Graduation Date
Spring 2026
Document Type
Thesis
Program
Master of Science degree with a major in Natural Resources, option Wildlife
Committee Chair Name
Sharon Kahara
Committee Chair Affiliation
Community Member or Outside Professional
Second Committee Member Name
Daniel Barton
Second Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Third Committee Member Name
Ho Yi Wan
Third Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Keywords
Amphibians, California Central Valley, Occupancy models, Western toads, American bullfrogs, Environmental DNA methods
Subject Categories
Wildlife
Abstract
Drastic losses of historic wetland habitat in the California Central Valley have caused widespread population declines and extirpations of aquatic breeding amphibians. Efforts to restore wetland habitat have included private land, which represents 70% of land tenure in the Central Valley. Management of these restored wetlands is focused on waterfowl with the assumption that they will provide habitat for native herpetofauna. This study used combined surveys to test the assumption that restored wetlands on private land provide habitat for native amphibians with the western toad (Anaxyrus boreas) as a case study. The objectives were to use environmental DNA (eDNA) survey and visual encounter survey (VES) methods to obtain presence/non-detection data for modeling the occupancy of restored wetlands by western toads in relation to local and landscape habitat characteristics as well as the presence of invasive American bullfrogs. Thirty sampling locations including 8 restored wetlands across three sub-basins in the Central Valley were surveyed 4 times for eDNA between May and October 2021. Vegetative cover, water depth, and water temperature were remotely measured at each wetland with uncrewed aerial vehicles (UAV) and data loggers. Surrounding land use was estimated using reclassified satellite imagery. A total of 57 water samples were successfully processed with digital droplet PCR analysis. Positive eDNA and VES detections were combined for the occurrence dataset and single-season occupancy modeling was applied to model influences of local and landscape habitat characteristics on western toad and American bullfrog occupancy. Western toad detections were minimal (n=5) during the study period while bullfrogs were detected extensively (n=25) in the study area. The use of combined survey methods increased detection probability for western toads while bullfrog occupancy may have been negatively influenced by water flow rate. Probability of bullfrog occupancy slightly decreased with the presence of tree cover and slightly positively increased with increasing water depth and presence of emergent vegetation. Probability of bullfrog occupancy may have a positive association with the proportion of wetland within a 2 km buffer. The small sample size of the data led to weak support for the top model parameter estimates.
Citation Style
JWM
Recommended Citation
Hernandez, Kaitlyn Breana, "Aquatic habitat use by western toads and American bullfrogs in California’s Central Valley: Effects of local habitat characteristics and landscape composition" (2026). Cal Poly Humboldt theses and projects. 2540.
https://digitalcommons.humboldt.edu/etd/2540