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Coast Redwood Forests as Refugia for Bats Under Global Change


Anthropogenic climate change poses an existential threat to biodiversity that is compounded by habitat conversion, disease, and other disturbances. Understanding the ecological needs of sensitive species and identifying priority habitat is vital for wildlife conservation. However, there is growing recognition that the traditional conservation emphasis on protected areas is not sufficient to sustain biodiversity. We also need to understand the habitat value of working lands and incorporate these areas in conservation planning. My dissertation explores these themes through the lens of bats in the California coast redwood ecosystem. Bats are extremely sensitive to climate and habitat disturbance, making them useful indicators of ecosystem health. Coast redwood forests may provide critical habitat for bats under climate change because coastal effects on temperature and fog patterns buffer the coast from the extreme temperatures and drought experienced inland. Although 13 species of bats are found in coast redwood forests, bats are an especially elusive taxon and much about their basic ecology remains unknown. In addition to climate change, hibernating bat populations in North America are threatened by an emerging disease called white-nose syndrome. The disease has newly spread to the west coast, heightening the urgency to understand the ecology of western bat populations.

I conducted research on the factors shaping habitat suitability for bats across the coast redwood ecosystem, the environmental drivers behind species activity, and how species activity patterns shift seasonally. In Chapter 1, a large landscape field study demonstrates that both working forests and protected areas provide valuable summer bat habitat, but species vary significantly in their sensitivity to microclimate and forest habitat conditions. In Chapter 2, I complement conventional, ground-level acoustic survey techniques with canopy-level monitoring. This comparison reveals that treetop deployment methods significantly increase the detection of tree-roosting and migratory species across all forest management types and all seasons, revealing new insights about niche partitioning and seasonal bat activity. In Chapter 3, the results of winter bat activity surveys were examined in relation to microclimate, and daytime maximum temperature was determined to be the variable that best explained variation in nightly probability of bat acoustic presence. The winter data also show that 11 bat species are detected in coast redwood forests during the winter, but species vary in their winter activity levels, which may have ramifications for population susceptibility to white-nose syndrome. In the final chapter, the implications for bat conservation of laws and policies regulating California timberland are discussed, and ways to improve protections are recommended based on scientific understanding of species ecology.

Taken together, the findings from this dissertation highlight the value of coastal forests as habitat for bat species threatened by climate change and emerging diseases. Species of special concern are present and active in coast redwood forests year-round. However, the presence of high-flying species was found to be systematically underestimated by ground-level survey methods, especially during the winter. Standard survey protocols may thus bias assessment of forest use and management impacts by primarily detecting species that make frequent use of understory habitat. The results of this research that includes year-round, canopy-level monitoring and advanced approaches to statistical analysis can be used to improve bat monitoring efforts and identify priority habitat to conserve bat species under global change.

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