Browsing by Subject "Competition"

The mycorrhizal mutualism is one of the earliest and most influential of all terrestrial symbioses. As the primary method used by most plants to acquire nutrients from the soil, mycorrhizal fungi help to shape the structure and composition of many ecosystems. Ectomycorrhizal (EM) fungi play an especially significant role because most EM fungi prefer a limited number of host taxa, and EM plant species likewise associate with only a fraction of the available EM fungi. This host preference issue, combined with the high diversity of EM fungi in forest ecosystems, complicates interspecies competition both among fungi and among plants, because these plant and fungal communities interact.

Despite recent attempts at documenting mycorrhizal fungi in the context of ecological succession, many questions remain about the underlying causal relationships among EM fungi, soil conditions, and plant community assembly. The succession of mycorrhizal fungi often mirrors the succession of plants, and ectomycorrhizal (EM) community composition may affect the outcome of competition among trees during succession. In a pine-oak seral system, we tested the ability of Pinus taeda and Quercus alba seedlings to associate with EM fungi when planted under both conspecific and heterospecific adults. We found that EM communities under pine and oak canopy were distinct regardless of seedling identity, indicating that the fungal associations of adult trees determine which EM species are available in the soil. In addition, pine seedlings planted under oak canopy showed decreased mycorrhization and growth compared to those planted under pine canopy, while oak seedlings showed no negative effects of heterospecific planting. This impaired ability of pine seedlings to associate with the EM community established under oaks may deter pine recruitment and facilitate the late-seral replacement of pines with oaks.

While EM fungal communities correlate with the dominant species of host tree, soil properties do as well, making it difficult to establish causality among these three variables. Soil was collected from oak- and pine-dominated stands and dried to kill off mature mycelium, leaving only the spore bank as a source of inoculum for pine and oak seedlings. EM root tips were collected for molecular identification of fungal species based on ITS barcoding, and soil samples from field and laboratory conditions were analyzed for fungal diversity using 454 sequencing. We found a reduced influence of canopy type and a more pronounced influence of seedling identity when compared to the EM communities on seedlings planted in the field, suggesting that adult trees do alter the availability of fungi by directly promoting the growth of their preferred EM associates.

The availability of EM fungi can also affect seedlings at the interface between EM- an AM-dominated forest. We tested the hypothesis that seedlings of Dicymbe corymbosa which recruit outside of monodominant stands have limited access to EM symbionts compared with those which recruit inside D. corymbosa stands. EM root tips and rhizosphere soil were collected from seedlings along two transects inside monodominant stands and three transects in the transition zone into mixed forest dominated by AM associates. Seedlings inside monodominant stands yielded both a greater quantity of mycorrhized root tips and a higher diversity of EM species than transition zone seedlings. Of the fungal families commonly found on adult roots, the Boletaceae were notably underrepresented on all seedlings. In the transition zones, high-throughput sequencing of soil also detected a decrease in EM diversity with distance from the parent tree.

Seedlings of D. corymbosa may benefit from recruiting within monodominant stands by tapping into common mycorrhizal networks (CMNs) to acquire low-cost nitrogen and, potentially, photosynthates produced by conspecific adults. Leaves of stand adults, stand seedlings, and mixed-forest seedlings were collected for stable isotope analysis to track the transfer of nitrogen and carbon through CMNs. The δ13C and δ15N results contradicted each other, suggesting that more complicated interactions may be playing out among adults, seedlings, and fungi.

The aim of this dissertation is to examine, model and estimate firm responses to

demand shocks by focusing on specific industries where demand shocks are well

identified. Combining reduced-form evidence and structural analysis, this dissertation

extends the economic literature by focusing on within-firm responses of firms

to two important demand shocks that are identifiable in empirical settings. First,

I focus on how firms respond to a decrease in effective demand due to competition

shocks coming from globalization. By considering China's accession to the World

Trade Organization in 2001 and its impact on the apparel industry, the aim of these

chapters is to answer how firms react to the increase in Chinese import competition,

what is the mechanism behind these responses, and how important they are in explaining

the survival of the Peruvian apparel industry. Second, I study how suppliers'

survival probability relates to the sudden disruption of their main customer-supplier

relationships with downstream manufacturers, conditional on suppliers' own idiosyncratic

characteristics such as physical productivity.

State and local governments are seeking to expand preschool programs for low-income children and to enroll an increasingly ethnically and culturally diverse population. To ensure that expanded preschools equitably serve populations who are currently under-represented in current preschool programs, policymakers must understand the contexts that influence parents’ enrollment decisions. This set of three mixed-methods studies examines the influence of inter-racial and inter-ethnic group relations on the preschool enrollment decisions of black and Hispanic parents in a region experiencing a burgeoning Hispanic population.

Paper 1: Historic and contemporary studies provide evidence that racial and ethnic relations affect parents’ selection in to and out of schools in the K-12 education context. However, no study examines whether racial and ethnic group relations that influence school enrollment in the K-12 setting may also influence parents’ selection in to or out of a public preschool program. This study addresses this gap by examining how intergroup relations are related to the public preschool enrollment status of low-income black and Hispanic parents in a region experiencing Hispanic population growth. Intergroup relations are measured through group identity, linked fate, competition and conflict. Through surveys of black and Hispanic parents in poverty (n=369), this study finds that linked fate has a negative relationship with enrollment for Hispanic parents. In addition, there is a positive relationship between parents’ report of conflict and Head Start enrollment.

With states and local government seeking to expand their preschool systems, findings that intergroup relations influence parents’ preschool enrollment decisions

will be an important policy, program, and outreach consideration for preschool expansion. Moreover, with the growth and dispersal of Hispanic populations into new regions and communities, intergroup relations will be a particularly important consideration for preschool expansion in a growing demographic context.

Paper 2: As state and local governments expand their public preschool systems targeting the enrollment of children from low-income families, understanding why some low-income families do not apply for public preschool may provide insights about barriers to enrollment. This study provides cluster analyses of data on 369 low-income parents with preschool-age children; 202 parents had children enrolled in preschool, while 167 parents did not apply for public preschool. Cluster analyses and a series of robustness checks reveal two distinct typologies of parents who do not apply for public preschool. The first group of parents exhibit several characteristics associated with non-enrollment. The second group of parents are distinguishable only by their lack of transportation. This exploratory analysis of types of non-enrollers is informative for the local community from which these data were derived and provides a blueprint for further examination of typologies of parents who do not enroll.

Paper 3: Through in-depth interviews with 34 low-income, black and Hispanic mothers in a southern city experiencing a burgeoning Hispanic population, this study examines racial/ethnic relations as mothers engage in constrained labor and public preschool markets. This study reveals that while there is indeed a sense of competition for scarce labor market resources, these sentiments are more strongly expressed by black mothers than by Hispanic mothers. However, Hispanic and black mothers express similar levels of competition for public preschool. Interviews reveal family structure and policy determinants of intergroup tensions.

Invasive species are rapidly expanding in riparian wetlands while concurrently anthropogenic causes are increasing nitrogen (N) into these ecosystems. Microstegium vimineum (Microstegium) is a particularly abundant invasive grass in the Southeast United States. To evaluate impacts of Microstegium on both plant diversity and N cycling in a riparian floodplain, paired plots of Microstegium hand-weeded and unweeded were established for three years. Plots without Microstegium increased from 4 to 15 species m-2 and 90% of the newly establishing species were native. The Microstegium community accumulated approximately half the annual N in biomass of the diverse community, 5.04 versus 9.36 g-N m-2 year-1, respectively (p=0.05). Decomposition and release of N from Microstegium detritus was much less than in the diverse community, 1.19 versus 5.24 g-N m-2 year-1. Rates of soil N mineralization estimated by in-situ incubations were relatively similar in all plots. While Microstegium invasion appears to greatly diminish within-ecosystem circulation of N through the under-story plants, it might increase ecosystem N losses through enhanced denitrification (due to lower redox potentials under Microstegium plots). Microstegium removal ceased in the fourth growing season and formerly weeded plots increased to 59% (± 11% SE) Microstegium cover and species richness decreased to <8 species m-2.

To learn how Microstegium responds to increased N, we conducted a greenhouse competition experiment between Microstegium and four native plants across an N gradient. There was a unique competition outcome in each species combination, yet Microstegium was most dominant in the high levels of N.

Last, we disturbed a floodplain similar to wetland restoration disturbance and tracked available N. We also established a native community of plants with and without Microstegium in three levels of N. Disturbance to the floodplain dramatically increased inorganic N, especially in the form of NO₃ which was five times higher in the disturbed floodplain than the undisturbed floodplain. N levels remained elevated for over a year. Microstegium was N responsive, but did not show negative effects to the planted vegetation until the second year. Ironically, restoration activities are increasing available N, and favoring invasive species which in turn detracts from restoration success.

Over the last few decades, there has been an increasingly large body of research focused on the ecology and function of the gut bacteria, collectively known as the gut microbiome. This work has focused on the human gut microbiome as well as other animals including livestock and model organisms that can be genetically and experimentally manipulated. These organisms include laboratory rodents, fruit flies, worms, and pigs, to name a few. The background and future directions of this field are reviewed in Chapter 1 of this thesis. My work in the laboratory of Dr. John Rawls has focused on several aspects of the gut microbiome in different contexts, including how the gut microbiome is affected by nutritional challenges, host diseases, and lifestyle interventions, as well as the factors affecting microbiome composition, which might inform how we can develop strategies to manipulate the microbiome.In Chapter 2, I focus on the question of what genetic factors affect microbiome composition. To do so, I focus on a specific gut microbe known as Phocaeicola (Previously Bacteroides) vulgatus, or Pvu. Pvu is among the most abundant Bacteroidaceae species. Pvu also has myriad health associations in human studies, is an early life colonizer, and an efficient long-term colonizer in both humans and mice. However, the genes required for Pvu to establish itself in a complex microbiome are unknown. To address this gap in knowledge, I present experiments using transposon mutagenesis and insertion sequencing (INSeq) to understand Pvu colonization of the mammalian gut. This reverse genetics approach identifies several potential pathways that Pvu might use to colonize and persist in a complex microbiome. I further elucidate the functions of a hypothetical secreted protein, Pvu777, that is required for competition in vivo in a complex microbiome. In vivo competition experiments using genetically engineered Pvu strains recapitulate these findings in Pvu777 as well as the downstream putative fatty acid transporter Pvu776. Comparative genomics suggests that the operon containing Pvu777, which consists of the predicted DNA Binding/Histone-like protein Pvu778, Pvu777, and Pvu776 may be unique to Pvu and closely related gut Bacteroides and Phocaeicola species. RNA Seq approaches link Pvu777 to outer membrane and envelope functions. In conclusion, we identify a variety of pathways required for Pvu to colonize and persist in a complex microbiome using an INSeq screen, and elucidate the potential functions of one of the genes emerging from this screen using a range of experimental approaches. These findings could be used to inform further fitness-based studies of Pvu, but could also be used to inform methods to control its in vivo abundance, in addition to suggesting mechanisms that could be used to design efficiently colonizing engineered gut bacteria. In Chapter 3, I focus on the question of how nutritional challenges affect the gut microbiome using a zebrafish model of starvation. Starvation is a widespread nutritional challenge for which animals possess many physiological adaptations. However, current research into animal starvation has focused mainly on tissue histopathologies associated with starvation, excluding the physiological changes in the GI tract as well as the gut microbiome. In Chapter 3, we used RNA sequencing and 16S rRNA gene sequencing to uncover changes in the intestinal transcriptome and microbiome of zebrafish subjected to long-term starvation and refeeding compared to continuously fed controls. Starvation over 21 days led to increased diversity and altered composition in the intestinal microbiome compared to fed controls, including relative increases in Vibrio and reductions in Plesiomonas bacteria. Starvation also led to significant alterations in host gene expression in the intestine, with distinct pathways affected at early and late stages of starvation. This included increases in the expression of ribosome biogenesis genes early in starvation, followed by decreased expression of genes involved in antiviral immunity and lipid transport at later stages. These effects of starvation on the host transcriptome and microbiome were almost completely restored within 3 days after refeeding. Comparison with published datasets identified host genes responsive to starvation as well as high-fat feeding or microbiome colonization, and predicted host transcription factors that may be involved in starvation response. Overall, the results presented in Chapter 3 demonstrate that there may be distinct stages of starvation that lead to specific changes in gut microbial ecology and host GI tract transcriptome. These stages of starvation are largely reversible upon refeeding and the ensuing changes in host gene expression and microbiome composition may be an adaptive response to recover from starvation. This work could thus inform future research investigating the roles of specific bacterial taxa in host starvation, as well as mechanistic studies looking at the roles of specific host genes in starvation and refeeding using genetically modified hosts. In Chapter 4, I suggest studies that could extend from the work presented in Chapter 2, focusing on the role of individual genes in the Pvu 777 operon in vivo as well as within Pvu. I also suggest potential roles for the predicted DNA-Binding/Histone-like protein Pvu778 in the regulation of Pvu gene expression and Pvu fitness. I conclude by considering the evolutionary conservation of the 777 operon among Pvu and close relatives, and methods to investigate the fitness requirements of the 777 operon in these related bacterial species as well. Structural studies of Pvu777 are also proposed, which would help clarify the function and potential binding partners for the Pvu777 hypothetical protein. Thus, the studies proposed in Chapter 4 would help provide a clearer picture of the functions, regulation, and evolutionary history of the 777 operon, which would underscore its importance as a potentially conserved operon involved in Pvu fitness in vivo.

Environmental factors are a crucial determinant of an animals fitness. The effects of environment on fitness are often mediated by behavioral mechanisms as well as mechanisms that are ‘under the skin,’ such as growth and physiology. In my dissertation work, I study how two environmental factors – dominance rank and early-life conditions – are associated with growth and physiology. My colleagues and I test these links in a population of wild baboons studied by the Amboseli Baboon Research Project. The Amboseli Baboons Research Project has been collecting behavioral and demographic data on the Amboseli baboons for over 50 years, fecal hormone data for over 20 years, and blood samples collected via brief anaesthetizations for nearly 10 years. We complemented these remarkable datasets with cross-sectional data of female baboon body size.

In Chapter 1, we address two gaps in our understand of female dominance rank: (1) do higher-ranking females experience fewer stressors than lower-ranking females, and (2) how should we best quantify female dominance rank? Using fecal glucocorticoid concentrations as a proxy for the intensity and/or frequency of stressors that a baboon experiences, we find that, indeed, higher-ranking females do experience fewer stressors than lower-ranking females. Surprisingly, we also find that the best way to understand this effect is by categorizing females into two groups: alpha females, who are the highest-ranking female in the group, and everyone else.

In Chapter 2, we then focus on differences in the competitive landscapes assumed by two common measures of dominance rank, ordinal and proportional ranks. We complement theoretical work with re-analysis of 20 prior Amboseli baboon studies to show that for males, ordinal rank (i.e., number of individual ranking above the focal animal) was always a better predictor of traits than proportional rank, whereas for females, some traits were better predicted by ordinal rank, and some were better predicted by proportional rank (i.e., proportion of the group that a focal animal dominates). Our results suggest that males compete for density-dependent resources, whereas females compete for a mix of density-dependent and density-independent resources. In addition, our study demonstrates a new way to learn about the nature of within-group competition.

In Chapter 3, we present two new methods to use with body size data collected via parallel-laser photogrammetry. One of these methods was developed by colleagues here at Duke University, and the other method was developed by colleagues at George Washington University. These methods automate part of the hand-measurement process – measuring the distance between the lasers – and effectively saves time while increasing accuracy and precision of the final body size measurement. Our two methods have different strengths and weaknesses, and we anticipate that researchers will gravitate toward one or the other depending on their dataset, with the ultimate goal of increasing the use, ease, and accuracy of parallel-laser photogrammetry in studies of behavioral ecology.

In Chapters 4, we use the method developed in Chapter 3 to test whether early-life adversity stunts body size in female baboons. While this effect has been found in humans and some nonhuman animals, data on inter-individual differences in body size are extremely rare in wild primates. Using a dataset of over 2,000 images of 127 female baboons, we present the first cross-sectional growth curve of wild female baboons from juvenescence throughout adulthood. We then test whether females exposed to three main sources of early-life adversity - drought, maternal loss, or a cumulative measure of adversity – are smaller for their age in juvenescence or adulthood. We find that early-life drought predicts smaller limb length but not smaller torso length; our other measures of early-life adversity do not predict differences in body size. Our results suggest that baboons grow plastically in response to energetic early-life stress, but that this plasticity seems limited to limb growth, not torso growth.

Finally, in Chapter 5, we test a component of the biological embedding hypothesis, which predicts that early-life adversity is associated with elevated baseline inflammation as well as heightened acute inflammation in adulthood. To our knowledge, these predictions have only been tested in humans. Using serum samples collected from 89 baboons via brief anaesthetization, we measured several biomarkers of baseline and acute inflammation: c-reactive protein, soluble urokinase plasminogen activator receptor, interleukin 6, interleukin 1-beta, and tumor necrosis factor alpha. We test two measures of early-life adversity: maternal loss and a cumulative measure that incorporates 5 different potential sources of adversity. In contrast to the predictions of the biological embedding hypothesis, we find that baboons who experienced early-life adversity have a mix of comparable or lower levels of baseline and acute inflammation compared to baboons who experience no adversity. Prior tests of the biological embedding hypothesis were performed in humans who generally had access to more calories, less active lifestyles, and lower pathogen burden than wild baboons. Our results highlight the varied effects that early-life adversity can have on an organism’s development depending on the broader environment in which that organism lives.

Research in organizational ecology demonstrates that an organization’s competitive position within its market is highly associated with its survival chances, and that patterns of competitive constraint among organizations influence how markets evolve. However, the literature’s conceptualization of market structure is relatively coarse and static; it does not explore how individual organizations’ competitive positions shift or how market offerings change over relatively short intervals of time. In this study, I use social network analysis to study the structure of organizations’ competitive relationships directly. I examine both how changes in the structure of an organization’s competitive environment influence its survival chances, and how the structure of organizations’ competitive relationships affect the stability of market offerings. With a combination of a large crowd-sourced restaurant dataset from Yelp.com and census tract information from the American Community Survey (Census Bureau, 2009; Yelp, 2019), I apply methods from social network analysis, text analysis, and geographic information systems to track how restaurants’ competitive relationships change over time and space, and to study how these changes influence restaurants’ survival chances and overall market stability. This study provides evidence for new mechanisms of competitive constraint among organizations (niche centrality and niche compression) and new mechanisms of market stability (niche redundancy), offers a new theoretic framework for studying market structure and organizational evolution, and has critical implications for theory in the field of organizational ecology.

This dissertation is comprised of three papers related to nonprofit market structure and its consequences. I begin with an essay that examines how the recent boom of nonprofit organizations affects giving using the context of the Combined Federal Campaign (CFC). I find that the nonprofit boom has not increased donations to nonprofit organizations. Since a fixed amount of charitable resources is split among more organizations, the average nonprofit receives less funding as the number of organizations grows. The second paper proposes a new definition of nonprofit markets based on individual-level donor behavior and donor-nonprofit network ties. Notably, the new market definition predicts donor substitution among organizations 58% more accurately than the standard nonprofit market definition based on an organization’s subsector and geographic location. The CFC data and this donor-based market definition are also used to examine an important nonprofit policy issue—the relationship between market concentration and nonprofit spending on overhead. In the final essay, I study one of the processes by which competition in the CFC has increased over time—changes in the structure of government contracts. I examine whether fewer, but larger, contracts change performance. I find that contract consolidation does not significantly improve performance. Furthermore, I find no evidence that economies of scale exist in workplace giving.

This dissertation explores the effects of private equity (PE) investment on product quality among healthcare providers. In the first essay, I study the determinants of PE manager behavior, focusing on the role of product market competition. Using the nursing home setting as a backdrop, I consider the broader question of whether and how product market competition shapes the impact of PE acquisitions on consumers. By studying acquisitions of skilled nursing facilities by PE firms, I find that PE-owned providers exhibit greater competitive sensitivity—in that they compete more aggressively when competitive incentives are comparatively strong, and exploit market power more aggressively when competitive incentives are comparatively weak.

To investigate whether PE managers respond differently than non-PE managers to competition, I consider two sources of variation in competitive incentives facing nursing homes. First, I exploit the fact that nursing homes compete with one another in geographically segmented markets to contrast facilities according to the levels of local competition they face. I find significant heterogeneity in the effect of PE ownership according to levels of local market concentration. In highly competitive markets, PE owners increase staffing by $101,783 worth of care annually (enough to increase registered nurse (RN) hours by 20.8% of the mean), while actually reducing staffing in less competitive markets. Second, I show that PE-owned nursing homes respond more strongly to policies intended to spur competition. I study the introduction of the Five-Star Quality Rating System, a policy that increased the salience of staffing for consumers. Following its introduction, PE-owned facilities increased their staffing by an average of $39,118 worth of care more than their non-PE counterparts. Moreover, PE managers more aggressively shift their staffing composition towards RNs in response to the rating system's specific emphasis on RN staffing (RN expenditure increasing by 14.7% of the mean, with licensed practical nurse (LPN) expenditure decreasing by 4.9% of the mean): in total, the share of RN staffing increased by 1.9 percentage points (17.3% of the mean) more than non-PE facilities.

In the second essay, I assess how PE acquisitions influenced the readiness and outcomes of nursing facilities during the onset of the COVID-19 pandemic. With over 40% of U.S. COVID-19 deaths occurring in nursing homes, long-term care is a critical setting in which we must better understand the impact of PE ownership during the coronavirus pandemic. I find PE ownership to be associated with a mean decrease in the probability of confirmed COVID-19 cases among residents by 7.1 percentage points and confirmed staff cases by 5.4 percentage points. PE was also associated with a decreased probability of PPE shortages—including N95 masks, surgical masks, eyewear, gowns, gloves, and hand sanitizer. However, facilities previously (but not presently) owned by PE firms did not fare similarly well. I observe that prior PE ownership may result in increased PPE shortages and a potentially greater likelihood of resident outbreaks. This suggests that the contribution of PE ownership to improved COVID-19 outcomes is a result of active management during the pandemic, rather than the legacy of interventions undertaken beforehand.

While phenotypic plasticity has been widely documented, the relative contribution of parent versus offspring environment to determining progeny phenotypes and the persistence of parental effects throughout progeny development under different environments remain unknown. In predictable and/or seasonal environments, parental effects are predicted to be favored during early life stages of offspring, while offspring environment is predicted to increase in relative importance as the accuracy of offspring perception increases over the course of development. Furthermore, as plastic responses to the environment are both transmissible across generations and dynamic across development, epigenetic mechanisms are likely involved in the regulation of parental effects and phenotypic plasticity. The neighboring community is an important environmental factor for plants since sessile plants cannot escape competition within their current generation. Whether interactions with neighbors are positive or negative likely depends on the environment, as competition is hypothesized to increase in favorable environments and the neighboring community of plants may change over a seasonal progression. For these reasons, neighbor environment is an interesting and ecologically important environmental variable that can be used to investigate how parental and progeny environment regulate progeny phenotypes throughout development. To test these predictions, this dissertation evaluated how progeny phenotypes responded to the combination of parental and offspring environments, quantified how parental and progeny methylation regulate offspring phenotypes, and examined their effects on plasticity. To address which generation’s environment and DNA methylation affect phenotypes in offspring, I manipulated simulated and real neighbor environments and DNA methylation within and across generations in Arabidopsis thaliana, a winter annual native to Eurasia widely introduced across North America. In Chapter 1, using chemical demethylation, I tested whether parental and progeny DNA methylation influences germination, and whether parental DNA regulates germination response to past and present simulated canopy. I found that germination of offspring is regulated by parental DNA methylation and is responsive to parent, not seed, environment for most genotypes. Furthermore, I confirmed using mutant lines that all contexts of DNA methylation were involved in the transmission of parental effects, but they may operate through different pathways controlling germination. In Chapter 2, I quantified how parental versus progeny methylation regulate progeny phenotypic responses to parental and progeny canopy shade. I found that both parent and offspring canopy affect offspring traits across development, but parental environment had stronger effects at the seedling stage. Both parent and offspring DNA methylation affected offspring response to canopy, but parental DNA methylation only affected traits at the seedling stage. Trait correlations were significantly altered by chemical demethylation of parents and offspring, indicating that DNA methylation of both generations are important in regulating development and integrating phenotypic response to canopy. Finally, in Chapter 3, I tested whether parent or offspring DNA methylation affected response to the heterospecific neighbor, Stellaria media, under simulated seasonal conditions. I found that growing with competitors decreased fitness for all genotypes, but genotypes varied in the effect of neighbors on morphology and fitness. Both parent and offspring DNA methylation had direct effects on growth and fitness in all genotypes, but genotypes varied in how DNA methylation influenced response to neighbors. In one genotype, plastic response to neighbors was unaffected by chemical demethylation treatments, indicating that neighbor-induced plasticity is not always mediated via DNA methylation. Together, these results indicated that offspring phenotypes are shaped by both parent and offspring environment, and that parental environment and parental DNA methylation are especially important in regulating offspring traits early in life. The genetic variation observed in the expression of phenotypic plasticity via parental and progeny DNA methylation suggests that the epigenetic regulation of progeny phenotypes has a genetic basis and may evolve.

Theoretical and empirical studies have consistently shown that the optimal timing of seed germination reduces exposure to physical stress and minimizes competitive interactions with neighbors. However, this research has not accounted for facilitative (positive) interactions among plants, which become more pronounced as environmental stress increases. Facilitation is more likely to occur early in a plant's life when it is more susceptible to stress. In seasonal environments, the stress a given individual experiences can change throughout the year, and some years are more stressful than others. These sources of temporal variation in stress will dictate the facilitation-competition balance that individuals experience. However, it remains unclear how this balance affects the optimal timing of germination. My dissertation research asks how the timing of germination responds to neighbors, how those responses affect the facilitation-competition balance individuals experience, and how that balance in turn affects fitness and demography. More generally, it asks how the timing of germination and other types of emergence affect the facilitation and competition that individuals experience throughout their lives.

I used laboratory, greenhouse, and field experiments to examine how the timing of germination in the winter annual Arabidopsis thaliana (Brassicaceae) responds to cues of neighbors and how those responses affect interactions with neighbors. I then developed a mathematical model of population growth in an annual plant to examine how intraspecific facilitation and competition over ontogeny affect the optimal degree of investment in dormancy (i.e., delayed germination) in variable environments.

My experiments revealed that seeds of A. thaliana typically delay germination in response to neighbors and that these responses can promote facilitative interactions and reduce competitive ones with neighbors. Selection against delayed germination, which occurs because of stress later in the season, can be mitigated by facilitation. Further, delaying germination can be beneficial by increasing the difference in sizes between seedlings and their neighbors, which may promote resource partitioning. In the theoretical study, I found that increasing the degree of investment in the fraction of dormant seeds (i.e., delaying germination) can promote the persistence of populations that experience both facilitation and competition in variable environments. This occurs because increased dormancy prevents high juvenile densities that promote facilitation and consequently limit reproduction in large populations. The findings of this research indicate that plant-plant interactions depend strongly on temporal context, and they reveal that the facilitation-competition balance determined by temporal variation in stress plays a key role in how germination and dormancy traits will evolve in variable environments.

Although we often study mutualisms (interactions in which both species benefit) at the level of the individual partners, mutualistic interactions take place in the context of populations and communities. Sharing mutualists with others in a population could result in indirect interactions in the form of mutualist-mediated competition or facilitation. In my dissertation work I asked whether intraspecific competition or facilitation for ants might occur in an extrafloral nectary-bearing (EFN) plant, and what the consequences would be for long-term population dynamics of the plant. My focal species was Colubrina spinosa (Rhamnaceae), a neotropical treelet on which I observed 69 ant species at La Selva Biological Station, Costa Rica.

Demonstrating intraspecific competition for mutualists requires that 1) neighbor densities affect mutualist visits to an individual, and 2) change in mutualist visits results in reduced benefit. To determine how mutualist density affects plant benefit, I experimentally manipulated ant abundances on plants over two years and measured growth and survival. To assess competition for mutualists, I excluded ants from conspecific neighbors and followed ant abundance on focal plants. To consider long-term facilitation, in which greater local nectar resources increase local ant abundance, I manipulated nectar resources in a two-year field experiment and estimated ant abundance on C. spinosa plants and on baits.

Considering local neighbor density both within a 1m radius and in 5x5 m plots, ant densities on C. spinosa plants showed evidence for a small-scale competition effect and a contrasting plot-level facilitation effect. The small-scale competition was sized-based; smaller plants lost ants to larger plants. Ant benefit to plants also depended on plant size. For larger plants, those with greater size-adjusted ant density had higher growth and survival than those with fewer ants than expected for their size.

To determine whether these contrasting competition and facilitation effects could impact population growth or densities, I modeled population dynamics with an integral projection model (IPM). Growth and survival were functions of ant density, which in turn depended on conspecific neighbors, plant size, and mean background ants. Results suggest that larger-scale facilitation of mutualists impacts long-term population growth more than small-scale competition. Population growth rate increased with increasing background ant density, which depended on facilitation at the 5x5m plot scale. In contrast, small-scale competition caused a redistribution of mutualist ants among plants of different sizes, but had very little effect on long-term population growth.

I thus conclude that on the scale of individuals there is evidence of intraspecific competition for ants as well as facilitation in the EFN plant C. spinosa, but only facilitation effects lead to appreciable changes in population dynamics. If mutualist-mediated facilitation effects tend to occur over long time scales in other systems as well, facilitation might prove to be more important than competition in other mutualisms.