<p> Unregulated hunting and habitat loss led to a near extirpation of moose (Alces alces) in New Hampshire in the 1800s. After state protection in 1901, the estimated population increased slowly to &sim;500 moose in 1977, then increased rapidly in the next 2 decades to &sim;7500 following an increase in browse habitat created by spruce budworm (<i>Choristoneura fumiferana </i>) and related timber salvage operations, and then halved from 1998-2016 despite highly available optimal habitat. The declining population was partially related to the specific management objective to reduce moose-vehicle collisions, and a possible change in deer hunter and moose behavior that influence population estimates. But given the substantial decline in productivity and condition of cows, and frequent episodes of high calf mortality in April, the primary cause of decline was presumed to be is an increase in winter tick abundance. </p><p> This study examined the relationships among moose density, optimal habitat, weather/ground conditions, winter tick abundance, and natal dispersal in northern New England. Comparing movement data from the previous (2002-2006) and current (2014-2016) productivity studies in New Hampshire and Maine, the distance of natal dispersal, home and core range size, and home and core range overlap did not significantly (<i>P</i> > 0.05) change despite an increase in optimal habitat and a decrease in moose density.</p><p> Geographic changes in tick abundance were related to an interaction between moose density, and the onset and length of winter. Annual changes in tick abundance in northern New Hampshire are driven by desiccating late summer conditions, as well as the length of the fall questing season. Lower precipitation (6.4 cm) and higher minimum temperatures (9.8 &deg;C) specifically concentrated during larval quiescence from mid-August through mid-September reduces winter tick abundance and the likelihood of an epizootic event. The onset of winter, defined by the first snowfall event (> 2.54 cm), influenced the length of the questing season relative to the date of long-term first snowfall event (14 November). In the epizootic region, average winter tick abundance on moose harvested in mid-October indicated a threshold of 36.9 ticks, above which an epizootic is like to occur unless an early snowfall event shortened the fall questing season. Optimal habitat created by forest harvesting was produced at an annual rate of 1.3% (1999-2011) and is not considered limiting in northern New Hampshire, but likely concentrates moose density locally (&sim;4 moose/km²) facilitating the exchange of winter ticks. In northern New Hampshire, snow cover late into April did not reduce tick abundance in the following year and cold temperatures (&lt; 17 &deg;C) that induced replete adult female mortality are extremely rare in April.</p><p> Given a continuation of warming climate and conservative moose harvest weather conditions and high local moose densities will continue to favor the life cycle of winter ticks, increasing the frequency of winter tick epizootics and shift the epizootic region slowly northward. Conversely, temporary reduction of moose density may substantially reduce parasite abundance and support a healthier and more productive moose population.</p><p>

<p> Indirect interactions have increasingly been recognized as important forces influencing population dynamics and structuring communities. Associational susceptibility is a form of indirect effect in which a focal plant experiences greater herbivore damage due to neighboring plant identity or diversity. These interactions remain poorly understood in the context of invasion ecology, though they may be responsible for huge impacts of invasive species on native communities. This dissertation investigates the potential mechanisms and consequences of associational susceptibility of a native perennial shrub, <i> Atriplex canescens,</i> driven by an invasive annual forb, <i>Brassica tournefortii,</i> and an invasive herbivorous stinkbug, <i>Bagrada hilaris.</i> In Chapter 1, a potential associational effect is experimentally demonstrated and a phenologically-driven trait is identified as a potential mechanism for this interaction. In Chapter 2, relative host plant quality is explored for its role in mediating the numerical response of the shared herbivore, and the herbivore&rsquo;s damage impact on <i>A. canescens. </i> In Chapter 3, neighbor density, herbivore presence and herbivore density were manipulated to identify their impacts on spillover timing, extent, and fitness consequences for <i>A. canescens.</i> Overall, potential mechanisms of <i>A. canescens</i> associational susceptibility to <i> Br. tournefortii</i> and <i>Ba. hilaris</i> identified include: <i> Ba. hilaris</i> accumulation on <i>Br. tournefortii</i> followed by <i>Br. tournefortii</i> senescence and depletion, triggering <i> Ba. hilaris</i> alternative host-seeking. Associational susceptibility of <i>A. canescens</i> could not be re-created under experimental conditions, but further study is required to ascertain whether this interaction is due to experimental limitations or ecological implausibility.</p><p>

In the first chapter, I introduce the study organisms, non-biting midges in the genus Axarus. These are large flies that spend the majority of their lifespan living aquatically in submerged clays and rotting wood in the Connecticut River. The adult lifespan is very brief, on the order of three days, while the larvae live for one year. There are two species so far discovered in the Connecticut River, both new to science, and very closely related to each other. Both species coexist in distinct populations separated by areas where there is no suitable habitat for them; the primary habitat being clay that was deposited during the existence of glacial Lakes Hitchcock and Coös, with a secondary use of rotting wood also evident. The population genetic evidence indicates that wood is only inhabited near clay deposits though, and there is genetic separation between the populations, a fact that is established in Chapter 3. Chapter 1 provides the foundational biological information necessary to use these species in a study of population genetics. In the second chapter I show that populations of one of the species, herein referred to as A. species varvestris, is polymorphic for 4 chromosomal inversions, and that natural selection is acting on one of these inversions. The inversion in question results in a significant decrease in larval size when heterozygous, and increases in frequency within one generation. This is evidence of natural selection, and size-selective predation is suggested as a mechanism for the selection. An association between this inversion and the sex determination of the species is also shown. The third chapter presents a large dataset of chromosomal mutation frequencies that establishes marked cytogenetic differentiation between the populations of Axarus species varvestris. I suggest that this differentiation has occurred within a very short time-frame, on the order of 10,000 generations, and that the structure of the population cytogenetic relationships may reveal and be correlated with the ancient separation between the post-Pleistocene lakes that were present in the river valley following the last ice age. (Abstract shortened by UMI.)

While pollinators and leaf herbivores have been a focus of research for decades, floral antagonists have been studied significantly less. Since floral antagonists can be as common as leaf herbivores and have strong impacts on plant reproduction, it is important to understand the role of floral antagonists in the ecology and evolution of flowers. I conducted four experiments to better understand the relationship between plants, floral traits, floral antagonists, and other plant-insect interactions. First, I manipulated resources (light and soil nutrients) that are known to have impacts on plants and floral traits to test how they affect floral antagonists and other plant-insect interactions. Plentiful resources increased the proportion of floral antagonists to visit flowers, but also increase tolerance of floral antagonists. Second, I manipulated flower bud gallers, a species-specific floral herbivore that destroys flowers, to test how it affected other plant-insect interactions, floral traits, and plant reproduction. Plants with flower bud gallers tended to have more pollinator visits, but this effect is due to a shared preference by gallers and pollinators for similar plants. Third, I manipulated florivory to examine how it affects subsequent plant-arthropod interactions, floral traits, and plant reproduction. Florivory had systemic effects on other plant-insect interactions, including leaf herbivores, and shifted the plant mating system towards more selfing. Additionally, I tested how several floral antagonists respond to floral attractive and defense traits to understand which floral traits are important in mediating antagonisms. Finally, I manipulated florivory, pollination, and nectar robbing to test for effects of multiple floral interactions on subsequent plant-insect interactions, floral traits, and plant reproduction. There were significant many-way interactions between the three treatments on subsequent plant-insect interactions and reproduction, indicating that the effect of one interaction depends on what other interactions are present. Understanding the role that floral antagonists play in plant ecology can help scientists determine which interactions are most important, and may help determine why some floral traits exist in their current state. Together, this work represents some of the most comprehensive research on the community consequences of floral antagonists, as well as the interplay between floral traits and floral interactions.

Mutualisms are major structuring forces in biological communities. However, the concept of the trophic cascade has rarely been explicitly applied to mutualisms. Antagonists of one mutualist have the potential to negatively affect the second mutualist through effects on their partner, and the magnitude of such effects should vary with mutualism strength. Bumblebees represent an ecologically and economically important mutualist pollinator group. They are attacked by a range of parasites, and visit a variety of plants that vary in reliance on bumblebees for pollination service. Using bumblebees and their parasites, I investigated whether mutualisms can mediate trophic cascades. I surveyed three parasites (Crithidia bombi, Nosema bombi and conopid flies) in Bombus spp. in Massachusetts and found that parasites are more common in wild bumblebees than previously believed (Chapter 1). To test whether infection by these parasites has top-down effects on pollination service to plants, I examined whether site-level parasitism rates correlated with pollination service to greenhouse-raised plants. I used several plant species which varied in their dependence on bumblebees for pollination. The relationship between parasitism and pollination service differed between plants and parasite species, and there was evidence that plants more reliant on bees for pollination experienced stronger negative indirect effects of parasites (Chapter 2). Finally, I developed an ordinary differential equation model of mutualist population dynamics incorporating a producer, its mutualist, and an antagonist of the mutualist. I varied the mutualism from obligate to facultative, and investigated effects of the antagonist on mutualist coexistence. My results highlight how mutualisms differ from traditional trophic cascades, and the importance of considering obligate, facultative, and asymmetric mutualisms in attempting to understand their interactions with the community (Chapter 3). My contributes to conservation issues and extends our theoretical understanding of basic ecology. I provided valuable data about the incidence and multitrophic effects of parasites in an important native pollinator. By combining mutualisms with the theoretical structure of the trophic cascade, two previously independent areas of ecological research that will benefit from integration, my research improves our understanding of how mutualisms structure the community as a whole.

<p> Streams along the Copper River Delta, southcentral Alaska, exhibit contrasting thermal and hydrologic variability associated with being primarily groundwater-fed (GWF) or surface water-fed (SWF). Groundwater-fed streams are predictable both thermally and hydrologically year round, whereas SWF streams are unpredictable and exhibit more variable thermal and hydrologic regimes. These differences may strongly influence aquatic insect community structure and secondary production. Four streams, two GWF and two SWF, were sampled twice monthly from late April 2013 through August 2013 and once seasonally in fall (September) and early winter (November). Aquatic insect community structure differed markedly in both hydrologic types. Taxa richness was significantly higher in SWF (43) than in GWF (39) streams and non-metric multidimensional scaling of community structure revealed two distinct groups corresponding to the two hydrologic types. Total secondary production was higher in GWF than in SWF streams with Orthocladiinae (Diptera: Chironomidae) representing 56% of insect secondary production in GWF streams. Results from this study have strong implications for aquatic insect communities in GWF and SWF streams because of differing susceptibilities of these systems to the potential effects of climate change. Due to their thermal stability, groundwater-fed streams are less likely to be impacted by climate change, whereas SWF streams are thermally variable and more likely to be influenced. The effects of altered aquatic insect communities can cascade to higher trophic levels such as salmon and ultimately impact stream ecosystem function and the ecosystem services they provide.</p>

<p> Adaptive phenotypic plasticity is a key mechanism by which organisms deal with variation in many different aspects of their environment. Adaptive plasticity can occur in any trait, from aspects of biochemistry and morphology to behaviors. Because so many different traits can be plastic, organisms often respond plastically to a given change in their environment, such as an increase in temperature, with adaptive changes in multiple traits. Nevertheless, how these different plastic responses interact with each other and evolve together has received little attention. My research addresses these potential interactions among plastic traits and proposes new hypotheses regarding the causes and consequences of these interactions. It does so by focusing on heat avoidance in the caterpillars of <i>Battus philenor</i> (the pipevine swallowtail) which involves two distinct plastic mechanisms. First, the caterpillars can change color when they molt, a form of morphological plasticity in which they develop a red color under high temperatures which cools them by absorbing less solar radiation. Second, when the caterpillars become too hot, they will leave their host to seek cooler thermal refuges, a case of behavior as a form of plasticity. In terms of function, I demonstrated through field research that these two responses to high temperatures are largely redundant. Behavior provides a much stronger and faster response than color change, and red coloration provides little additional cooling when on a refuge. Instead, the primary benefit of color change is that it reduces the use of refuge seeking behavior, allowing the caterpillars to stay on their hosts longer. Using laboratory experiments, I demonstrated that this change in the use of refuge-seeking behavior with color occurs because color changes the cue for the behavior, body temperature, rather having any effect on how the caterpillar responds to that cue. Alternatively, similar experiments on caterpillars of varying sizes show that developmental size change lowers the body temperature at which caterpillars leave their host, demonstrating a change in the response to the cue (although larger caterpillars are also warmer, so both mechanisms are likely relevant for how size changes the expression of behavior). All of my research to this point was conducted on the local population in southern Arizona, which experiences quite high temperatures, but <i>B. philenor</i> is also found in much cooler environments, such as the Appalachian Mountains. Given this variation in their thermal environment, I used common garden experiments to compare the capacity for color change and refuge-seeking among B. philenor caterpillars from across the species range. Both color change and refuge seeking not only occurred in all populations, but also had the same reaction norms, occurring at the same temperatures and to the same degree. This is particularly notable for color change, which is not observed in the wild in northeastern populations, and thus has persisted despite minimal if any use. Overall, I have shown that studies of plasticity need to account for plasticity in different traits as well as the interactions between these forms of plasticity. My research on B. philenor provides a model for how to address these interactions, which future research can extend to additional organisms and environmental circumstances. </p>

Every organism needs to survive and successfully reproduce in the face of changing environmental conditions in which variation in resource availability can seriously limit performance. Organisms can respond to the variation in quality or availability of food resources with behavioural and physiological accommodations going from the baseline physiological response to environmental stressors to complex life-history strategies like migration and diapause. In insects, one avenue to cope with the resources’ variation is to plastically tune the reproductive system to the environmental conditions in order to shift resources away from reproduction during unfavourable periods but maximize it when resources are abundant. I studied the role of reproductive physiology in both males and females in mediating a response to challenging conditions determined by a lack of food resources or the presence of qualitatively different diets using the milkweed bug, Oncopeltus fasciatus, as model species. I studied the role of oosorption, a plastic physiological response through which resources can be recovered and redirected to body maintenance and survival, in shaping behavioural strategies to cope with challenging environments. I also studied the effects of diet quality on male’s sexual behaviour and how these modulate the trade-offs between reproduction and survival. I then investigated how the effects of diet quality, sexual maturation and rearing conditions influence the occurrence of reproductive diapause in both males and females. I found that females exposed to different diets plastically adapt their schedule of reproduction depending on diet quality: this also influences the occurrence of oosorption in the ovary mediating the amount of resources that are directed to reproduction or survival. Diet quality influences males’ sexual behaviour so that even after a long-term adaptation on an alternative artificial diet, they invest more in reproduction at the expenses of survival when fed on an ancestral high quality diet; this is achieved with a shift in the trade-off between reproduction and survival. The occurrence of reproductive diapause in both males and females is a function of several factors: the quality of food resources ultimately modulates sexual maturation in adult individuals determining the occurrence of diapause or reproduction. Finally, oosorption may be involved in the evolution of alternative condition-dependent strategies as an adaptive physiological mechanism to cope with stressful environments; thus females from different populations may be able either to migrate in favourable areas where they can exploit abundant food resources or remain residents and perform high levels of oosorption to cope with the seasonal shortage of food.