Bat Use of Hollows in California’s Old-Growth Redwood Forests: From DNA to Ecology

SIMPLE SUMMARY: The extent of the use of tree hollows by bats is relatively unknown. However, these are vital habitats for cavern-dwelling bats to use when seeking rest, protection, hibernation, or reproduction opportunities. We collected bat guano from nearly 200 tree hollows to determine which species were present, using genetic markers, and which habitat characteristics influenced roost use. Our results indicate a different species composition than previously known in redwood trees (Sequoia sempervirens) on the North Coast of California. Based on the quantity of guano collected, more bats roosted in hollows with high ceilings and in forests with fewer small trees. Researchers may use the techniques of guano collection and DNA analysis presented here for the management and conservation of bat populations. ABSTRACT: The loss of roosting resources, either through disturbance or removal, negatively affects bats. Identifying sensitive species and determining roost requirements are critical components in conserving their habitat. Cavity-roosting bats on the North Coast of California are known to use hollows in large redwood trees. In this study, we examined the factors determining the use of basal tree hollows by different bat species at eight redwood forest sites in Del Norte, Humboldt, and Mendocino Counties, California. Bat guano was collected from 179 basal hollow roosts from 2017 to 2018, and guano mass was used as an index of roosting activity. Nine bat species and one species group were identified by analysis of DNA in guano. We made a total of 253 identifications from 83 hollows into the 10 species categories. The most prevalent species were Myotis californicus (California myotis; 28.5% of all identifications), the Myotis evotis-Myotis thysanodes group (17.4%), Corynorhinus townsendii (17.0%), and Myotis volans (15.0%). We evaluated the extent to which habitat variables at the scales of the hollow, vicinity, and site influenced the level of roost use. The correlations between guano mass and habitat variables were examined using generalized additive mixed models. At the hollow scale, guano mass increased with ceiling height above the opening. At the vicinity scale, guano mass increased with less cover of small trees. At the site scale, there was no association between guano mass and distance to foraging areas, elevation, or the number of nearby hollows. These tree hollow roost preferences can inform land managers when planning the management and conservation of redwood forests.