This dissertation addresses the escalating threat of aridification to global freshwater ecosystems due to anthropogenic climate change, focusing on South-Central Texas, USA, using a space-for-time approach along a precipitation gradient from semi-arid to sub-humid. Over the 2017-2020 survey period, I integrated community, stable isotope, climate, and hydrologic data.In Chapter 2, my initial assessment of fish and invertebrate communities along the precipitation gradient unveiled compositional shifts and other nuanced responses. Positive correlations between fish diversity and rainfall, coupled with unexpected invertebrate diversity patterns, underscored the role of water quality in shaping fish assemblages. Drier conditions imposed abiotic filters, reducing diversity and favoring taxa with specialized adaptations, particularly in semi-arid systems with a dominance of euryhaline and live-bearing taxa. Chapter 3 examined differences in allochthonous and autochthonous dependencies as well as overall food-web structure across the precipitation gradient. In drier climates, a shift from insect predation to herbivory was apparent, supported by stable isotope data indicating increased autochthonous assimilation, wider resource use, and reduced trophic levels. Invertebrate metrics suggested limited bottom-up effects, while niche dimensions indicated top-down control at Sub-Humid and competition at Semi-Arid and Transition sites to be important drivers of food web structure. In summary, aridification intensifies autochthonous production, fish herbivory, and invertebrate dietary overlap in semi-arid streams. Chapter 4, exploring ecological connectivity in coastal rivers, focused on inconspicuous amphidromous species. Stable isotope analysis quantified substantial estuarine assimilation and highlighted dam impacts on connectivity. Significant downstream to upstream connectivity, driven by completely amphidromous taxa, emphasized vulnerability to interruptions by dams, urbanization, and climate change. Dominance of Fundulidae, Cyprinidontidae, and Palaemonidae families facilitated estuarine nutrient subsidies. Chapter 5 addressed ecological impacts of hydrological disturbances, challenging prevailing notions about flood effects. Results revealed nuanced relationships influenced by long-term precipitation patterns, with varied impacts of drought events based on precipitation regimes. These findings contribute to a refined understanding of climate, hydrology, and fish communities, offering insights into how different precipitation regimes shape responses to hydrological disturbances. Specifically, my data highlighted Poeciliid resilience and reduced centrarchid abundances during hydrological droughts in hot and arid summers. This dissertation unveils nuanced ecological dynamics within semi-arid ecosystems, where abiotic filters, influenced by water quality, shape fish communities. The prevalence of euryhaline and live-bearing taxa, along with amphidromous species, underscores the vital role of estuarine connectivity in maintaining resilient coastal rivers, especially in arid climates. My findings elucidate intricate ecological responses to floods, droughts, and seasonality, contingent upon long-term precipitation patterns. As regions teeter on the brink of transitioning from mesic to semi-arid climates, my results foreshadow analogous transformations in stream ecosystems. This scholarly pursuit represents a substantial stride in advancing ecological understanding, offering vital insights for adept stewardship amid the challenges presented by climate-induced alterations.

Anthropogenic noise, which is increasing globally, affects birds from gene expression up through alteration of community composition. as urbanization pushes further into undisturbed habitat, noise often disperses birds away from the point source. The impacts of this dispersal on surrounding quieter areas is not well understood. Therefore, in the first chapter, we sought to understand how noise-related dispersal affected the sociality of groups of songbirds as they moved away from the source of noise. as the displaced birds would likely be forced to occupy a smaller area that may already have resident individuals, we predicted that displaced birds would show a tighter clustered social network that may include new individuals, and that individuals within the flock would have more social connections overall. in the second chapter, we were interested in chronic, inescapable noise, the kind that birds living in urbanized areas might experience, and how that affects sociality. We predicted that birds would distribute themselves along a noise gradient and prefer the quieter areas and that the resulting density of birds would increase both flock clustering and individual sociality. We tested these ideas in two songbird systems, free-living red-backed fairy wrens (Malurus melanocephalus) to study dispersal and captive domesticated zebra finches (Taeniopygia guttata) to study chronic noise exposure. Contrary to our prediction, the free-living red-backed fairywrens became less social when dispersing from noise, as measured by node strength, during experimental noise treatments. Additionally, these birds tended to shift their space use away from the sound source. in our captive system, our hypothesis was supported that both flock cluster and individual sociality increased with noise treatments. However, the birds did not alter their distribution in accordance to the noise gradient, thus we propose support for the Increased Threat Hypothesis and suggest that increased vigilance was responsible for flock clustering and sociality. If social networks are altered consistently, there may be implications for future breeding success, detection of communication signals, and even for pathways of disease transmission among individuals.

Accelerated sea-level rise poses a significant threat to coastal habitats. Salt marshes are critical coastal ecosystems, providing a host of services such as storm protection, food production, and carbon storage. Persistence of salt marshes in the face of rising sea levels relies, in part, on vertical accretion. Current ecogeomorphic models and empirical studies emphasize the importance of the positive relationship between plant production and vertical accretion via sediment trapping by stems aboveground and belowground organic matter production. Thus, changes in plant production influence salt marsh persistence with sea-level rise. However, studies and models of marsh accretion do not consider the effects of animal-mediated changes in plant production. Here, I tested how two co-occurring marsh crustaceans, Uca pugnax (marsh fiddler crab) and Sesarma reticulatum (purple marsh crab), which have contrasting effects on smooth cordgrass (Spartina alterniflora) production, indirectly influence sediment deposition and belowground organic matter contribution, through observational surveys and field manipulation. S. reticulatum feeds directly on S. alterniflora, while U. pugnax facilitates S. alterniflora production through burrowing and biodeposits. I found that U. pugnax facilitated S. alterniflora biomass in some marshes, but not others. However, this facilitation of S. alterniflora biomass did not enhance sediment deposition. U. pugnax had no effect on belowground components of vertical accretion (i.e. root production and decomposition). These results suggest that in isolation, U. pugnax has little impact on saltmarsh geomorphic processes. S. reticulatum reduced S. alterniflora above- and belowground biomass; however, sediment deposition increased as S. alterniflora biomass decreased, contrary to models of ecogeomorphology. This trend was likely due to sediment being resuspended by crab bioturbation, as U. pugnax abundances were higher in S. reticulatum-grazed areas than in non-grazed areas. When U. pugnax occurred in areas of low S. reticulatum grazing, S. alterniflora biomass and sedimentation was similar to areas with only U. pugnax. I suggest that the negative impacts of S. reticulatum are exaggerated when intense grazing results in completely unvegetated areas and subsequent increases in U. pugnax density, where bioturbation erodes sediments. Thus, while S. reticulatum can increase the susceptibility of marsh sediments to physical erosion by removing vegetation, it may also do so by facilitating U. pugnax bioturbation. However, when S. reticulatum grazing intensity is low, facilitation of S. alterniflora growth by U. pugnax can mitigate the negative effect of grazing, which suggests that the net effect of these species may depend on their relative abundance. This study demonstrates that ecological interactions, in addition to physical processes, have significant effects on marsh persistence as sea level rises, and merit incorporation into ecogeomorphic models and empirical studies of marsh accretion.

The supply of new individuals into a population is one of the most important factors impacting species distributions and ecological interactions within a community. For marine invertebrates with complex life cycles, the supply of new individuals into a population can be influenced by factors experienced throughout their life history—before, during, or after metamorphosis. In recent years, scientists have begun to take a more holistic approach to understanding marine population assemblages by considering links between early life stages. When experiences in the pre-metamorphic life stages impact post-metamorphic life stages, this is known as carry-over effects. Because carry-over effects impact fitness of individuals, they could determine which individuals are recruited into the population and ultimately influence adult population structure. Using the keystone sea star Asterias forbesi, I tested how carry-over effects of larval food environment influence post-metamorphic performance in juveniles. I also tested whether carry-over effects could be compensated for if juvenile sea stars are fed juvenile mussels. Larvae were reared to metamorphosis under high larval food concentration and low larval food concentration. to test for carry-over effects of larval food concentration, my response variables at metamorphosis were survival, age, juvenile area, and juvenile spine number. to test if carry-over effects could be compensated for, each juvenile sea star was reared for 2-3 weeks on a juvenile feeding treatment of unfed, 1 juvenile mussel week-1, 3 juvenile mussels week-1, or 6 juvenile mussels week-1. My main response variables for the juvenile feeding experiment were mussel mass consumed and juvenile growth rate. I predicted that juveniles that settled early would experience the most severe carry-over effects, so I conducted the juvenile feeding experiment on the first settlers (“early”) and settlers that delayed their metamorphosis relative to the first settlers (“late”). Overall, I found that A. forbesi larvae reared under low food concentration took longer to reach metamorphosis and settled as smaller juveniles with fewer spines compared to those juveniles reared on high larval food concentration. For early settlers, juveniles from low larval food background metamorphosed at smaller sizes, so they reduced feeding and had lower mean growth rates compared to juveniles from high larval food background. Therefore, carry-over effects significantly impacted early settler performance, and this could not be overcome through juvenile feeding. However for late settlers, there was no significant difference in area at settlement between juveniles reared from high versus low larval food background. Therefore, carry-over effects of larval food environment were not present among late settlers, and thus there were no differences observed in juvenile performance. The differences observed between early and late settlers suggest that there may be a trade-off between larval duration time (i.e. delaying metamorphosis) and post-metamorphic performance.

Plants play an important role in structuring ecological communities from the bottom up through interactions with herbivores, and environmental variation can affect these interactions. We use the interaction between common milkweed (Asclepias syriaca) and the monarch butterfly (Danaus plexippus) to examine 1) the role of environmental variation in dictating plants traits, and 2) how those variations affect herbivores. We quantified intraspecific trait variation in 53 natural common milkweed populations, then remeasured these traits when population representatives were regrown in a common garden to control for environmental variation. We then measured growth, performance, and survival of monarch larvae feeding on these same plants. Our findings indicate distinct spatial patterns in traits throughout the range of A. syriaca, but these patterns dissipate when genets are regrown in a common environment. When monarch larvae are raised on these milkweeds, those fed on plants from the Northeast gain more weight than those fed on plants from the Northcentral and Southcentral regions. These results can better inform monarch conservation efforts; current conservation efforts have been focused on milkweed restoration in the Midwest, but an increased focus on milkweed restoration in the Northeast may be beneficial. Furthermore, we demonstrated plasticity in specific plant traits in response to environmental change, which could have theoretical implications in light of current and projected changes in climate. *The most recent version of this work is published in The American Journal of Botany in March 2021 (v. 108, iss. 3). Doi: https://doi.org/10.1002/ajb2.1630

Foundation species, such as seagrasses, provide many ecosystem functions in coastal habitats and support diverse food webs. However, environmental changes and human impacts to coastal ecosystems worldwide are threatening these biogenic habitats. When these changes cause shifts in the identity of foundation species, whole food webs dependent on those habitats can be altered. Seagrass meadows in Chesapeake Bay are an ideal study system to investigate these shifts, because the identity of the dominant seagrass in the lower Bay is currently shifting due to climate change. As water temperatures have risen, the once dominant species, Zostera marina, has been declining, while Ruppia maritima has been expanding. We performed quantitative surveys on epifaunal, infaunal, and nektonic communities of meadows with different seagrass species composition to examine how the structure and function of faunal communities differ between these two seagrass species. We found that seagrass bed type impacted epifaunal, infaunal, and nektonic abundance, biomass, secondary production, and diversity differently depending on trophic level. Epifaunal abundance was higher in Ruppia beds, but evenness was higher in Zostera beds. Furthermore, while epifauna biomass and secondary production per unit plant biomass was higher in Ruppia beds, biomass did not differ per unit area and secondary production was comparatively higher in Zostera beds suggesting that shifts to larger fauna and higher Zostera plant biomass per unit area offsets the increased abundance of small fauna in comparatively low biomass Ruppia beds. And while species composition of deep dwelling infauna differed by bed type, we observed few differences in infaunal abundance, biomass, and production between bed types. Based on current bay wide seagrass coverage, Ruppia has approximately 28.6% less gigagrams standing biomass of epifauna and infauna and 22.2% less gigagrams secondary production than Zostera. Moreover, in the water column, nektonic abundance, biomass, and richness all increased with the proportion of Zostera coverage relative to Ruppia coverage. We interpret these patterns to show that replacement of Zostera by Ruppia in Chesapeake Bay will reduce overall biomass and production of invertebrates and mobile fauna and shift invertebrate size distributions to smaller species. Overall, this study advances our understanding of how the shift in seagrass species occurring in Chesapeake Bay impacts the food web and serves as a case study for predicting how changes in the identity of foundation may affect community structure in other estuaries.

The Antarctic fish fauna is unique, with approximately 88% of the ~375 known species endemic to the Southern Ocean. One group, the perciform suborder Notothenioidei, dominates fish richness and biomass. These fishes possess several traits that allow existence in frigid conditions, but also limit their ability to cope with temperature increases due to climate change. In the western Antarctic Peninsula (WAP) region, a combination of anthropogenic impacts, including climate change and fishing pressure, threaten these fishes. The WAP is one of the most rapidly warming regions globally (3.4 °C per century) and has the highest fishing pressure for Antarctic krill in the Southern Ocean, which can result in the bycatch of larval nototheniods. The goal of my dissertation is to utilize interdisciplinary techniques to better understand the diversity of these unique fishes, as well as more accurately predict how their early life stages will respond to future conditions. Data in my dissertation are sourced from the Palmer Antarctica Long-Term Ecological Research (Palmer LTER) program, established in 1990 to study the pelagic ecosystem of the WAP. Larval fishes are captured annually as bycatch in Palmer LTER zooplankton net tows and represent the longest running (30+ years), fisheries-independent time series of Antarctic fishes. In Chapter 2, I utilize this novel time series to model the impacts of environmental change on Pleuragramma antarctica (Nototheniidae) larvae. Adult P. antarctica use sea ice as spawning habitat and are important prey for penguins and other predators. I show that warmer sea surface temperature and decreased sea ice are associated with reduced larval abundance, indicating these keystone fish could disappear regionally in the near future. In Chapter 3, I employ a combination of morphological and genetic analyses to describe unknown early life stages of a rare notothenioid genus (Akarotaxis; Bathydraconidae), which led to the discovery of a new species, Akarotaxis n. sp., that is described in Chapter 4. These two Chapters provide information on the evolution of notothenioids and biology of Akarotaxis n. sp., which appears to be endemic to the WAP region. In Chapter 5, I model biotic and abiotic drivers of phenology and growth of the larval fish assemblage near Palmer Station on the WAP. Several fish species are positively correlated with copepod and diatom abundances, which are modulated by interannual changes in sea ice. In addition, I model dispersion pathways of nototheniid yolk-sac larvae to locate their potential spawning areas. In Chapter 6, I use thermal tolerance experiments to study the physiological vulnerability to warming of larvae from three notothenioid families (Nototheniidae, Channichthyidae, and Artedidraconidae). Thermal tolerance increases with body length in larval channichthyids, suggesting that younger, less motile larvae may be especially susceptible to rapid warming events such as marine heatwaves. Results also indicate that the artedidraconid species Neodraco skottsbergi possesses one of the highest thermal tolerances reported for endemic notothenioids of any life stage. I offer ecological and evolutionary hypotheses that may explain this remarkable tolerance. My research provides multifaceted information on the vulnerable early life stages of fishes in a rapidly changing ecosystem. These insights inform future research priorities and will significantly contribute to the ongoing development of Marine Protected Areas in the Southern Ocean.

Prenatal acoustic communication affects embryonic development in ways that modify, and potentially optimize, the postnatal phenotype. In zebra finch (Taeniopygia guttata), prenatal exposure to a heat-induced parental call affects the behavior, growth, and ultimate fitness of individuals in hotter environments. However, observing the effects of heat calling on the postnatal phenotype does little to inform our understanding of how embryos are immediately perceiving and reacting to this signal. Here, we tested whether prenatal exposure to heat calls induces an immediate response from embryos that can be detected by changes in vibrations at the egg’s surface. More specifically, we used laser Doppler vibrometry (LDV) to measure the frequency and amplitude of egg vibrations to assess (i) whether unstimulated eggs exhibit a generalized vibration profile, and (ii) whether this profile changes in response to heat calls played back at four different amplitudes. We found variation in vibration intensity across many frequencies in unstimulated live eggs compared to dead eggs, suggesting randomness or idiosyncrasy in embryonic movements. However, a discernible peak in vibration amplitude between 40 to 45 Hz could be an important identifier for eggs at this stage of development. Importantly, exposure to heat calls affected a two-fold increase in the proportion of eggs exhibiting changes in the intensity of 10 to 15 Hz vibrations. Although the mechanism affecting this response remains unclear, this result provides evidence that developing altricial bird embryos are capable of both perceiving and responding to acoustic stimuli. Moreover, this immediate vibration responses to parental calls supports the potential for communication between parents and their embryos in the nest. Although we acknowledge that anthropogenic sources of noise and other substrate-borne vibrations may affect the transmission and subsequent benefit of this subtle form of communication, egg vibrations may nevertheless be an important tool for measuring embryonic perception.

Plants, unlike animals, lack the ability to quickly move to desirable locations, which poses a challenge when faced with climate change. Here, we use anthropogenic forest canopy loss in the eastern United States as a model for understanding the effects of climate change. Removal of the forest canopy significantly alters the abiotic habitat, as does climate change, by increasing the variability of moisture, light, and temperature. One powerful mechanism plants can use to cope with environmental change is hybridization, as it provides novel genetic variation. Hybridization, followed by introgression, makes it possible to pass along adaptive traits between species, which is crucial for safeguarding populations in challenging climates. Our study investigates the system of Asclepias syriaca (common milkweed) and Asclepias exaltata (poke milkweed) to understand how plants are responding to anthropogenic landscape changes. In areas where the forest canopy has been removed, these two species meet at an abrupt edge. Previous investigation found evidence of prolific gene flow and potential introgression between the two species, suggesting an adaptive benefit to having A. syriaca alleles. To explore further, we defined the native habitats of A. exaltata and A. syriaca and examined the adaptive potential of milkweed hybrids using both wild and purchased seed. Our results demonstrate that hybrids are not only intermediate in morphology but also physiologically, particularly in responses to light, herbivory, and desiccation stress. The plasticity these hybrids show suggest that they have the tools to adapt and pass on beneficial traits through gene flow. It also demonstrates how in a single generation, plants can show adaptive responses, which is imperative to the survival and genetic longevity of sessile organisms. These findings have important implications as anthropogenic activity is likely to continue altering light availability through forestry and urban development. Additionally, there will be pressure from a more variable climate, which will have trickle-down effects on plant-insect interactions. Our study highlights the adaptive potential of hybridization in plants and its importance in facilitating their response to climate change.

The Atlantic States Marine Fisheries Commission (ASMFC), the governing body responsible for managing fisheries on the U.S. East Coast, formally adopted the use of Ecological Reference Points (ERPs) for Atlantic menhaden, Brevoortia tyrannus. Scientists and stakeholders have long recognized the importance of menhaden and predators such as ospreys, Pandion haliaetus, that support the valuable ecotourism industry and hold cultural significance. Landings in the reduction fishery are at their lowest levels and menhaden is facing potential local depletion. Mobjack Bay, located within the lower Chesapeake Bay, has been a focus of osprey research since 1970 and represents a barometer for the relationship between osprey breeding performance and menhaden availability. Since local levels of menhaden abundance were not available, we conducted a supplemental feeding experiment on osprey pairs during the 2021 breeding season. Our main objective was to determine if the delivery rate of menhaden had an influence on nest success and productivity. Nest success (χ2 = 5.5, df = 1, P = 0.02) and productivity (β = 0.88, SE = 0.45, pseudo r2 = 0.14, CI = 0.049, 1.825, P = 0.048) were significantly higher within the treatment group. The added average biomass/d/nest (β = 0.03, SE = 0.009, pseudo r2 = 0.59, CI = 0.01, 0.05, P = 0.02) and energy content/d/nest (β = 0.02, SE = 0.005, pseudo r2 = 0.64, CI = 0.006, 0.03, P = 0.02) had an influence on pairs reaching maintenance reproductive rates (1.15 young/pair). Reproductive rates within the control group were low and unsustainable suggesting that current menhaden availability is too low to support a demographically stable osprey population.