engagement in the Kenyan economy, and frequent drought, are also pushing and pulling pastoralists to adjust their livelihood strategies. These drivers have created an atmosphere of unprecedented change in Maasailand, Kenya -- a situation with negative implications for pastoral well-being and resilience. How pastoral households cope with these challenges is the central question of this PhD study. Three key responses are identified and analyzed: economic diversification, livestock intensification and livestock mobility.
Open Access
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
Hybridization between spotted knapweed (Centaurea stoebe) and diffuse knapweed (Centaurea diffusa): patterns and implications for invasion
(Colorado State University. Libraries, 2008) Blair, Amy C., author; Hufbauer, Ruth, advisor
Hybridization is an evolutionary force that has the potential to alter invasion dynamics. Centaurea diffusa Lam. (diffuse knapweed) and C. stoebe L. (spotted knapweed) are two problematic invasive weeds in western North America. Anecdotal information suggested these two plants were hybridizing in the introduced range. The overall goal of my dissertation was to examine the patterns of hybridization in the introduced and native ranges in the field and at the molecular level, and determine if hybridization was altering the invasion of either species.
Open Access
Genetic studies of Northern Goshawks (Accipiter gentilis): genetic tagging and individual identification from feathers, and determining phylogeography, gene flow and population history for Goshawks in North America
(Colorado State University. Libraries, 2008) Bayard de Volo, Shelley, author; Antolin, Michael F., advisor; Reynolds, Richard T., committee member; Douglas, Marlis R., committee member; Burnham, Kenneth P., committee member
The Northern Goshawk (Accipiter gentilis) is a large, highly mobile, mostly nonmigratory and widespread forest raptor. It ranges across the Boreal forests from Alaska to Newfoundland, and south into forests of the Great Lakes, and montane regions of the Appalachian, Cascade, Sierra Nevada, Rocky Mountain, and Sierra Madre Occidental. There has been much interest in the population and taxonomic status of Northern Goshawks, especially for populations occurring west of the 100th meridian. The objectives of my dissertation were to (1) determine whether alternative methods to capture-recapture methods were feasible; and (2) evaluate the genetic relationships among goshawk populations across their continental U.S. range. The genetic "marking" studies (Chapters 1 and 2) provided feasible and cost-effective alternative methods for capture-recapture, and are logistically easier to implement. If used, these methods have the potential to standardize the collection of demographic data across the species' range. Assessment of the genetic status among goshawks (Chapter 3) indicated populations are subdivided at a regional scale, with some gene flow within and among regions.
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
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.