USING MARK-RELEASE-RECAPTURE AND DISTANCE SAMPLING METHODS TO ESTIMATE OREGON SILVERSPOT BUTTERFLY POPULATION SIZE AND DEMOGRAPHY

Butterflies are in decline wordwide. In the face of habitat loss and fragmentation, understanding population dynamics and dispersal behavior is critical for implementing effective conservation strategies. However, for many at-risk species, basic biological information is lacking, which limits our understanding of species’ needs and the success of recovery efforts. Additionally, commonly used monitoring strategies often fail to provide the information needed to adapt management approaches. This thesis addresses key knowledge gaps for the federally threatened Oregon silverspot butterfly (Argynnis = Speyeria zerene hippolyta), focusing on population size, adult demography, and dispersal. In Chapter One, we use two monitoring methods – distance sampling and mark-release-recapture (MRR) – to estimate population size and assess the relative reliability of current index count-based monitoring. We also investigate adult demography across three occupied sites and compare survival rates between captive-reared individuals released for population augmentation with wild origin individuals. We found that counts did not consistently scale with population size and that distance sampling and index counts showed opposing population trends, emphasizing the importance of accounting for detection probability to improve the reliability of population estimates. MRR also revealed substantial variation in lifespan across sites and lower survival rates for captive-reared individuals compared to wild individuals. These findings inform the development of a long-term monitoring strategy using distance sampling to reliably estimate population size and evaluate the ongoing effectiveness of augmentation. In Chapter Two, we used MRR data to assess adult dispersal in Oregon silverspot metapopulations occupying patchy landscapes. We compared total movement distances and patch-to-patch movements with a literature review of 18 other at-risk Nymphalid species. We found that Oregon silverspot butterflies have moderately strong dispersal capacity – similar to or greater than other medium-sized fritillaries – with total movement distances of several kilometers and frequent patch-to-patch movements. These findings suggest that management should focus on expanding the overall size of habitat networks to support larger population sizes. Together, these chapters emphasize the importance of species-specific data on movement, demography, and population size for informing habitat restoration and long-term monitoring. By using field-based research to support conservation management, this work provides key insights to advance evidence-based conservation of Oregon silverspot butterflies and other at-risk species in fragmented ecosystems.