Browsing by Author "Hart, Sarah, committee member"

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Open Access
Occupancy dynamics of barred owls on a spotted owl landscape
(Colorado State University. Libraries, 2024) McGee, Kaitlin R., author; Doherty, Paul, Jr., advisor; Franklin, Alan B., advisor; Bailey, Larissa, committee member; Hart, Sarah, committee member
Invasive barred owls (Strix varia) pose a significant threat to native northern spotted owls (Strix occidentalis caurina) in western North America. Barred owls compete with northern spotted owls for resources and habitat, leading to displacement and subsequent declines in northern spotted owl populations. This impact highlights the need for conservation efforts to mitigate the negative consequences of barred owl presence in spotted owl habitats. In my thesis, I explored barred owl habitat use in northern California using data on both lethal removals of barred owls and barred owl occupancy of non-removal areas from long-term studies. These studies provided a unique opportunity to study associations of barred owl land use both during initial colonization and over time. Barred owl land use was influenced by landscape factors such as topographic water accumulation, forest age, tree composition, and canopy cover as well as interspecific effects of northern spotted owls. I used these results to predict variation in barred owl use of landscapes, which can then focus barred owl management efforts in California.
Open Access
The effect of multiple stressors on Kaua'i's forest birds: demography, health, and trophic cascades
(Colorado State University. Libraries, 2024) Romero, Ashley Cozette, author; Pejchar, Liba, advisor; Koons, David, committee member; Hart, Sarah, committee member
Island ecosystems harbor unparalleled biodiversity, providing habitat for a disproportionate share of the world's species relative to their area. Nevertheless, the vulnerability of islands to disturbances that result in negative consequences renders them among the most endangered environments. Bird populations on islands are particularly susceptible and often strongly affected by invasive species, habitat loss, climate change, and disease, all of which has resulted in widespread decline and extinction. In the Hawaiian Islands, native forest bird populations confront numerous threats, with the majority of remaining native bird populations classified as threatened, endangered, or teetering on the brink of extinction. To sustain and recover island bird populations, it is essential to understand the direct and indirect impacts of anthropogenic changes on forest bird populations, and to what degree each driver of change influences bird health, demographics, and habitat quality. The goals of this project were to quantify how various stressors influence the body condition and sex ratios of Kaua'i's forest birds, and to assess if primary and secondary forest regeneration processes that affect forest bird resource and habitat availability are influenced by invasive rodent control. To address these questions, I collected and analyzed three years of forest bird banding data in the Alaka'i Wilderness Preserve on Kaua'i's Island (Hawai'i, U.S.A.) from 2021 to 2023. Birds were banded in and outside of areas that have been subject to intensive rodent control for the past 5-9 years. I focused on four focal species of birds, three native and one non-native, and collected data to assess the body condition and sex of these captured birds. Mixed effects models were used to determine whether body condition was significantly associated with rodent control intensity on a local and landscape scale, malarial infection status, mean temperature during the peak mosquito breeding season, frequency of extreme rain events, canopy density, and topographic wetness index (TWI). Similarly, I used general linear models to test if rodent control intensity, malarial infection status, or date of capture was associated with a higher or lower likelihood of encountering a female. I found that body condition was negatively associated with positive malarial status for all but one of the focal species, and higher average temperatures and TWI were negatively associated with body condition for all species. None of the predictor variables evaluated were strong predictors of the sex of captured birds. To evaluate the effects of rodent control on forest regeneration processes that sustain bird habitat in the Alaka'i Wilderness Preserve, I examined whether rodent control was associated with changes in seed rain, fruiting density, and seedling emergence. Seed traps were established in February of 2023 and were checked periodically over a six-month period to obtain seed rain data from within and outside of areas of rodent control. Seeds collected from traps were identified to the lowest possible taxonomic level. During each visit to collect seed trap contents, the number of fruits on all fruiting plants within five meters of each seed rain trap was estimated. These estimates were used to calculate relative fruiting densities in treatment and reference plots. To examine seedling emergence, emergence plots in treatment and reference plots were cleared of all vegetation in July of 2022. I used Mixed Effects Models to compare seed rain, fruiting density, and seedling emergence between sites with and without rodent control and found no differences in seed rain, seedling emergence, or fruiting density within and outside of rodent control. My findings suggest that variables that are related to disease transmission may be more influential in predicting the health of forest birds than variables related to predation risk or food and resource availability. These findings also suggest that rodent control, at least at current levels of intensity, may not play a significant role in forest regeneration in Kaua'i's 'Alaka'i Plateau. Further research is needed to understand the effectiveness of rodent control in this system, but reducing the prevalence of avian malaria and mitigating climate change will be critical to alleviating sub-lethal effects on Kaua'i's forest birds. This study contributes to our understanding of the sometimes complex and synergistic effects of invasive predators, disease, forest structure, and climate on forest birds and the dynamics of insular ecosystems. My findings and approach could have implications for conservation and restoration in the many areas globally where rat invasion, climate change, and disease may interact to pose similar threats.
Open Access
Wildfire effects on host-parasite interactions in freshwater streams
(Colorado State University. Libraries, 2024) Svatos, Emma C., author; Preston, Daniel, advisor; Hart, Sarah, committee member; Wells, Caitlin, committee member
Wildfires are increasing in intensity and frequency globally, accentuating the need to understand the implications of fire on community interactions. While previous research has focused on fire effects on free-living species, our knowledge of how wildfires influence parasite interactions with hosts and predators remains limited, especially in freshwater ecosystems. This thesis addresses this knowledge gap and presents results from two distinct multi-year observational field studies that explore how wildfires influence interactions among parasites, hosts, and predators in freshwater streams. In Chapter 1, I used a Before-After-Control-Impact design to compare freshwater snail (Juga plicifera) host populations and trematode parasite communities in Oregon streams before and after wildfire disturbance. In Chapter 2, I investigated host-parasite-predator interactions involving mermithid nematode parasites (Family Mermithidae), mayfly hosts (Order Ephemeroptera), and trout predators (Salvelinus fontinalis, Salmo trutta, and Oncorhynchus clarkia) in the southern Rocky Mountains after severe wildfires. Despite substantial changes to stream habitat, snail host populations and trematode infection patterns and community structure remained relatively stable following fire disturbance in Oregon streams; however, I observed subtle taxon-specific responses to fire, suggesting changes in abundance or behavior of definitive hosts. In Rocky Mountain watersheds, mermithid parasite patterns varied considerably over time in burned streams, which coincided with similar responses in mayfly host densities, suggesting that wildfire indirectly affected mermithid infection patterns through host-density changes in the stream. Host attributes also influenced parasite interactions, as intermediate-size Baetidae mayflies experienced the highest probability of infection. Furthermore, infection prevalences of mayflies consumed by trout were positively related to mayflies in the benthos; yet, infection prevalences in trout stomachs were lower on average, potentially due to parasite-induced behavioral changes in infected mayfly hosts that reduced susceptibility to predation. Wildfire did not seem to affect rates of this predator-parasite interaction, as instances of concomitant predation remained consistent in burned streams over time. Comparing two different host-parasite systems sheds light on how environmental variables and host-parasite ecologies mediate wildfire effects on parasite interactions with hosts and predators. Together these findings expand our knowledge of parasite ecology in aquatic macroinvertebrate hosts, offer insights into the role of parasites in energy flow through food webs and as bioindicators of environmental change, and help integrate parasite interactions into our understanding of disturbance ecology in freshwater streams.