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In food webs, which are composed of antagonistic species interactions (with negative component effects), predators frequently suppress herbivores and indirectly benefit plants via "trophic cascades". Yet, ecological webs commonly contain multiple types of species interactions. Mutualistic species interactions (with only positive component effects) are ubiquitous across ecosystems and often co-occur with antagonistic relationships. A mechanistic understanding of cascades in ecological webs with both mutualistic and antagonistic species interactions is largely lacking, especially for cascades across four trophic levels. This dissertation focuses on how mutualistic interactions between predators and herbivores affect trophic cascades from fourth trophic level predators to plants. I used field experiments to evaluate the mechanisms leading to plant responses to predators in an ecological web that includes a mutualism between predacious ants and herbivorous treehopper insects. In this ecological web, ants have both mutualistic and antagonistic interactions with herbivores, resulting in a net effect of ants on plants that depends on the magnitude of positive and negative indirect pathways. These ant effects provide opportunities for testing the relative importance of antagonistic and mutualistic interspecific interactions for cascades. I tested the following predictions for cascades of effects on plants: 1) antagonistic interactions are more important than mutualistic interactions, 2) predators in the fourth trophic level will indirectly benefit plants, and 3) elevated nitrogen deposition will cause stronger effects from predators to plants. Among studies that have evaluated the effects of mutualisms on tri-trophic cascades, paths of antagonistic effects were generally considered stronger than indirect effects including mutualism. To test this expectation, I first demonstrated that ants simultaneously had a positive effect on treehoppers, and a negative effect on beetles, resulting in less beetle damage to plants hosting the mutualism. Then I compared interaction strengths through the entire insect herbivore community on plants with and without ants. The ant's mutualism with treehoppers was the sole strong interaction contributing to the net indirect effect of ants on plants. Predation, herbivory, and facilitation were weak, and the net effect of ants was reduced plant reproduction. This net indirect effect was also partially due to behavioral changes of herbivores in the presence of ants. An additional treehopper manipulation showed that the treehopper's effect on ant foraging was largely responsible for the ant's net effect on plants; ant workers were nearly ten times as abundant on plants with mutualists and effects on other herbivores were similar to those in the ant-manipulation experiment. This study shows that mutualistic interactions can be strong relative to negative direct interspecific interactions, and can drive indirect ecological dynamics. I next asked whether a predator in the fourth trophic level indirectly benefits plants in this ecological web. Fourth trophic level predators consume lower-level predators, which can indirectly harm plants when herbivores are released from predator suppression. However, I found that black bears, which are predators of ants, were beneficial for plants. Plants near bear-damaged ant nests had greater reproduction than those near undamaged nests, due to weaker ant protection for herbivores, which allowed herbivore suppression by arthropod predators. These results suggest that bears and other predators should be managed with the understanding that they can influence primary producers through many paths. Lastly, I asked whether low-level nitrogen (N) deposition alters the cascade from bears to plants. Widespread, low-level N deposition often enhances plant quality for herbivores and increases herbivory, but it is not known if this small anthropogenic nutrient enrichment affects trophic cascades. A previous study found that high levels of N enrichment caused this ant species to more strongly decrease beetle damage to plants; therefore I expected the indirect effect of bears on plants to become stronger under low-level N enrichment. I found that low-level N enrichment and bear damage to nearby ant nests had interacting effects on plants. The cascade was weak in N-enriched conditions because plants were more resistant to herbivores and predator effects were not stronger. This research indicates that cascades are currently being disrupted by N deposition. Overall, my studies show that mutualistic species interactions can be influential for trophic cascades. These results highlight the need to integrate mutualisms into trophic cascade theory, which is based primarily on antagonistic relationships. Positive and negative species interactions frequently co-occur and may often have interdependent effects on ecological dynamics. Progress in community ecology may be greatly promoted by adopting a perspective that integrates multiple types of species interactions in the same ecological network. / A Dissertation submitted to the Department of Biological Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester, 2014. / October 29, 2014. / ants, bears, ecological web, mutualism, nitrogen deposition, trophic cascade / Includes bibliographical references. / Nora Underwood, Professor Co-Directing Dissertation; Brian Inouye, Professor Co-Directing Dissertation; Thomas Miller, Committee Member; Walter Tschinkel, Committee Member.

Projected changes in climate for the northeastern U.S. over the next century include increased air temperatures and reduced snowpack, leading to increased frequency of soil freeze/thaw cycles (FTC) in winter. Forests of the northeastern U.S. currently offset up to 35% of regional carbon (C) emissions and water uptake by trees in these forests constitutes the majority of evapotranspiration. In addition, nitrogen (N) is an essential element and often limiting nutrient for net primary production in temperate ecosystems, but recent declines in atmospheric deposition of N and changes in climate have led to concerns about N supply not meeting demands by temperate forest trees in the future. While much is known about the effects of climate change in winter and the growing season independently on water, C, and N cycling in temperate forests, little is known about the combined effects on these processes. In Chapter 2, I utilize a soil temperature manipulation experiment and demonstrate that rates of transpiration and leaf-level C uptake by Acer rubrum increase with rising growing season soil temperatures, but increased rates of C uptake are offset by increased frequency of FTCs, while increased transpiration rates are maintained. In Chapter 3, I demonstrate that net N mineralization and foliar N in trees are elevated with soil warming and not affected by FTCs. In Chapter 4, I show that trees access shallow water (< 30 cm depth) in the early growing season. Further, trees that experience FTCs take up equal amounts of water from all soil depths, while those without FTCs switch to a deeper source (> 90 cm depth) that has greater water potential in the late growing season. In Chapter 5, my synthesis of the published literature demonstrates that the majority of studies utilizing stable isotopes of water to determine water sources for vegetation occurred using no experimental manipulation, in forests and grasslands, and in arid climates. Overall, results of my dissertation demonstrate that biogeochemical cycling of C, N, and water are affected by projected changes in climate across seasons in ways that would not have been apparent from examining only one season alone. / 2022-02-03T00:00:00Z

Heuweltjies (Afrikaans for "little hills") are non-anthropogenic, regularly dispersed earth mounds up to 32 meters in diameter and approximately 1.4 meters in height, that dot about 25% of the land surface of south-western southern Africa. The zoogenic "termite" hypothesis has been widely accepted as a hypothesis of heuweltjie origin. However, the recent "vegetation-patch-erosion" (VPE) hypothesis suggests an equally likely explanation for heuweltjie formation. The aim of this investigation was firstly to determine the influence of dust deposition and carbonates on heuweltjie formation in order to gain insight into the formative process of heuweltjies. The second part of this investigation sought to further test the VPE hypothesis by examining whether erosion alone could give rise to heuweltjies within a reasonable time frame. Soil surveys were conducted on 8 heuweltjies and at 2 inter-heuweltjie areas at depths of up to 0.9m. Textural analysis of soil samples collected from the 8 heuweltjies as well as an additional 11 heuweltjies was conducted. Bulk density was also evaluated for each horizon of each heuweltjie and inter-heuweltjie. Carbonates in all soil samples were determined by acid digestion of soils. A digital elevation model of the Clanwilliam area was constructed and used to obtain heuweltjie area and elevation. There was no difference in average soil texture moving from the centre of a heuweltjie to its periphery (indicating that they formed by layered stratification rather than radially from the centre). There was an apparent difference in texture and carbonates between heuweltjie and inter-heuweltjie zones. Mass of fines (silt and clay) and carbonates were both positively correlated with heuweltjie mass, but were also collinear. Non-parametric regression of heuweltjie mass against both fines and carbonates revealed a clear linear trend. However, the trend between fines and carbonates suggested that fines played a central role in both heuweltjie and carbonate formation and that carbonate took on only a secondary role in heuweltjie formation. Erosion alone would have taken 11000 years to form the heuweltjies. However, some calcretes within heuweltjies have been found to be older than 11000 years, suggesting that erosion has not had a major influence on heuweltjie formation. These findings support the vegetation-patch-erosion hypothesis. However they support dust deposition rather than erosion as a primary contributor to heuweltjie formation.

The Iceland Basin in the N. Atlantic has attracted considerable recent attention because the spring bloom in this region acts as the most powerful biological carbon pump anywhere in the world oceans. Furthermore, the link between primary productivity and herbivory by the calanoid copepod, Calanus finmarchicus provides the trophic link between primary producers and the important commercial fishery for Atlantic Cod. However, one result of ocean warming is that the distribution of the cold water loving Calanus finmarchicus is being pushed northwards which adversely affects the food supply to juvenile Cod. Perhaps of more serious concern are concerns that the biological carbon pump may weaken as a result of increased thermal stratification and therefore reduce upward nitrate flux to drive nitrate-based export production. This was in fact confirmed at least in the study region which is dominated by regenerated production (f-ratio &lt;0.5) which in turn suggests a shift in community structure from larger celled organisms to smaller celled organisms, characterised by long inefficient food chains. This project makes use of summer cruise data to the Iceland Basin last summer (2007) where measurements of 15N derived new production (NP) were made to add to previous seasonal (spring, winter) measurements of NP in the region.

Plants can resist herbivore pressure through structural or chemical defence or both. The ultimate goal of defence against herbivory is to reduce the amount of damage to biomass, but more specifically to protect against damage to meristematic tissue. The defences employed depend on the type of herbivory experienced, which is contingent on the herbivore and its mouthparts. This investigation was concerned with structural defence presented by spines. This type of defence protects against mammalian herbivores such as browsers. Spiny species do not dominate in low-light deep forest environments. Therefore the aim of this study was to determine the constraints on spines as a defence strategy under shaded conditions by assessing the effect of reduced light on spine efficiency. Spine efficiency was defined as the amount of defence afforded the plant given the resources available. Spines require carbon to be built and need to be arranged properly in order to present an adequate defence. Thus two non-mutually exclusive hypotheses were proposed: Light limitation reduces the ability of spines to present an adequate defence against browsers due to the architectural strategy employed and/or its influence on carbon gain. The spinescent plant chosen for study was Carissa macrocarpa (Ecklon) A.DC. Light condition of plants was determined using hemispherical photography. Spine efficiency of sun and shade plants was determined using a bite test and was evaluated using architectural and physiological analyses. Architectural analysis involved identifying levels of organisation within the plant across ontogeny and indentifying sun and shade growth strategies. Physiological analysis involved determining carbon gain of sun and shade individuals using gas-exchange measurements, as well as the measurement of biomass allocation by harvesting and oven drying different plant parts. Results showed that biomass allocation patterns of C.macrocarpa did not change in sun and shade but total biomass increased from shade to sunlit conditions. Architectural analysis revealed that in the sun the plant adopted a stout dense structure with high spine efficiency, while in the shade it was more elongated with lower spine efficiency. Therefore C.macrocarpa adapts to the light environment by adopting either the shade or the sun architectural strategy. The way in which this works is that light affects carbon gain, which either increases or decreases biomass and in turn leads the plant to adopt the sun or shade architectural strategy. The architectural strategy affects spine efficiency such that plants in the sun have higher spine efficiency than plants in the shade. Thus, spinescent plants do not do well in light limited environments because they are architecturally constrained to elongate in such conditions. This constraint would put them at higher risk of browser damage than plants in light-sufficient conditions, ultimately decreasing their fitness. If the patterns observed in C. macrocarpa prove to be general, then it helps to explain why spiny plants are more commonly found in open, sunlit environments than in deep shade.

Biocrusts have been shown to affect the ecological conditions in a variety of habitats, involving abiotic mechanisms such as soil moisture retention, nitrogen-rich biomass additions, changing soil physical properties, as well as direct biotic mechanisms such as impeding seedling growth. In coastal dunes, low-lying interdune habitats experience occasional flooding, which promotes the formation of microbial biocrusts. While storm patterns are known to be effective drivers of coastal dune plant community composition, the interaction between microbial communities and vegetation in the context of storm patterns is not well understood. I investigated the role of biocrusts on native coastal dune community, how environmental conditions, particularly storm patterns, affect interactions between crusts and vegetation, as well as how the presence of crusts affects plant-plant interactions. To explore correlative patterns between biocrusts, plants and abiotic factors in the field, I analyzed a long term dataset of a coastal dune plant community on St George Island, FL. The presence of several plant species was correlated with crust, and included both negative and positive associations. Additionally, I found correlative relationships with crusts and total plant cover, as well as soil moisture; the latter supports previous research in other systems on the effects of crusts on soil characteristics. To test the effect of crusts on plant species, I conducted a greenhouse experiment that simulated various environmental conditions, including several water and salt treatments, with and without biocrust in a factorial design. Crusts did have an effect on the growth and survival of some of these species, and this effect varied from negative to positive depending on the species. Additionally, I found trends in the data which suggest that crusts may interact with rain and salt, and this interaction may change the effect of crust on plant species in various environmental conditions. Lastly, I performed a target-neighbor competition experiment using four dominant coastal dune plant species in a full factorial design to test the effect of crusts on plant-plant interactions. I found that the selected species often strongly compete, and these competition interactions vary among species. I also found a significant effect of crust on one species, Schizachyrium maritimum. / A Thesis submitted to the Department of Biological Science in partial fulfillment of the requirements for the degree of Master of Science. / Summer Semester 2016. / July 5, 2016. / Biocrusts, Coastal dunes, Plant communities, Plant-Plant interactions / Includes bibliographical references. / Thomas E. Miller, Professor Directing Thesis; Alice A. Winn, Committee Member; Brian D. Inouye, Committee Member.

The importance of predators in influencing community structure is a well-studied area of ecology. However, few studies apply ecological hypotheses of predation when studying multi-predator systems, and even fewer apply these theories to microbial communities. The phytotelma community found within the leaves of the purple pitcher plant, Sarrecenia purpurea, exhibits a simple trophic structure that includes the micro-scale interactions of protozoan predators and microbial prey, making this microcosm ideal for studying how predator diversity and composition affects microbial community structure. Replicate inquiline microbial communities were maintained for seven days with five protozoan species (four ciliates and one flagellate) in a factorial design. We sought to determine how predator diversity (up to four species) affects prey community diversity, whether different predators target distinct microorganisms, and how interactions among protozoans affect resource (microorganism) use. In so doing, we sought to determine if predators followed ecological patterns of keystone predation or generalist predation. Microbial community structure was analyzed by 16S rRNA gene sequencing (iTag). The data revealed that Archaea were rare, with Proteobacteria and Bacteriodetes dominating regardless of treatment. Of the different protozoan species, two ciliates and one flagellate distinctly altered their communities as compared to one another (p<0.05). The ciliates followed patterns of keystone predation by increasing microbial evenness, while the flagellate left a community statistically similar to the control and followed generalist patterns. When in pairwise treatments, remaining prey communities more closely resembled those of the respective ciliate than that of the flagellate (as compared to these communities in monoculture). The relative abundance of one bacterial Operational Taxonomic Unit (OTU) classified as an Aquitalea sp. decreased from the control when in the presence these two ciliates. This OTU is 98% similar to an Aquitalea sp. associated with chitin degradation, an important function for this plant's fitness. Associated predicted metabolic functions revealed a positive correlation with the relative abundance of this OTU and chitinase, as well as an inverse correlation with community evenness. Taken together, the data demonstrated that specific predator behaviors may have a greater affect on prey diversity and relative abundances than overall predator diversity. Based on metabolic predictions, these trends could have an affect on microbial community function with consequences that reverberate throughout the trophic structure. / A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Sciences in partial fulfillment of the requirements for the degree of Master of Science. / Spring Semester 2017. / April 14, 2017. / Includes bibliographical references. / Olivia U. Mason, Professor Directing Thesis; Thomas E. Miller, Committee Member; Sven Kranz, Committee Member.

Coral reefs and many of the coral species that construct them are in a state of global decline, and this rapid decline of reef corals has incited worldwide conservation, restoration, and research efforts. One growing area of research involves exploring the value of reef-associated habitats, like mangroves, for the survival of particular coral species and the resilience of nearby reefs. Mangroves have long been regarded as inhospitable habitat for corals due to their combination of low light levels, low water clarity, and fluctuating water temperatures. However, roughly half of the coral species living on Caribbean reefs have been documented to inhabit mangrove habitats. If mangroves constitute a critical component of the ecological niche for some corals, then they should support viable, self-sustaining populations. If mangroves contribute directly to the resilience of reef coral populations, then corals must be able to exploit both habitat types coping with varying environmental conditions. Despite the growing number of documented mangrove-coral communities, these questions have not been studied. Here I use two Caribbean coral species from the genus Porites that are known to inhabit both reef and mangrove habitats (P. astreoides and P. divaricata) to explore the contribution of mangrove habitats to coral survival via the following objectives: 1) Characterize the survival, growth, and spatial-distribution of a mangrove-dwelling population of P. divaricata through a multi-year field study; 2) Determine if the population characterized in objective 1 is capable of self-recruitment and map dispersal through a population genomic study using 2bRAD markers; 3) Determine how key phenotypic traits vary between reef and mangrove habitats in P. divaricata and P. astreiodes, using a cross-habitat comparative field-study. Results suggest that mangroves may serve as important habitat for some reef corals due to their ability to support a viable, stable, and healthy coral population that is self-sustaining largely through asexual reproduction. Subsequently, mangrove and reef corals display predictable differences in phenotype, that could help reveal how a “reef coral” can exploit darker mangroves. This work can inform the design of marine protected areas, whereby both mangroves as well as the connection between mangroves and reefs is preserved to facilitate coral survival. / 2024-03-02T00:00:00Z

Among oscine songbird species, the chipping sparrow, Spizella passerina, has perhaps one of the simplest song structures. Each male sparrow has one single song type with one type of syllable repeated many times. To acquire the simple song, juvenile males learn from a neighboring tutor either during the hatching fall or the first spring, perhaps depending on their hatching date. In nature, a male may use either an instructive or a selective mechanism to precisely imitate a song from a close neighbor. Learning song from only one neighbor, a limited sensitive phase, frequent territory shifting or dispersal, and the continuous geographic distribution of chipping sparrows may lead to highly diverse song types within each population and the lack of song differentiation among populations. Male sparrows are able to flexibly change their simple songs at different times of day and under different social circumstances. Singing at dawn and during the daytime are different in acoustic structure, behavioral contexts, social situations, developmental patterns, and, putatively, in function. Territorial males also respond differently to the playback of dawn or daytime song. Removal experiments further suggest that the dawn chorus is associated with interactions among neighbors, whereas daytime song may function in long distance territory advertisement, particularly for female attraction. Further examination of the dawn chorus reveals that male sparrows can use their dawn songs to signal dominance relationships among close neighbors. The dominant male appears to inhibit the singing of his subordinate neighbor. In addition, frequent alternating or overlapping of songs at dawn may provide specific information, such as motivational or social status, for interactions among close neighbors.