Impact of restoration and habitat type on small mammal abundance and bat occupancy in California’s Sacramento-San Joaquin Delta

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Kristen A. Zumdahl, Michelle Mah, Danielle T. Fradet and Andrew Engilis Jr., Wildlife, Fish, and Conservation Biology, University of California, Davis, CA, Molly Ferrell, California Department of Water Resources, Sacramento, CA

Background/Question/Methods
The California Department of Water Resources (DWR) is restoring subsided agricultural lands on islands in California’s Sacramento-San Joaquin Delta to historic freshwater marsh and riparian forest habitat. We are monitoring volant and non-volant mammals in order to assess changes in species diversity, abundance and occupancy as a result of restoration actions. Our investigation into how multiple taxa occupy and respond to different habitats in the California Delta aims to inform restoration practices and will lay the foundation for long-term biomonitoring in the region.
During fall 2020 we conducted small mammal trapping and passive bat acoustic surveys across 35 study sites in DWR project locations. At each site, we established two passive bat acoustic survey stations and a trap line consisting of 50 Sherman live-traps. Both surveys were run concurrently for three consecutive nights. We used single-species occupancy models to measure habitat use by bats, and mark-recapture models to estimate small mammal abundance.
Results/Conclusions
We captured a total of 704 small mammals including California meadow vole (Microtus californicus), deer mouse (Peromyscus maniculatus), and western harvest mouse (Reithrodontomys megalotis), and introduced black rat (Rattus rattus) and house mouse (Mus musculus). At each of the 35 study sites, small mammal abundance was determined. Small mammal abundance was greater in restored habitat types, while abundance was significantly lower in unrestored agricultural/pasture habitat. House mouse and deer mouse abundance was highest in freshwater marsh sites. Western harvest mouse abundance was highest in riparian forests. California meadow voles had their highest abundance in unrestored agricultural/pasture habitat.
We analyzed 24 nights of acoustic recordings using Sonobat version 4. We examined how different habitat types determine bat occupancy in the California Delta. Brazilian free-tailed bats (Tadarida brasiliensis) had the highest predicted occupancy across the study area (0.93), followed by western red bats (Lasiurus blossevillii; 0.74), and silver-haired bats (Lasionycteris noctivagans; 0.25). Brazilian free-tailed bats had the highest habitat use in unrestored agricultural/pasture sites, while silver-haired bats preferred riparian forest and freshwater marsh sites. Yuma myotis (Myotis yumanensis), Little brown bat (Myotis lucifugus), and hoary bat (Lasiurus cinereus) were also identified, but had relatively low predicted occupancy ( <0.2) across the study area during the fall season.
Our preliminary results provide evidence for diversity and abundance gains for mammals and bat occupancy in restored areas. Additional spatial-temporal data in subsequent years will help us better measure trends related to the continuation of habitat restoration in the California Delta.