Bat roosting strategies and torpor expression in a wildfire-affected landscape during summer

Abstract

Abstract Background Climate change has caused several alterations in the frequency, intensity, and severity of wildfires globally, particularly in the western United States. Wildfire can dramatically change the microclimate experienced by animals who inhabit fire-prone areas, with implications for energy expenditure, particularly for heterothermic species. Heterothermic mammals manage energy expenditure by selecting optimal microclimates and regulating their body temperature. Because bats frequently use torpor, they are a useful model organism to understand how wildfires affect heterothermic mammal communities. In 2020, Sequoia and Kings Canyon National Parks were subjected to a severe wildfire that spanned 3636 ha within the boundaries of the park. We tracked eight California myotis (Myotis californicus) captured in Sequoia National Park to 22 roost sites in a gradient of burn severities in the summer from June–August 2021. We also quantified the torpor expression of bats on this post-burn landscape by measuring the roosting skin temperature of four M. californicus captured in a fire-affected site using temperature-sensitive radio telemetry. Results M. californicus showed preference for taller and larger diameter trees with more exfoliating bark, although they did not show preference for whether individual trees were burned or unburned. At the roost habitat-scale, 21 out of 22 M. californicus roosts were within or < 5 m from the burn mosaic, despite availability of unburned landscape. M. californicus most frequently roosted in low-severity burned areas, as opposed to unburned or severely burned areas. Additionally, bats avoided areas without a canopy and used areas with taller trees in proportion to their availability. Myotis californicus used torpor during the coolest periods of the morning before sunrise and regularly used torpor in low-severity burn area roosts. Conclusions This research indicates that while fire may be beneficial to some bat species, those that are clutter-tolerant could be negatively affected by severe wildfires that cause major reductions in vegetative complexity. Bats in our study appear to use both habitat selection and torpor to manage their energy and water budgets. Protecting large diameter trees with exfoliating bark, like those used by bats in our study, could be key to promoting the persistence of M. californicus on this landscape as severe wildfire may reduce these roosts. In addition, restoring historical mixed-severity fire regimes, thereby creating a fire mosaic containing low severity burned areas, may be critical for maintaining both roosting and foraging habitat suitable for our study species in the fire-adapted coniferous forests of North America’s Sierra Nevada.