VTechWorks Repository :: Browsing by Author "Belden, Lisa K."

Browsing by Author "Belden, Lisa K."

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Abiotic Factors Underlying Stress Hormone Level Variation Among Larval Amphibians
Chambers, David L. (Virginia Tech, 2009-03-31)
Anthropogenic disturbances can alter the abiotic composition of freshwater systems. These compositional changes can act as physiological stressors towards system inhabitants. However, little is known about how these altered abiotic factors influence stress hormones (corticosterone) in larval amphibians. Throughout the following chapters, I examined the effects of several abiotic factors on baseline and stress-induced corticosterone levels in the larvae of four amphibian species: Jefferson salamander (Ambystoma jeffersonianum), spotted salamander (A. maculatum), wood frog (Rana sylvatica), and grey treefrog (Hyla versicolor). Chapter II examined corticosterone level differences throughout development in A. jeffersonianum and R. sylvatica larvae under field, mesocosm, and laboratory venues. Baseline corticosterone levels in R. sylvatica increased near metamorphic climax in all venues, but not in A. jeffersonianum. Rather, baseline corticosterone levels differed with respect to venue throughout development in A. jeffersonianum. Chapter III examined corticosterone level differences among free-living A. jeffersonianum populations and possible abiotic factors underlying these hormone differences. Corticosterone levels significantly differed across populations. Increased baseline corticosterone levels significantly correlated to low pH. There was also a trend for increased baseline corticosterone levels to be positively correlated with chloride levels and negatively correlated with conductivity. Chapter IV examined the effects of laboratory manipulated pH on corticosterone levels in A. jeffersonianum, A. maculatum, R. sylvatica, and H. versicolor. There was a significant correlation between increased baseline corticosterone levels to low pH in all four species. Prey consumption (in both Ambystoma species) and survival (in A. jeffersonianum, A. maculatum, and R. sylvatica) were also negatively correlated to low pH. Chapter V examined the effects of increased conductivity on corticosterone levels in A. jeffersonianum, R. sylvatica, and H. versicolor. Increased conductivity exposure significantly correlated to increased baseline corticosterone levels in A. jeffersonianum and R. sylvatica. Prey consumption in A. jeffersonianum was also negatively correlated to increased conductivity. My dissertation shows that abiotic factors, such as pH and conductivity, can influence corticosterone levels in larval amphibians. These results suggest that corticosterone levels in larval amphibians may be a suitable biomarker reflective of altered freshwater habitat quality. However, my results also suggest that one should use a high degree of caution when using corticosterone levels in larval amphibians as a means to infer the health status of a population.
Antibiotic perturbation of gut bacteria does not significantly alter host responses to ocular disease in a songbird species
Weitzman, Chava L.; Belden, Lisa K.; May, Meghan; Langager, Marissa M.; Dalloul, Rami A.; Hawley, Dana M. (PeerJ, 2022-06-10)
Bacterial communities in and on wild hosts are increasingly appreciated for their importance in host health. Through both direct and indirect interactions, bacteria lining vertebrate gut mucosa provide hosts protection against infectious pathogens, sometimes even in distal body regions through immune regulation. In house finches (Haemorhous mexicanus), the bacterial pathogen Mycoplasma gallisepticum (MG) causes conjunctivitis, with ocular inflammation mediated by pro- and antiinflammatory cytokines and infection triggering MG-specific antibodies. Here, we tested the role of gut bacteria in host responses to MG by using oral antibiotics to perturb bacteria in the gut of captive house finches prior to experimental inoculation with MG. We found no clear support for an impact of gut bacterial disruption on conjunctival pathology, MG load, or plasma antibody levels. However, there was a non-significant trend for birds with intact gut communities to have greater conjunctival pathology, suggesting a possible impact of gut bacteria on pro-inflammatory cytokine stimulation. Using 16S bacterial rRNA amplicon sequencing, we found dramatic differences in cloacal bacterial community composition between captive, wild-caught house finches in our experiment and free-living finches from the same population, with lower bacterial richness and core communities composed of fewer genera in captive finches. We hypothesize that captivity may have affected the strength of results in this experiment, necessitating further study with this consideration. The abundance of anthropogenic impacts on wildlife and their bacterial communities, alongside the emergence and spread of infectious diseases, highlights the importance of studies addressing the role of commensal bacteria in health and disease, and the consequences of gut bacterial shifts on wild hosts.
Assessing age, breeding stage, and mating activity as drivers of variation in the reproductive microbiome of female tree swallows
Hernandez, Jessica; Hucul, Catherine; Reasor, Emily; Smith, Taryn; McGlothlin, Joel W.; Haak, David C.; Belden, Lisa K.; Moore, Ignacio T. (Wiley, 2021-07)
Sexually transmitted microbes are hypothesized to influence the evolution of reproductive strategies. Though frequently discussed in this context, our understanding of the reproductive microbiome is quite nascent. Indeed, testing this hypothesis first re-quires establishing a baseline understanding of the temporal dynamics of the reproductive microbiome and of how individual variation in reproductive behavior and age influence the assembly and maintenance of the reproductive microbiome as a whole. Here, we ask how mating activity, breeding stage, and age influence the reproductive microbiome. We use observational and experimental approaches to explain variation in the cloacal microbiome of free- living, female tree swallows (Tachycineta bicolor). Using microsatellite- based parentage analyses, we determined the number of sires per brood (a proxy for female mating activity). We experimentally increased female sexual activity by administering exogenous 17ß-estradiol. Lastly, we used bacterial 16S rRNA amplicon sequencing to characterize the cloacal microbiome. Neither the number of sires per brood nor the increased sexual activity of females significantly influenced female cloacal microbiome richness or community structure. Female age, however, was positively correlated with cloacal microbiome richness and influenced overall community structure. A hypothesis to explain these patterns is that the effect of sexual activity and the number of mates on variation in the cloacal microbiome manifests over an individual's lifetime. Additionally, we found that cloacal microbiome alpha diversity (Shannon Index, Faith's phylogenetic distance) decreased and community structure shifted between breeding stages. This is one of few studies to document within-individual changes and age- related differences in the cloacal microbiome across successive breeding stages. More broadly, our results contribute to our understanding of the role that host life history and behavior play in shaping the cloacal microbiomes of wild birds.
Assessing Diversity, Culturability and Context-dependent Function of the Amphibian Skin Microbiome
Medina Lopez, Daniel Christofer (Virginia Tech, 2018-08-17)
Emergent infectious diseases are a major driver of the accelerated rates of biodiversity loss that are being documented around the world. Global losses of amphibians provide evidence of this, especially those associated with chytridiomycosis, a lethal skin disease caused by the fungus Batrachochytrium dendrobatidis (Bd). Amphibian skin can harbor diverse bacterial communities that, in some cases, can inhibit the growth of Bd. Thus, there is interest in using skin bacteria as probiotics to mitigate Bd infections in amphibians. However, experiments testing this conservation approach have yielded mixed results, suggesting a lack of understanding about the ecology of these microbial communities. My dissertation research aimed to assess basic ecological questions in microbial ecology and to contribute to the development of probiotics using amphibian skin bacteria. First, to assess whether environmental conditions influence the function of amphibian skin bacterial communities, I conducted a field survey across low and high elevation populations of an amphibian host to assess their skin bacterial communities and metabolite profiles. I found that similar bacterial communities produced different metabolites at different locations, implying a potential functional plasticity. Second, since culturing is critical for characterizing bacteria, I aimed to identify the culture media (low vs high nutrient concentration) that recovers the most representative fraction of the amphibian skin bacterial community. I found that media with low nutrient concentrations cultured a higher diversity and recovered a more representative fraction of the diversity occurring on amphibian skin. I also determined that sampling more individuals is critical to maximize culture collections. Third, I assessed the diversity of the amphibian skin fungal community in relation to Bd infection across eight amphibian species. I determined that amphibian species was the most important predictor of fungal diversity and community structure, and that Bd infection did not have a strong impact. My dissertation highlights the importance of environmental conditions in the function of amphibian skin bacteria, expands our knowledge of the understudied fungal component of the amphibian skin microbiome, and complements current efforts in amphibian conservation.
Behavioral Heterogeneity and Disease Dynamics in House Finches (Haemorhous mexicanus)
Moyers, Sahnzi C. (Virginia Tech, 2017-06-16)
Infectious disease is a ubiquitous aspect of life on earth; however, parasites and pathogens are not distributed equally among individual hosts. Due to its ability to shape the way that individuals interact with other potential hosts and the environment, behavior is one of the most salient ways through which host biology varies in the context of disease. Variation in animal behavior can impact both transmission and the extent of a host's pathogen acquisition, and thus can have important consequences for infectious disease dynamics. Additionally, in this world of rapid urbanization where landscapes and wildlife resources are being altered, it is important to understand the ways in which human activity impact wildlife behavior, and in turn, disease dynamics. Here, we used both observational and experimental studies in field and laboratory settings to investigate the relationships among host behavior and physiology, anthropogenic food sources, and disease transmission in a natural host-pathogen system. First, we examined the relationship between house finch (Haemorhous mexicanus) stress physiology, exploratory behaviors, and social behaviors in the wild. We provided evidence that more exploratory house finches interact with more individuals in the wild, and have higher baseline concentrations of circulating stress hormones. Next, we found evidence that the amount of time spent on bird feeders drives both the acquisition and transmission of the bacterial pathogen Mycoplasma gallisepticum (Mg), indicating that variation in host foraging behavior has important transmission consequences in this system. Lastly, we found that the density of bird feeders available to house finches predicts the extent of Mg transmission in captivity. Taken together, these results highlight the important role that behavioral heterogeneity can play in the acquisition and spread of pathogens, as well as the potential impacts of human behavior on wildlife disease dynamics. Future work should seek to identify specific physiological mechanisms driving Mg acquisition and transmission as they relate to variation in host behavior, and the ways in which bird feeders impact disease-relevant behaviors in the wild.
Biotic Interaction of Invasive, Early-Succession Trees and Their Effects on Community Diversity: a Multi-Scale Study Using the Exotic Invasive Ailanthus altissima and the Native Robinia pseudoacacia in the Mid-Appalachian Forest of Eastern United States
Bao, Zhe (Virginia Tech, 2015-04-28)
Invasive plants can displace native species, deteriorate native forest, and change plant communities and ecosystem functions. Native plant populations are fundamentally impacted by invasive species because of the interactions between invasive species and native plants. This study focuses on understanding the extent, mechanisms and consequences of interaction between a non-indigenous invader Ailanthus altissima and its functionally similar native species Robinia pseudoacacia in the Mid-Appalachian region, from an individual scale to a regional scale. These two subject species are common and coexist in early-successional eastern deciduous forest. The interactions between these two common species are important to community structure and canopy tree regeneration. To address the type and extent of interactions of these two species, a greenhouse experiment utilizing various species proportions, nutrient levels and seed sources was performed. In addition, a common-garden experiment with various species densities and proportions over three consecutive growing seasons was performed in a more natural condition than that of the greenhouse experiment. We found at the seedling stage, the dominant interaction was competition, and R. pseudoacacia was the winner both above- and belowground. The allelopathic compounds of A. altissima may have inhibited nodulation of R. pseudoacacia. Ailanthus altissima seedlings from its native region had slightly stronger competitive abilities compared with the seedlings from its invaded range. In the common garden experiment, R. pseudoacacia plants grew quicker than A. altissima, but A. altissima inhibited the growth of R. pseudoacacia by interspecific competition. The negative impact of A. altissima on R. pseudoacacia became larger as time progressed. To assess the community-level consequences of the two species, we conducted a forest mapping and a complete target-tree-based forest survey, and analyzed regional-scale data from the Forest Inventory Analysis Data Base. The two target species were significantly associated with themselves and with each other. Community species composition and diversity were significantly different across sites. A negative impact of both species on the understory community diversity and tree regeneration at the neighborhood scale was detected; while at a regional level, tree diversity in the FIA plots with either A. altissima or R. pseudoacacia was higher than the reference plots.
Classic Hoarding Cages Increase Gut Bacterial Abundance and Reduce the Individual Immune Response of Honey Bee (Apis mellifera) Workers
Gregory, Casey L.; Fell, Richard D.; Belden, Lisa K.; Walke, Jenifer B. (Oxford University Press, 2022-03-01)
Laboratory experiments have advanced our understanding of honey bee (Apis mellifera) responses to environmental factors, but removal from the hive environment may also impact physiology. To examine whether the laboratory environment alters the honey bee gut bacterial community and immune responses, we compared bacterial community structure (based on amplicon sequence variant relative abundance), total bacterial abundance, and immune enzyme (phenoloxidase and glucose oxidase) activity of cohort honey bee workers kept under laboratory and hive conditions. Workers housed in the laboratory showed differences in the relative abundance of their core gut taxa, an increase in total gut bacterial abundance, and reduced phenoloxidase activity, compared to bees housed in hives.
The Cloacal Microbiome Changes with the Breeding Season in a Wild Bird
Escallón, C.; Belden, Lisa K.; Moore, Ignacio T. (Oxford University Press, 2018-09-29)
The symbiotic microbial communities, or “microbiomes,” that reside on animals are dynamic, and can be affected by the behavior and physiology of the host. These communities provide many critical beneficial functions for their hosts, but they can also include potential pathogens. In birds, bacteria residing in the cloaca form a complex community, including both gut and sexually-transmitted bacteria. Transmission of cloacal bacteria among individuals is likely during the breeding season, when there is direct cloacal contact between individuals. In addition, the major energetic investment in reproduction can draw resources away from immune responses that might otherwise prevent the successful establishment of microbes. We assessed dynamic variation in the cloacal microbiome of free-living rufouscollared sparrows (Zonotrichia capensis) through sequential breeding and non-breeding seasons. We found that the cloacal bacterial communities differed between the sexes when they were in breeding condition. Further, in males, but not in females, the bacterial community became more diverse with the onset of reproduction, and then decreased in diversity as males transitioned to non-breeding condition. Individuals sampled across sequential breeding seasons did not accumulate more bacterial taxa over seasons, but bacterial community composition did change. Our results suggest that the cloacal microbiome in birds is dynamic and, especially in males, responsive to breeding condition.
Community ecology of aquatic insects in forested headwater streams in the southern Appalachians
Sokol, Eric R. (Virginia Tech, 2009-09-04)
Competing paradigms of community assembly emphasize different mechanisms for predicting patterns in biogeography. Niche assembly emphasizes the role of environmental gradients as filters that organize a metacommunity by locally selecting colonizers with similar functional traits, whereas dispersal assembly emphasizes the importance of source pool characteristics and dispersal limitation in organizing a metacommunity. In this study, I developed a framework that uses spatially explicit patterns in taxonomic and functional measures of community composition as diagnostics for community assembly processes for benthic macroinvertebrates in headwater streams in the southern Appalachians. Distance decay in taxonomic and functional similarity was used to determine the scales at which taxonomic turnover occurred within functional niches. Trait-neutral models of community composition were used as null models to assess which functional traits were the best candidates to explain how community composition was influenced by environmental gradients: an assessment of niche-based community assembly. Regional scale patterns suggested that niche-based community assembly was the dominant mechanism organizing community composition in headwater streams at local scales (<30km). Therefore, I compared how well trait-neutral models identified functional traits as relevant to community sorting against how well observed trait distributions correlated with environmental variation at a local scale, in the Ray Branch catchment (<10km study extent). Functional traits exhibiting non-random distributions within the Ray Branch watershed were most strongly correlated with environmental variation. Lastly, I assessed how the influences of niche and dispersal assembly on benthic macroinvertebrate community composition were affected by disturbance (shelterwood logging). Environmental variables defining the habitat template, and macroinvertebrate community composition, were measured before and after the disturbance; and path analysis was used to quantify the disturbance effect. The relationship between environmental variation and functional composition increased following logging, indicating disturbance augmented the influence of environmental filters, and consequently, the importance of niche-based community assembly. My dissertation provides the framework for a novel assessment of taxonomic and functional community composition data to infer the types of ecological dynamics that organize communities in the landscape. Additionally, this work provides a theoretical basis for assessing how dominant ecological processes change, in predictable ways, in response to changes in the habitat template.
Community Structure and Function of Amphibian Skin Microbes: An Experiment with Bullfrogs Exposed to a Chytrid Fungus
Walke, Jenifer B.; Becker, Matthew H.; Loftus, Stephen C.; House, Leanna L.; Teotonio, Thais L.; Minbiole, Kevin P. C.; Belden, Lisa K. (PLOS, 2015-10-07)
The vertebrate microbiome contributes to disease resistance, but few experiments have examined the link between microbiome community structure and disease resistance functions. Chytridiomycosis, a major cause of amphibian population declines, is a skin disease caused by the fungus, Batrachochytrium dendrobatidis (Bd). In a factorial experiment, bullfrog skin microbiota was reduced with antibiotics, augmented with an anti-Bd bacterial isolate (Janthinobacterium lividum), or unmanipulated, and individuals were then either exposed or not exposed to Bd. We found that the microbial community structure of individual frogs prior to Bd exposure influenced Bd infection intensity one week following exposure, which, in turn, was negatively correlated with proportional growth during the experiment. Microbial community structure and function differed among unmanipulated, antibiotic-treated, and augmented frogs only when frogs were exposed to Bd. Bd is a selective force on microbial community structure and function, and beneficial states of microbial community structure may serve to limit the impacts of infection.
Comparative genomics of bacteria from amphibian skin associated with inhibition of an amphibian fungal pathogen Batrachochytrium dendrobatidis
Wax, Noah David (Virginia Tech, 2021-06-22)
Chytridiomycosis is a fungal skin disease in amphibians that is primarily caused by Batrachochytrium dendrobatidis (Bd). We analyzed whole genome sequences of 40 bacterial isolates that had been previously cultured from the skin of four amphibian species from Virginia, USA, and tested for their ability to inhibit Bd growth via an in vitro challenge assay. These 40 isolates spanned 11 families and 13 genera. The aim of this study was to identify genomic differences among the amphibian skin bacterial isolates and generate hypotheses about possible differences that could contribute to variation in their ability to inhibit the growth of Bd. We identified sixty-five gene families that were present in all 40 isolates. We also looked for the presence of biosynthetic gene clusters. While this set of isolates contained a wide variety of biosynthetic gene clusters, the two most abundant clusters with potential anti-fungal activity were siderophores (N=17) and Type III polyketide synthases (N=20). We then analyzed the isolates belonging to the phylum Proteobacteria in more detail. We identified 197 gene families that were present in all 22 Proteobacteria. We examined various subsets of the Proteobacteria for genes for specific compounds with known activity against fungi, including chitinase and violacein. We identified a difference in the number, as well as amino acid sequences, of predicted chitinases found in two isolates belonging to the genus Agrobacterium that varied in their inhibition of Bd. After examining the annotated genomes, we identified a predicted chitinase in a Sphingomonas isolate that inhibited the growth of Bd that was absent from the five Sphingomonas isolates that did not inhibit Bd growth. The genes vioA, vioB, vioC, vioD and vioE are necessary to produce violacein, a compound which inhibits the growth of Bd. Differences in these genes were identified in three out of the four Janthinobacterium isolates. Of these three isolates, two showed strong inhibition of Bd growth, while the third inhibited Bd growth to a lesser extent. Using comparative genomics, we generated several testable hypotheses about differences among bacterial isolates that could contribute to variation in ability to inhibit Bd growth. Further work is necessary to test the various mechanisms utilized by amphibian skin bacterial isolates to inhibit Bd.
Comparative genomics of Lactobacillaceae from the gut of honey bees, Apis mellifera, from the Eastern United States
Bradford, Emma L.; Wax, Noah; Bueren, Emma K.; Walke, Jenifer B.; Fell, Richard; Belden, Lisa K.; Haak, David C. (Oxford University Press, 2022-12-01)
Lactobacillaceae are an important family of lactic acid bacteria that play key roles in the gut microbiome of many animal species. In the honey bee (Apis mellifera) gut microbiome, many species of Lactobacillaceae are found, and there is functionally important strain-level variation in the bacteria. In this study, we completed whole-genome sequencing of 3 unique Lactobacillaceae isolates collected from hives in Virginia, USA. Using 107 genomes of known bee-associated Lactobacillaceae and Limosilactobacillus reuteri as an outgroup, the phylogenetics of the 3 isolates was assessed, and these isolates were identified as novel strains of Apilactobacillus kunkeei, Lactobacillus kullabergensis, and Bombilactobacillus mellis. Genome rearrangements, conserved orthologous genes (COG) categories and potential prophage regions were identified across the 3 novel strains. The new A. kunkeei strain was enriched in genes related to replication, recombination and repair, the L. kullabergensis strain was enriched for carbohydrate transport, and the B. mellis strain was enriched in transcription or transcriptional regulation and in some genes with unknown functions. Prophage regions were identified in the A. kunkeei and L. kullabergensis isolates. These new bee-associated strains add to our growing knowledge of the honey bee gut microbiome, and to Lactobacillaceae genomics more broadly.
Culture Media and Individual Hosts Affect the Recovery of Culturable Bacterial Diversity from Amphibian Skin
Medina, Daniel; Walke, Jenifer B.; Gajewski, Zachary J.; Becker, Matthew H.; Swartwout, Meredith C.; Belden, Lisa K. (Frontiers, 2017-08-24)
One current challenge in microbial ecology is elucidating the functional roles of the large diversity of free-living and host-associated bacteria identified by culture-independent molecular methods. Importantly, the characterization of this immense bacterial diversity will likely require merging data from culture-independent approaches with work on bacterial isolates in culture. Amphibian skin bacterial communities have become a recent focus of work in host-associated microbial systems due to the potential role of these skin bacteria in host defense against the pathogenic fungus Batrachochytrium dendrobatidis (Bd), which is associated with global amphibian population declines and extinctions. As there is evidence that some skin bacteria may inhibit growth of Bd and prevent infection in some cases, there is interest in using these bacteria as probiotic therapy for conservation of at-risk amphibians. In this study, we used skin swabs from American toads Onaxyrus americanus) to: (1) assess the diversity and community structure of culturable amphibian skin bacteria grown on high and low nutrient culture media. (2) determine which culture media recover the highest proportion of the total skin bacterial community of individual toads relative to culture-independent data, and (3) assess whether the plated communities from the distinct media types vary in their ability to inhibit Bd growth in in-vitro assays. Overall, we found that culture media with low nutrient concentrations facilitated the growth of more diverse bacterial taxa and grew distinct communities relative to media with higher nutrient concentrations. Use of low nutrient media also resulted in culturing proportionally more of the bacterial diversity on individual toads relative to the overall community defined using culture-independent methods. However, while there were differences in diversity among media types, the variation among individual hosts was greater than variation among media types, suggesting that swabbing more individuals in a population is the best way to maximize culture collections, regardless of media type. Lastly, the function of the plated communities against Bd did not vary across culture media type or between high and low nutrient media. These results inform current efforts for developing a probiotic-based approach for amphibian conservation and help to ensure that culture collections are capturing the majority of the important diversity in these systems.
Dimension Reduction for Multinomial Models Via a Kolmogorov-Smirnov Measure (KSM)
Loftus, Stephen C.; House, Leanna L.; Hughey, Myra C.; Walke, Jenifer B.; Becker, Matthew H.; Belden, Lisa K. (Virginia Tech, 2015)
Due to advances in technology and data collection techniques, the number of measurements often exceeds the number of samples in ecological datasets. As such, standard models that attempt to assess the relationship between variables and a response are inapplicable and require a reduction in the number of dimensions to be estimable. Several filtering methods exist to accomplish this, including Indicator Species Analyses and Sure Information Screening, but these techniques often have questionable asymptotic properties or are not readily applicable to data with multinomial responses. As such, we propose and validate a new metric called the Kolmogorov-Smirnov Measure (KSM) to be used for filtering variables. In the paper, we develop the KSM, investigate its asymptotic properties, and compare it to group equalized Indicator Species Values through simulation studies and application to a well-known biological dataset.
Disentangling the influence of dispersal on community assembly and stability
Cathey, Sara Elizabeth (Virginia Tech, 2023-01-31)
With the introduction of metacommunity theory, the field of community ecology expanded its scope to include patterns and processes beyond the scale of local communities. Dispersal, or the movement of organisms between sites, can play an influential role in generating patterns of community assembly and stability. However, little is known about the role of dispersal in structuring and stabilizing freshwater communities. For my dissertation, I conducted a literature review of dispersal in stream metapopulations and metacommunities. Our current knowledge of the movement of freshwater taxa is limited due to difficulties in accurately monitoring dispersal. We have inferred the role of dispersal based primarily on organismal-based and graph-based proxies, although the body of work in modeling and experimental research is growing. Future research should incorporate innovative methods to directly monitor dispersal at finer spatial and temporal scales. To address this knowledge gap, we experimentally manipulated dispersal mode (aerial and drift) alongside the magnitude of dispersal (network location as a proxy) to investigate the role of these components of dispersal in community assembly and multiple metrics of stability. The results of my experiment suggest both factors may play a role in community assembly and stability patterns in stream metacommunities. Lastly, I conducted a mesocosm experiment with zooplankton mesocosms to investigate if biodiversity can generate asynchronous patterns of community dynamics that contribute to stability. There was a positive biodiversity-asynchrony relationship that, in turn, generated higher levels of stability. This effect was strongest in communities connected via dispersal. Overall, my dissertation demonstrates that dispersal plays a role in the assembly and stability of freshwater communities.
Effects of Atrazine and Metolachlor on Snails, Tadpoles, and Their Trematode Parasites
Griggs, Jennifer Lynn (Virginia Tech, 2006-11-30)
The widespread use and subsequent release of pesticides into aquatic environments have sparked concerns about how organisms within these aquatic systems are affected by pesticide pollution. While many studies have examined the effects of pesticides on individual organisms, in a series of experiments, I investigated the effects of a pesticide mixture on members of a complex host-parasite system and on host susceptibility to infection. In my first experiment in the laboratory, I examined changes in survivorship when trematode parasites (Echinostoma trivolvis) and their first intermediate host, Planorbella trivolvis snails, were exposed to a low concentration (10 ppb: 15 ppb) and high concentration (85 ppb: 100 ppb) mixture of atrazine and metolachlor, respectively. There was a significant decline in parasite survivorship in the high concentration treatment at 14 hours, while snail survivorship was unaffected across all treatments. In my second experiment, prior to infection, I exposed the parasites and/or second intermediate hosts, Rana clamitans and Rana sylvatica tadpoles, to the pesticide mixtures and examined subsequent infection levels in the tadpoles. The atrazine and metolachlor mixtures had no significant effects on parasite load in the laboratory. Newly shed parasites were more likely than 10 hours old parasites to infect tadpoles, regardless of pesticide exposure. In my final experiment, I utilized outdoor mesocosms to expose parasites, snail hosts, and Rana sylvatica tadpoles to the pesticide mixture, and I examined differences in parasite load within the tadpoles after two weeks. The pesticides had no significant effect on parasite loads in the field. Overall, my findings suggest the atrazine and metolachlor mixtures used in this study had no significant effects on disease dynamics in a system involving Echinostome parasites, snails, and tadpoles.
Effects of Bird Feeder Density on the Behavior and Ecology of a Feeder-Dependent Songbird: Patterns and Implications for Disease Transmission
Aberle, Matthew A. (Virginia Tech, 2018-09-18)
Anthropogenic resource provisioning of wildlife has increasingly been hypothesized to alter pathogen spread. Although bird feeding is the most widespread form of intentional wildlife provisioning, we know relatively little about how the degree of anthropogenic feeding at a site impacts wild birds in ways relevant to disease transmission. We manipulated the density of bird feeders (low versus high) available at otherwise similar sites and tracked the local abundance, body condition (scaled-mass index), feeding behavior, and movement across the landscape in wild house finches (Haemorhous mexicanus), a feeder-dependent species subject to outbreaks of a contagious pathogen commonly spread at feeders. The local abundance of house finches was significantly higher at sites with high feeder density but, surprisingly, finches at high-density feeder sites had poorer body condition than those at low-density sites. Behaviorally, birds at high-density feeder sites had longer average feeding bouts and spent more time per day on feeders than birds at low-density feeder sites. Further, birds first recorded at low-density feeder sites were more likely to move to a neighboring high-density feeder site than vice versa. Overall, because local abundance and time spent on feeders have been linked with the risk of disease outbreaks in this species, effects of bird feeder density on both traits may, in turn, influence disease dynamics in house finches. Our results suggest that heterogeneity in the density of bird feeders can have diverse effects on wild birds, with potential consequences for disease transmission.
The Effects of Hypoxia on Zooplankton Communities in Lakes and Reservoirs
Doubek, Jonathan Patrick (Virginia Tech, 2018-06-19)
Global change is altering the community composition, variability, and behavior of organisms in a diverse suite of ecosystems. Because of climate change and eutrophication, freshwater lakes and reservoirs are experiencing an increase in low dissolved oxygen concentrations (hypoxia) in their bottom waters (hypolimnion), which can disrupt ecological communities. Zooplankton, important aquatic organisms for regulating water quality and food webs, are one group of organisms affected by hypoxia since zooplankton need oxygen to respire. My research shows that hypoxia may disrupt zooplankton behavior and increase the variability of zooplankton communities. Zooplankton ubiquitously exhibit diel vertical migration, where the majority of the population resides in the hypolimnion during the daytime to escape predation from fish and damage from ultraviolet radiation. At night, many zooplankton ascend to the surface waters to feed on phytoplankton, when there is decreased risk of predation and radiation. My results from intensive 24-hour sampling campaigns suggest that hypolimnetic hypoxia may alter zooplankton migration, biomass, and behavior, which may in turn exacerbate water quality degradation due to the critical role zooplankton play in freshwater ecosystems. In addition, field surveys in four reservoirs over three years revealed that hypoxia may increase the variability of zooplankton communities compared to oxic conditions. Consequently, as lakes and reservoirs experience increased extent and duration of hypoxia in the future, it is critical to understand how more variable zooplankton communities alter freshwater ecosystem functioning.
Effects of Land Use on Hellbenders (Cryptobranchus alleganiensis) at Multiple Levels and Efficacy of Artificial Shelters as a Monitoring Tool
Jachowski, Catherine M. (Virginia Tech, 2016-07-01)
Understanding how species respond to anthropogenic changes and why species respond in the way that they do can help focus conservation planning. Hellbenders (Cryptobranchus alleganiensis) are a freshwater species of increasing conservation concern that are suspected to have declined due to loss of forest cover. However, quantitative evidence of land use effects on hellbenders is lacking. I used a multilevel approach to understanding whether hellbenders respond to land use by examining physiological indices, demographics and patterns of species occurrence as endpoints. My study area included two major river drainages in Virginia which mark a largely understudied portion of the species' range. In Chapter I, I described hellbender distribution and tested the hypothesis that hellbenders would be more likely to occur in heavily forested landscapes. Surprisingly, hellbenders occupied a relatively wide land use gradient (range = 50-90% forest in an upstream catchment) and current land use was an unreliable predictor of occurrence. In Chapter II I examined hellbender abundance and demographics at a subset of study sites stratified across a land use gradient. Abundance of sub-adult/adult hellbenders increased as forest cover increased in collective upstream riparian areas, primarily as the result of increased recruitment of new adults to local populations (rather than increased apparent survival of adults). Populations in lesser forested areas were declining and composed largely of relatively old adults, indicating that land use can lead to changes in hellbender distribution given sufficient time. In Chapter III, I examined three indices of physiological condition (body condition, hematocrit and leukocyte profiles) in hellbenders captured across a land use gradient. I found evidence suggesting low reproductive success may explain reduced recruitment in areas of low forest cover and evidence suggesting hellbender endocrinology during the breeding season may vary with land use. In Chapter IV I examine efficacy of artificial shelters as a monitoring tool and demonstrate their potential as tool to further our understanding of mechanisms underlying demographic responses of hellbenders to land use. I synthesize my findings in Chapter V and conclude that loss of forest cover in riparian areas poses a formidable threat to hellbender population viability in Virginia.
Environmental Controls Over the Distribution and Function of Antarctic Soil Microbial Communities
Geyer, Kevin M. (Virginia Tech, 2014-07-15)
Microbial community composition plays a vital role in soil biogeochemical cycling. Information that explains the biogeography of microorganisms is consequently necessary for predicting the timing and magnitude of important ecosystem services mediated by soil biota, such as decomposition and nutrient cycling. Theory developed to explain patterns in plant and animal distributions such as the prevalent relationship between ecosystem productivity and diversity may be successfully extended to microbial systems and accelerate an emerging ecological understanding of the "unseen majority." These considerations suggest a need to define the important mechanisms which affect microbial biogeography as well as the sensitivity of community structure/function to changing climatic or environmental conditions. To this end, my dissertation covers three data chapters in which I have 1) examined patterns in bacterial biogeography using gradients of environmental severity and productivity to identify changes in community diversity (e.g. taxonomic richness) and structure (e.g. similarity); 2) detected potential bacterial ecotypes associated with distinct soil habitats such as those of high alkalinity or electrical conductivity and; 3) measured environmental controls over the function (e.g. primary production, exoenzyme activity) of soil organisms in an environment of severe environmental limitations. Sampling was performed in the polar desert of Antarctica's McMurdo Dry Valleys, a model ecosystem which hosts microbially-dominated soil foodwebs and displays heterogeneously distributed soil properties across the landscape. Results for Chapter 2 indicate differential effects of resource availability and geochemical severity on bacterial communities, with a significant productivity-diversity relationship that plateaus near the highest observed concentrations of the limiting resource organic carbon (0.30mg C/g soil). Geochemical severity (e.g. pH, electrical conductivity) primarily affected bacterial community similarity and successfully explained the divergent structure of a subset of samples. 16S rRNA amplicon pyrosequencing further revealed in Chapter 3 the identity of specific phyla that preferentially exist within certain habitats (i.e. Acidobacteria in alkaline soils, Nitrospira in mesic soils) suggesting the presence of niche specialists and spatial heterogeneity of taxa-specific functions (i.e. nitrite oxidation). Additionally, environmental parameters had different explanatory power towards predicting bacterial richness at varying taxonomic scales, from 57% of phylum-level richness with pH to 91% of order- and genus-level richness with moisture. Finally, Chapter 4 details a simultaneous sampling of soil communities and their associated ecosystem functions (primary productivity, enzymatic decomposition) and indicates that the overall organic substrate diversity may be greater in mesic soils where bacterial diversity is also highest, thus a potentially unforeseen driver of community dynamics. I also quantified annual rates of soil production which range between 0.7 - 18.1g C/m2/yr from the more arid to productive soils, respectively. In conclusion, the extension of biogeographical theory for macroorganisms has proven successful and both environmental severity and resource availability have obvious (although different) effects on the diversity and composition of soil microbial communities.

Browsing by Author "Belden, Lisa K."

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