Browsing by Author "Wright, Justin P"

Human welfare depends heavily on ecosystem services like water purification and nutrient cycling. Many of these ecosystem services, in turn, rely on reactions performed by microbes and yet remarkably little is known about how anthropogenic impacts are affecting the structure and function of microbial communities. To help address this knowledge gap, this dissertation uses field surveys and laboratory experiments to examine how watershed urbanization affects microbial communities in receiving streams. We focus on a specific functional group and its associated function - the denitrifying bacteria and denitrification. Denitrifying bacteria use reactive nitrogen and organic carbon as substrates to perform denitrification. Denitrification is one of the few ways to permanently remove reactive nitrogen from ecosystems. Since excess reactive nitrogen in water contributes to serious water quality and human health problems like toxic algal blooms and bowel cancer, denitrification in streams can be considered a valuable ecosystem service. Watershed urbanization, however, may alter the structure of denitrifying bacteria communities in ways that constrain their capacity to remove reactive nitrogen from streams.

Watershed urbanization leads to drastic changes in receiving streams, with urban streams receiving a high frequency of scouring flows, together with increased nutrient (nitrogen and carbon), contaminant (e.g., heavy metals), and thermal pollution. These changes are known to cause significant losses of sensitive insect and fish species from urban streams. Microbes like denitrifying bacteria may be similarly affected. In the first part of this dissertation, we describe results from four repeated surveys of eight central North Carolina streams affected to varying degrees by watershed urbanization. For each stream and sampling date, we characterized both overall and denitrifying bacterial communities and measured denitrification potentials. Differences in overall and denitrifying bacteria community composition were strongly associated with the urbanization gradient. Denitrification potentials, which varied widely, were not significantly associated with substrate supply. By incorporating information on the community composition of denitrifying bacteria together with substrate supply in a linear mixed-effects model, we explained 45% of the variation in denitrification potential (p < 0.001). Results suggest that 1) watershed urbanization can lead to significant changes in the composition of bacterial communities in streams and 2) such changes may have important functional consequences.

The second part of this dissertation examines how urbanization-driven changes to the structure of denitrifying bacteria communities might affect the way they respond to stress or disturbance. Some communities can resist changes to functionality in response to disturbance, potentially as a result of previous exposure and subsequent adaptation (legacy hypothesis) or high diversity (insurance hypothesis). We compare the resistance of two structurally distinct denitrifying bacteria communities to experimental disturbances in laboratory microcosms. Communities originated from either a polluted, warm urban streams or a relatively pristine, cool forest stream. In this case, the two communities had comparable compositions, but forest communities were more diverse than their urban counterparts. Urban communities experienced significant reductions in denitrification rates in response to the most severe increased pollution and temperature treatments, while forest communities were unaffected by those same treatments. These findings support the insurance, but not the legacy hypothesis and suggest that the functioning of urban streams may be more susceptible to further environmental degradation than forest streams not heavily impacted by human activities.

In the third part of this dissertation, we discuss results from a one-time survey of denitrifying bacteria communities and denitrification potentials in 49 central North Carolina streams affected to varying degrees by watershed urbanization. We use multivariate statistics and structural equation modeling to address two key questions: 1) How do different urban impacts affect the structure of denitrifying bacteria communities and 2) How do abiotic (e.g., temperature) versus biotic (denitrifying bacteria community structure) factors affect denitrification potentials in urban streams? Denitrifying bacteria community structure was strongly affected by the urban impacts measured. Community composition responded to increased temperatures, substrate supply, and contamination, while diversity responded negatively to increased temperatures and hydrologic disturbance. Moreover, increased temperatures and substrate supply had significant positive effects, while urbanization-driven changes to denitrifying bacteria community structure had significant negative effects on denitrification potential. The structural equation model captured 63% of the variation in denitrification potential among sites and highlighted the important role that microbial community structure can play in regulating ecosystem functioning. These findings provide a novel explanation for recent observations of decreasing denitrification efficiency with increasing urbanization. Ultimately, we hope findings from this dissertation will help inform more effective stream management and restoration plans and motivate ecologists to consider including microbial community structure in ecosystem models of microbe-mediated processes.

Microbes are the foundation of all ecosystems and crucial players in major ecosystem processes. However, most of our ecological theory was developed for plants and animals and thus may not help us understand these important communities. Previous syntheses have found mixed evidence for ecological patterns in observational data of microbes. In my dissertation, I combine observation and experiments to identify forces structuring microbial communities and how these are similar or different from those at play in systems of macroorganisms. In two chapters, I test new ecological hypotheses in soil microbial communities. In later chapters, I draw on similar ecological theory to explore the relative importance of host and microbial control for the gut microbiota.

In chapter one, I analyze insect, fungal, and bacterial responses to urbanization and habitat fragmentation. This study is the first of its kind to compare scaling relationships between macro- and micro-organisms in the same habitats. I find that microbial communities were vastly more immune than even the smallest of animals to human perturbation.

In chapter two, I seek to identify the drivers responsible for microbial community assembly during secondary succession. I use a fully factorial microcosm experiment that manipulates both biotic and abiotic factors in microcosms emulating old-fields. I find that both plant community and soil conditions are important for determining microbial community composition but that unique taxa respond to each driver.

In chapter three, I aim to quantify and document the impact of gut nitrogen availability on the microbiota. I find that stoichiometric mismatch between microbes and gut resources is pervasive for mammals, indicating that nitrogen may be limiting. Furthermore, I show that nitrogen availability in the gut is under host control, with host secreted nitrogen serving as a dynamic means for the host to manipulate microbial composition.

In chapter four, I use both in and ex vivo approaches to document shifts in composition and changes in the environment in the gut following antibiotic treatment. I find that the most significant abiotic shift is an increase in redox potential, which is due primarily to changes in microbial metabolism rather than a host response. Feedbacks between the environment and the microbial community, as well as dispersal limitation, then contribute to compositional change during post-antibiotic succession.

Climate change is driving vegetation community shifts in coastal regions of the world, where low topographic relief makes ecosystems particularly vulnerable to sea level rise, salinization, storm surge, and other effects of global climate change. Salinization has clear effects on vegetation, as few plant species can survive in brackish water, and these shifts in vegetation lead to declines in biomass carbon stocks, as well as significant changes in habitat structure and biodiversity. The rate and extent of these impacts on other wetland ecosystem properties and function is far less certain. This dissertation investigates the ecosystem consequences of saltwater intrusion in coastal wetlands, from shifting vegetation at the landscape scale, to soil biogeochemistry and wetland carbon cycling.Coastal plant communities globally are highly vulnerable to future sea-level rise and storm damage, but the extent to which these habitats are affected by the various environmental perturbations associated with chronic salinization remains unclear. In 2016, a series of vegetation plots across the Albemarle-Pamlico Peninsula that had been surveyed 7-13 years earlier were revisited in order to measure changes in tree basal area and community composition over time. I found reduced tree basal area in plots at lower elevations and with higher current soil salt content, while these factors explained only a small fraction of the measured changes in tree community composition. While tree basal area increased in the majority of plots, I measured declines in basal area in multiple sites with high soil salt content or low elevation. This decadal comparison provides convincing evidence that increases in soil salinity and saturation can explain recent changes in tree biomass, and potential shifts in community composition in low-elevation sites along the North Carolina coast. In Chapter 3, I quantified land and land cover change in the Alligator River National Wildlife Refuge (ARNWR), North Carolina’s largest coastal wildlife preserve, from 1985 to 2019 using classification algorithms applied to a long-term record of satellite imagery. Despite ARNWR’s protected status, and in the absence of any active forest management, 32 % (31,600 hectares) of the refuge area has changed land cover classification during the study period. A total of 1151 hectares of land was lost to the sea and ~19,300 hectares of coastal forest habitat were converted to shrubland or marsh habitat. As much as 11 % of all forested cover in the refuge transitioned to ghost forest, a unique land cover class that is characterized by standing dead trees and fallen tree trunks. This is the first attempt to map and quantity coastal ghost forests using remote sensing. These unprecedented rates of deforestation and land cover change due to climate change may become the status quo for coastal regions worldwide, with implications for wetland function, wildlife habitat and global carbon cycling. Salinization of freshwater wetlands is a symptom of climate change induced sea level rise. The ecosystem consequences of increasing salinity are poorly constrained and highly variable within prior observational and experimental studies. Chapter 4 presents the results of the first attempt to conduct a salinization experiment in a coastal forested wetland. Over four years, marine salts were applied to experimental plots several times annually with the goal of raising soil salinity to brackish levels while soil porewater in control plots remained fresh. Each year I measured aboveground and belowground vegetation biomass along with soil carbon stocks and fluxes. Despite adding more than 1.5 kg of salt per m2 to our experimental plots over four years, the ecosystem responses to salt treatments were subtle and varied over the multi-year experiment. In the final year of the experiment, soil respiration was suppressed, and bulk and aromatic soil carbon became less soluble as a result of salt treatments. The more stable carbon pools—soil organic carbon and vegetation associated carbon—remained unaffected by the salt treatment. This experiment demonstrates substantial ecosystem resistance to low dose salinity manipulations. The inconsistent soil carbon responses to experimental salinization I observed in the field led me to question how differences in soil pH and base saturation might alter the impacts of salinity of soil microbial activity. To test this, I performed a salt addition experiment on two series of wetland soils with independently manipulated salt concentrations and solution pH to tease apart the effect of these seawater components on soil carbon cycling (Chapter 5). Microbial respiration and dissolved organic carbon solubility were depressed by marine salts in both soils, while pH manipulation alone had no effect. Salinity treatments had a far greater effect on soil pH than did our intentional pH manipulation and there was a strong interaction between salt treatments and soil type that affected the magnitude of soil carbon responses. Site soils varied significantly in pH and base saturation, suggesting that the interaction between salinity and edaphic factors is mediating soil carbon processes. The degree of salinization and the effective pH shift following seawater exposure may vary widely based on initial soil conditions and may explain much of the variation in reports of salt effects on soil carbon dynamics. I suggest that these edaphic factors may help explain the heretofore inconsistent reports of carbon cycle responses to experimental salinization reported in the literature to date.

Understanding how biotic organisms are affected by abiotic conditions and, in turn, affect the functioning of their environment is one of the most unifying goals of ecological research. Particularly in the context of a changing environment, a generalizable understanding of the factors which underpin plant functioning has crucial implications for predicting how and why ecosystems may function differently under future climate scenarios. In my dissertation, I assess the implications of two fundamental abiotic drivers - soil resource supply and disturbance regime - on community and ecosystem dynamics. I use a combination of greenhouse and field experiments to iteratively examine how responses to these drivers at the levels of individual and species scale up to influence competitive interactions, biomass regeneration, and productivity.

In chapter two, I quantify a short-lived increase in nitrogen availability following prescribed fire. In chapter three, I then test the extent to which a similar nitrogen pulse is utilized by co-occurring plant species, and relate their nitrogen uptake to regrowth when planted with a stronger or weaker competitor. This chapter demonstrates that species differ significantly in their ability to capture a pulse of nitrogen but that this has no effect on their competitive ability during resprouting. Instead, both functions are correlated in opposite directions with the same root trait, suggesting that they may exemplify opposing life history strategies. In chapter four I examine the role of nitrogen availability on biomass regrowth in complex field communities to assess how nitrogen supply and disturbance history affect community responses to subsequent disturbances. I found that historical disturbance frequency had a much stronger impact on disturbance response than nitrogen availability, although nitrogen availability mediated the disturbance response of some species. Together these findings suggest that root traits can be used to predict competitive strategies for plants in this frequently-disturbed and pulse-driven ecosystem. If competitive strategies are optimized either following nitrogen pulses or disturbance, species abundances may fluctuate over time and under future conditions depending on the frequency of each abiotic driver. Finally, in chapter five I extend these questions to water-limited conditions: in mesocosms planted with tree saplings that form associations with either arbuscular or ectomycorrhizal fungi, I compare the resistance and recovery of belowground respiration to drought and rewetting. I find that mycorrhizal association affects respiration resistance to drought, but both mycorrhizal types fail to recover following rewetting. The association of trees with mycorrhizal fungi may, then, have implications for the drought tolerance of host tree species.

Root carbon exudation is a critical element of the soil carbon cycle, and how both environmental conditions and plant traits influence exudation remains uncertain. I studied relationships between environmental conditions, plant traits, and carbon exudation in four herbaceous plant species: Asclepias incarnata, Microstegium vimineum, Panicum virgatum, and Scirpus cyperinus. Mature individuals were given short-term factorial light and N treatments, and exudates were collected from 8-hour carbon-free hydroponic incubations. I measured size traits (biomass, leaf area, root length, and root volume), photosynthesis (leaf-level and whole-plant), and tissue N traits (root, stem, and leaf percent N and C:N ratio). Neither light nor N treatments affected exudation, while exudation varied with species and traits. Species alone substantially explained mass-specific exudation (estimated R2 = 0.38). Size strongly predicted both total and mass-specific exudation, interacting with species (estimated R2 = 0.52 and 0.48, respectively). Generally, larger individuals exuded more overall but less per unit mass, although larger M. vimineum plants exuded more per unit mass. Whole-plant photosynthetic rate was weakly related to total exudation (estimated R2 = 0.17), and tissue N concentration moderately predicted mass-specific exudation (estimated R2 = 0.23). Other researchers have found that high light and low nitrogen availability stimulate exudation; my results indicate that this relationship is not straightforward. Plant traits, however, significantly explained variation in exudation, including some variation across species, supporting trait-based analyses of plant species' effects on ecosystem processes.

Species invasions are more prevalent than ever before. While the addition of a species can dramatically change critical ecosystem processes, factors that mediate the direction and magnitude of those impacts have received less attention. A better understanding of the factors that mediate invasion impacts on ecosystem functioning is needed in order to target which exotic species will be most harmful and which systems are most vulnerable. The role of invasion on nitrogen (N) cycling is particularly important since N cycling controls ecosystem services that provision human health, e.g. nutrient retention and water quality.

We conducted a meta-analysis and in-depth studies focused on the invasive grass species, Microstegium vimineum, to better understand how (i) plant characteristics, (ii) invader abundance and neighbor identity, and (iii) environmental conditions mediate the impacts of invasion on N pools and fluxes. The results of our global meta-analysis support the concept that invasive species and reference community traits such as leaf %N and leaf C:N are useful for understanding invasion impacts on soil N cycling, but that trait dissimilarities between invaded and reference communities are most informative. Regarding the in-depth studies of Microstegium, we did not find evidence to suggest that invasion increases net nitrification as other studies have shown. Instead, we found that an interaction between its abundance and the neighboring plant identify were important for determining soil nitrate concentrations and net nitrification rates in the greenhouse. In field, we found that variability in environmental conditions mediated the impact of Microstegium invasion on soil N pools and fluxes, primarily net ammonification, between sites through direct, indirect, and interactive pathways. Notably, we detected a scenario in which forest openness has a negative direct effect and indirect positive effect on ammonification in sites with high soil moisture and organic matter. Collectively, our findings suggest that dissimilarity in plant community traits, neighbor identity, and environmental conditions can be important drivers of invasion impacts on ecosystem N cycling and should be considered when evaluating the ecosystem impacts of invasive species across heterogeneous landscapes.

Environmental changes, such as climate change, can have differential effects on species, with important consequences for community structure and ultimately, for ecosystem functioning. In the Arctic, where ecosystems are experiencing warming at twice the rate as elsewhere, these effects are expected to be particularly strong. A proper characterization of the link between warming and biotic interactions in these particular communities is of global importance because the tundra's permafrost stores a vast amount of carbon that could be released through decomposition as greenhouse gases and alter the global rate of climate change. In this dissertation, I examine how arthropod communities are responding to warming in the Arctic and how these responses might be affecting ecosystem functioning.

I first address the question of whether and how long-term changes in climate are affecting individual groups and overall community structure in a high-arctic arthropod food web. I find that increasingly warm springs and summers between 1996-2011 differentially affected some arthropod groups and that this led to major changes in the relative abundances of different trophic groups within the arthropod community. Specifically, spring and summer warming are associated with relatively more herbivores and parasitoids and fewer detritivores within the community. These changes are particularly pronounced in heath sites, suggesting that arthropod communities in dry habitats are more responsive to climate change than those in wet habitats. I also show that herbivores and parasitoids are sensitive to conditions at subzero temperatures, even during periods of diapause, and that all trophic groups benefit from a longer transition period between summer and winter. These results suggest that the projected winter and springtime warming in Greenland may have unexpected consequences for northern arthropod communities. Moreover, the relative increase in herbivores and loss of detritivores may be changing the influence of the arthropod community over key ecosystem processes such as decomposition, nutrient cycling, and primary productivity in the tundra.

Predator-induced trophic cascades have been shown to impact both community structure and ecosystem processes, yet it is unclear how climate change may exacerbate or dampen predator effects on ecosystems. In the second chapter of my dissertation, I investigate the role of one of the dominant tundra predators within the arctic ecosystem, wolf spiders, and how their impact might be changing with warming. Using results from a two-year-long field experiment, I test the influence of wolf spider density over the structure of soil microarthropod communities and decomposition rates under both ambient and artificially warmed temperatures. I find that predator effects on soil microarthropods change in response to warming and that these changes translate into context-specific indirect effects of predators on decomposition. Specifically, while high densities of wolf spiders lead to faster decomposition rates at ambient temperatures, they are associated with slower decomposition rates in experimentally warmed plots. My results suggest that if warming causes an increase in arctic wolf spider densities, these spiders may buffer the rate at which the massive pool of stored carbon is lost from the tundra.

Wolf spiders in the Arctic are expected to become larger with warming, but it is unclear how this change in body size will affect spider populations or the role of wolf spiders within arctic food webs. In the third chapter of my dissertation, I explore wolf spider population structure and juvenile recruitment at three sites of the Alaskan Arctic that naturally differ in mean spider body size. I find that there are fewer juveniles in sites where female body sizes are larger and that this pattern is likely driven by a size-related increase in the rate of intraspecific cannibalism. These findings suggest that across the tundra landscape, there is substantial variation in the population structure and trophic position of wolf spiders, which is driven by differences in female spider body sizes.

Overall, this dissertation demonstrates that arctic arthropod communities are changing as a result of warming. In the long-term, warming is causing a shift in arthropod community structure that is likely altering the functional role of these animals within the ecosystem. However even in the short-term, warming can alter species interactions and community structure, with important consequences for ecosystem function. Arthropods are not typically considered to be major players in arctic ecosystems, but I provide evidence that this assumption should be questioned. Considering that they are the largest source of animal biomass across much of the tundra, it is likely that their activities have important consequences for regional and global carbon dynamics.