Organisms with complex life cycles may experience diverse stressors during their development. Stressors experienced in early life stages may influence the quantity and quality of individuals in later life stages. However, it is unclear if these effects persist later in life and how they may influence population dynamics. This dissertation uses two amphibian species, the Western spadefoot toad (Pelobates cultripes) and the spotted salamander (Ambystoma maculatum) to explore how biotic and abiotic factors experienced in aquatic and terrestrial environments influence phenotype and survival. We use a combination of field mesoscosm studies, laboratory studies and modeling to explore how early life history stressors persist in diverse environmental contexts. In Chapter 1, pond drying and larval density negatively influence multiple aspects of phenotype in the Western spadefoot toad. In Chapter 2, reduced body size due to larval stressors persisted in the first year of life in spotted salamanders in both high and low terrestrial resource environments. Additionally, there was no relationship between size at metamorphosis and post-metamorphic terrestrial survival. In Chapter 3, low terrestrial moisture levels reduced post-metamorphic growth rates by reducing food intake in juvenile spotted salamanders from both high and low larval density treatments. In Chapter 4, we scale up the effects of reduced body size in the Western spadefoot toad to explore how reduced body size due to pond drying may influence population densities using a stage specific matrix model. Reduced body size, by delaying maturity, may reduce adult densities in the Western spadefoot toad. This dissertation suggests that life stages are highly interconnected, as stressors experienced early in life stage may persist through their effects on phenotype in the absence of compensatory mechanisms. Variation in later life stages may also influence phenotype, but may not completely erase effects of early environments. Stressors experienced early in life may also translate to population level consequences, especially when organisms experience multiple stressors across life stages.

Individual differences in fecundity often serve as proxies for differences in overall fitness, especially when it is difficult to track the fate of an individual's offspring to reproductive maturity. Using fecundity may be biased, however, if density-dependent interactions between siblings affect survival and reproduction of offspring from high- and low-fecundity parents differently. To test for such density-dependent effects in plants, we sowed seeds of the wildflower Ipomopsis aggregata (scarlet gilia) to mimic partially overlapping seed shadows of pairs of plants, one of which produced twice as many seeds. We tested for differences in offspring success using a genetic marker to track offspring to flowering multiple years later. Without density dependence, the high-fecundity parent should produce twice as many surviving offspring. We also developed a model that considered the geometry of seed shadows and assumed limited survivors so that the number of juvenile recruits is proportional to the area. Rather than a ratio of 2:1 offspring success from high- versus low-fecundity parents, our model predicted a ratio of 1.42:1, which would translate into weaker selection. Empirical ratios of juvenile offspring and of flowers produced conformed well to the model's prediction. Extending the model shows how spatial relationships of parents and seed dispersal patterns modify inferences about relative fitness based solely on fecundity.

Multiple stressors have caused a decline in coral populations. Broadcast spawning corals once dominated the Florida Reef Tract (FRT), but since their decline, smaller brooding corals, soft corals, and macroalgae are replacing them. Brooding corals are more resilient to current threats in part because they are reproductive throughout much of the year and their larvae are competent to settle after release. Despite the ubiquity of brooders on Florida reefs, much of their reproductive strategy remains unknown. This study aimed to examine paternity as a function of colony size and density in Porites astreoides, a common brooding coral in the FRT. Porites astreoides colonies were configured in arrays at three densities that were replicated three times. A focal colony was surrounded by six other colonies, separated from the focal colony at different distances (1m, 7m, and 15m) representing high, moderate, and low population densities, respectively. All arrays were placed in the field but were separated from the reef and naturally occurring P. astreoides colonies by at least 50 m. Four days before the new moon, colonies were transported to the laboratory for larval collection. Over a four day period, a total of 3,184 larvae were collected from 24 colonies, 13 of which released larvae over consecutive days. The resulting larvae were genotyped using seven microsatellite markers. All larvae had the exact genotypes of the colony from which the larvae were collected, i.e. maternal- egg donor. This suggested the larvae were parthenogenically produced and no sperm was used to fertilize the eggs. This is the first study to suggest that parthenogenesis is occurring in P. astreoides. In today's oceans that have been depleted of corals, parthenogenesis may be an advantageous reproductive strategy used to boost populations. However, parthenogenesis reduces the genetic diversity which could hinder successful sexual reproduction in the future causing fragmented populations.

Agrilus planipennis, the emerald ash borer (EAB), is a destructive invasive forest pest decimating North American ash trees. Population-wide management of EAB focuses on biological control, with the introduction of four parasitic wasps; one egg parasitoid, Oobius agrili and three larval parasitoids- Spathius galinae, Spathius agrili and Tetrastichus planipennisi. This thesis examines some of the factors influencing the establishment of these larval biocontrol agents. Chapter 1 examines the relationship between woodpeckers and the parasitoids S. agrili and T. planipennisi. Both woodpeckers and these parasitoids attack the larval stage of EAB, which means their impacts overlap and potentially interact. To examine this relationship, I established parasitized larvae on ash trees and then used screening to exclude woodpeckers from some sections of the tree. Results show that while there is no evidence of discriminatory feeding for or against parasitized larvae, the presence of parasitized larvae changes woodpecker feeding behavior at a stand-level. I hypothesize that this change is due to these larval parasitoids being a low-food reward and that parasitism contributes to a change and decrease in patch quality, causing woodpeckers to quit foraging sooner than usual. My second chapter focuses on Spathius galinae, which was recently approved for release in the north central and northeastern US in 2015, to provide additional population control. Spathius galinae’s long ovipositor (4-5.3mm) is theoretically expected to help target EAB in ash with larger diameters and bark thicknesses. Using experimentally infested logs of varying thicknesses in the laboratory I tested the limits and preferences for oviposition of S. galinae, to understand its potential impact on EAB. My results demonstrated that although parasitism by S. galinae drops significantly when bark thickness reaches 8 mm, this prevents S. galinae only from reaching EAB larvae in my largest ash trees (S. galinae will play a vital role in providing additional control and in supporting ash regeneration in aftermath areas of EAB invasions.

To investigate the density-dependent factors that may be partly responsible for the apparent stability of hemlock woolly adelgid (HWA; Adelges tsugae Annand) populations in central New England, we infested 64 eastern hemlock (Tsuga canadensis Carrière) trees with varying densities of HWA sistens ovisacs in a typical eastern hemlock forest in western Massachusetts. We subsequently documented HWA density, fecundity, and the amount of new growth on experimental trees over two consecutive years. We used a 2 x 2 randomized block design using previously and newly infested hemlocks divided into 1 m tall saplings and branches of mature trees. There was a density-dependent decline in the survival and fecundity of HWA in both the spring and winter generations. This response was a function of both previous infestation by HWA and current years crawler density in the spring generation. Additionally, the production of sexuparae in the spring generation played a key role in the overall density-dependent survival of HWA, suggesting that sexuparae production is strongly linked to developing crawler density. In Chapter 2, varying densities of HWA were manipulated on 16 previously uninfested eastern hemlocks in an open field plantation. In contrast to HWA populations in our forest experiment, there was no evidence of density-dependent survival on a tree-wide basis in the plantation in the springtime progrediens generation. However, there were comparable density-dependent survival of settled crawlers and sexuparae production when samples of the population were examined from branches with high density. Plantation hemlocks had 9.3 times more foliage and ten times lower HWA densities per cm than forest hemlocks. Despite the lack of density-dependence in the progrediens generation of this study, HWA populations of the subsequent sistens generation went extinct on 13 out of the 16 trees. These results show that density-dependent processes may only exist when HWA density/cm reaches a certain threshold, and that high mortality may occur during the late-summer aestivation phase at densities below this threshold, perhaps due to high summertime temperatures in HWA’s sistens generation. These results may help explain the slow process of HWA establishment in our region.

There are thousands of species of mosquitoes, but only a handful of these species carry pathogens that cause human diseases. Here, we study two species, Aedes albopictus and Aedes aegypti, which transmit infections such as dengue, Zika, Mayaro virus, and La Crosse virus. Curtailing these diseases is a good reason to consider control of mosquito populations. However, mosquitoes are quite hardy and spraying of pesticides is typically a short-term solution. Thus, more long-term solutions require careful thought about mosquito populations, including early juvenile aquatic stages: egg, larva, and pupa. In this dissertation, we examine the factors that affect the dynamics of aquatic stages by creating mathematical models. The goal is to assess what key biological features most impact the total population. Both Aedes albopictus and Aedes aegypti lay eggs in small containers, producing limitations on space and food. We investigate how restricting resources changes development time, survival to adulthood, and body mass at emergence. The interactions between these changes are complicated, so to disentangle their effects we create three different mathematical models. The first model is discrete in time and focuses on the best way to incorporate the influence of larval density. We compare the impact of larval density by inputting seven different functional forms altering survival and development time. Larval density used in the model is determined from the average of the population size over the past one to thirty-six days. The second model is also discrete in time but focuses on the interaction between survival, development time, and mass. This model considers three levels of mass. Here, we use the density-dependent function determined from our first model and fit the maximum value for development time from experimental data. Survival values are fit using constants and a density-dependent function. Finally, growth is built in as a function of food. Food decreases at each time point as a function of the total larvae in the environment. We compare between model formulations with Akaike information criterion. The third model examines the ramifications of constricting resources on growth verses death. We employ a partial differential equation that has three independent variables: time, age, and mass. We find that density dependence is highly influential in the maturation of mosquitoes, and it is more essential to include its impacts on development time than on survival. These findings can be incorporated into a larger framework of disease dynamics, and give insight into better control of mosquitoes and disease spread. / Doctor of Philosophy / There are thousands of species of mosquitoes, but only a handful of these species spread human diseases. We specifically study mosquitoes that transmit diseases such as dengue and Zika, which plague large portions of the world. One way to reduce disease spread by mosquitoes is to eliminate the mosquito population, but mosquitoes are becoming resistant to commonly used insecticides. Thus, additional ways to combat mosquitoes are needed. To do so requires a better understanding of how mosquito populations change. This necessitates incorporating information from all life stages of the mosquito, including the juvenile stages that live in the water. In this dissertation, we examine various factors that affect the population size and characteristics of young mosquitoes by creating mathematical models. The goal is to assess what key biological features most impact the population. As young mosquitoes live in aquatic environments, there are limitations on space and food. We investigate how restricting resources changes the time it takes for mosquitoes to develop into adults, how many survive to adulthood, and what their body mass is as new adults. These outcomes interact in complicated way. To disentangle this, we create three different mathematical models. The first model focuses on the best way to incorporate the influence of limiting space or density on population size. The second model focuses on the interaction between mosquito body mass, survival, and time spent as a juvenile. The third model examines how resources alter growth and death and the ensuing mass of mosquitoes. Models are fit to experimental data and validated based on how accurately they describe known patterns. We find that population density is highly influential in the maturation of juvenile mosquitoes and its most important effect is on time spent as a juvenile mosquito. By correlating growth of juvenile mosquitoes to density and resources, our second model is better able to reproduce data. These findings provide important understanding on mosquito populations, which provides insight into how to better control mosquitoes and the diseases they spread.

Habitat loss is the leading cause of species extinction. Protecting and managing habitat quality is vital to an organism's persistence, and essential to endangered species recovery. We conducted an investigation of habitat quality and potential impacts from climate change to piping plovers (Charadrius melodius) breeding on the barrier island ecosystem of New York, during 2003-2005. Our first step in this analysis was to examined the relationship between two common measures of habitat quality: density and productivity (Chapter 1). We used both central and limiting tendency data analysis to find that density significantly limited productivity across many spatial scales, especially broader scales. Our analysis of plover habitat quality (Chapter 2) focused on 1) identifying the spatial scaling of plovers to their environment; 2) determining the relative importance of four aspects of the environment (land cover, predation, management, and disturbance); and 3) determining the key environmental variables that influence productivity. We found that plover habitat selection occurred within a narrow range of spatial scales that was unique to each environmental variable. Further, we found that management and predation variables influenced population-level productivity relatively more than land cover and disturbance. Environmental variables with a significant positive influence on habitat quality were land management units, plover conservation educational signs, and symbolic string fencing erected around plover nesting areas. We found a significant negative relationship among density of people on ocean beaches, herring gull density, and land cover degradation. To quantify possible impact to plover habitat from future climate change (Chapter 3), we examined the extent of habitat change resulting from different estimates of sea-level rise (SLR) and storminess over the next 100 years. We found that the particular SLR estimate, habitat response, and storm type used to model climate changes influenced the amount of potential habitat available. Importantly, we observed synergy between SLR and storms resulting in the increasing impact of SLR and storms on plover habitat over the next 100 years. Finally, we found that coastal development contributed considerably to habitat loss when combined with climate changes. Our findings raise concerns regarding current plover recovery goals and management strategies. Density-dependent productivity may threaten the goal of a joint increase in both plover population and productivity. We advocate density monitoring and allocation of alternative nesting areas to provide the relief of possible high-density limitations. Based on our analysis of habitat selection and climate change threats, we call for a shift in management focus away from known breeding areas, towards ecosystem processes. Long-term conservation of piping plover habitat quality is more likely through protecting and promoting natural barrier island dynamics (i.e. overwash and migration) and minimizing human development on the barrier islands of New York State.

Current eastern wild turkey (<I>Meleagris gallopavo silvestris</I>) harvest models assume density-independent population dynamics despite indications that populations are subject to a form of density dependence. I suggest that both density-dependent and independent factors operate simultaneously on wild turkey populations, where the relative strength of each is governed by population density. I attempt to estimate the form of the density dependence relationship in wild turkey population growth using the theta-Ricker model. Density-independent relationships are explored between production and rainfall and temperature correlates for possible inclusion in the harvest model. Density-dependent and independent effects are then combined in the model to compare multiple harvest strategies. To estimate a functional relationship between population growth and density, I fit the theta-Ricker model to harvest index time-series from 11 state wildlife agencies. To model density-independent effects on population growth, I explored the ability of rainfall, temperature, and mast during the nesting and brooding season to predict observed production indices for 7 states. I then built a harvest model incorporating estimates to determine their influence on the mean and variability of the fall and spring harvest. Estimated density-dependent growth rates produced a left-skewed yield curve maximized at ~40% of carrying capacity, with large residuals. Density-independent models of production varied widely and were characterized by high model uncertainty. Results indicate a non-linear density dependence effect strongest at low population densities. High residuals from the model fit indicate that extrinsic factors will overshadow density-dependent factors at most population densities. However, environmental models were weak, requiring more data with higher precision. This indicates that density-independence can be correctly and more easily modeled as random error. The constructed model uses both density dependence and density-independent stochastic error as a tool to explore harvest strategies for biologists. The inclusion of weak density dependence changes expected harvest rates little from density-independent models. However, it does lower the probability of overharvest at low densities. Alternatives to proportional harvesting are explored to reduce the uncertainty in annual harvests. / Ph. D.

<p>New and old mosquito-borne diseases have emerged and re-emerged in temperate regions over the recent past, but an understanding of mosquito population dynamics, a fundamental step toward disease control, remains elusive. In particular, we are still lacking reliable predictive models of mosquito abundance in temperate areas due to the subtle links between the fluctuation of mosquito population and highly heterogeneous environmental drivers. Hence, this doctoral dissertation presents an interdisciplinary approach towards an improved understanding and prediction of the fluctuations in mosquito abundance in temperate regions. In the first part of this dissertation a hierarchical Gompertz-based model is used to assess the relative importance of endogenous (density dependence) and exogenous (environmental forcings) controls and their interactions in regulating the dynamics of a West Nile Virus vector (Culex pipiens) in the Po River delta in Italy. The results clearly detect the effects of density-dependence in the observed population dynamics for the mosquito species analyzed and highlight the controls exerted by environmental forcings and habitat conditions. Subsequently, the characteristic scales of temporal variability in mosquito populations, and the representativeness of observations at different sampling resolutions, are investigated using a 10 year daily mosquito sample from Brunswick County, North Carolina. The species present in the sample (among which Aedes vexans and Culiseta melanura are addressed in greater detail, as vectors of East Equine Encephalitis and West Nile Virus) are investigated using a combination of time series analysis, individual based simulations, and density-dependent modeling approaches. Significant population fluctuations with characteristic periodicity between 2 days and several years are found in response to different regulation mechanisms. In particular, the observed fast fluctuations are importantly determined by a varying mosquito activity, rather than by reproduction/mortality processes, driven by rapid changes in meteorological conditions. Finally, in the third part of this study, a state space reconstruction (SSR) approach is used to understand how the predictability of mosquito abundance varies with aggregation time scale and with the prediction horizon, and how much can the prediction of mosquito abundance be improved by using daily observations compared to the commonly used once-per-week samples. The results show that the predictability of mosquito abundance decreases as the time scale of the models increases from one week to one month, while the predictability of per capita growth rate increases together with the modeling scale. It is also shown that the prediction of mosquito per capita growth rate can be improved using daily abundance observations. Furthermore, many mosquito models compare the observed and predicted abundance as a measure of model performance. However, my results suggest that short term forecasts of mosquito abundance may appear to have a significant capability due to the positive autocorrelation between abundance in subsequent time steps, even when the model's ability to predict the abundance change is low. Model capability should thus be evaluated comparing observed and modelled per capita rates of change.</p> / Dissertation

Measuring and assessing vital rates such as births and deaths are prerequisites for understanding population dynamics. Vital rates may be affected by the density of individuals, even though the importance of density dependence on population dynamics has been debated for a long time. The mallard Anas platyrhynchos is one of the foremost game species in the Holarctic, with millions of birds in hunters’ bags annually. Still, basic knowledge about regulation of mallards’ vital rates is poor, and experimental studies on this topic are rare. In this thesis I have studied survival patterns and density dependence in mallards breeding in Sweden and Finland. Long-term ringing data from both countries were analysed for mortality patterns and causation, as well as for e.g. survival rate estimation. Most of the studies were, though, experiments run over two years involving manipulations of the density of nests, broods and/or adults, in southern and northern Sweden, comprising different biotic regions. Common response variables were survival of nests, ducklings and hens, mainly analysed with program MARK. About 90% of the recovered mallards in Finland and Sweden were hunting kills. However, survival rates were high, ranging from 0.66 to 0.81 for most groups (sex*age). The generality of density dependence was evident since such processes were detected in all studies. Consequently, depredation rate was higher in high nest density compared to low nest density. Survival of ducklings was density-dependent in both boreal and nemoral biotic regions, with food limitation being evident in the former region but not in the latter. In spite of their generality, density-dependent patterns varied within as well between years, and for nest predation rates also between landscape types. The findings about density dependence in breeding mallards in this thesis are novel since they are based on experiments. They are potentially of general interest for management because they embrace a variety of lakes in two geographically distant areas, each being representative for large temperate areas in the northern hemisphere. Detection of density dependence at the local scale may be important at larger scales, too, following the principle of ‘ideal preemptive distribution’ in a source-sink dynamic system.