Calls, clutter and combustion: linking bat traits to forest structure and fire regime

Blog written by Rachel V. Blakey. Read the full paper here.

While this paper was in review, the deadliest and most destructive wildfire in Californian history spread across the western boundary of our study area, Plumas National Forest. Due to a multitude of factors including climate change, drought and fire suppression, coniferous forests of the Western US are experiencing increasingly more frequent and severe fires. For this reason, conservationists and natural resource managers seek to understand how wildlife are likely to respond to wildfire events of varying frequency and severity. My co-authors and I set out to better understand the relationships between bat communities and fire in the Sierra Nevada.

Why bats? They are a diverse group of mammals, with 33 species in the Western US alone, can fly long distances to take advantage of dynamic landscapes, respond to booms in prey availability following fire, and some species even preferentially roost in burned snags. Bats are also highly sensitive to forest structure, with diverse adaptations in their body size, wing and ear morphology and echolocation call characteristics that equip them to navigate and capture prey in a variety of structural habitats. For example, a larger bat with long, narrow (high aspect ratio) wings and a loud, long duration, low‐frequency call is well-adapted to the fast-paced aerial pursuit of beetles above the canopy. Conversely, a smaller bat with large ears, broad (low aspect ratio) wings and a soft, high frequency wide bandwidth call can detect and glean a moth from a leaf in a complex forest. Fire can dramatically change forest structure in both the short and long-term, so it follows that bat species, given different adaptations to forest structure, will respond differently to burned landscapes.

Using acoustic detections of bat calls to identify species at sites across a heterogeneously burned landscape in > 400,000 ha of National Forest, we recorded 17 bat species. These species differed in body size by an order of magnitude, from the smallest (canyon bat < 5 g) to largest (western mastiff bat > 50 g) bats in the United States, with calls and wing morphology also variable across the community. We investigated individual relationships between bat species occupancy probability (the likelihood of finding that species in a particular location or not), fire regime and forest structure variables for our 9 most commonly recorded species. Our results indicated that changes to forest structure were the main driving force of bat species relationships to burned habitats. However, one species, the little brown bat (Myotis lucifugus) was positively associated with more recently burned habitats, potentially because this species forages for aquatic emergent prey, that experience booms in productivity after fire.

While relating bat species to habitat is interesting, we can only hypothesize about why these relationships exist. Our next step was to employ trait ~ environment analyses to try to get closer to the “why” of these relationships. Could we identify adaptations that might make bats more or less likely to use burned areas of different severity or fire history? We found that bats with traits adapting them to forage in open areas were associated with higher severity and more frequent fires, while bats that were adapted to more cluttered environments were negatively associated with fire frequency and severity and positively associated with denser forests. Call traits (e.g. call bandwidth and duration) were the most strongly associated with the environment. The figure below, with illustrations by IBP Biologist Lauren Helton, illustrates how traits relate to bat usage of different forest structure and fire regimes. Examining trait ~ environment relationships has the added benefit of giving relevance to findings beyond the species or communities of the study region, making findings more relevant for international audiences.

Bat figureThough our study provides much-needed insight into how foraging bats use burned landscapes, many aspects of fire ecology, as related to bats, continue to elude us. Firstly, we know very little about how fire alters availability and quality of roosting habitat for bats. The majority of the world’s bats roost in vegetative structures including tree cavities, crevices, foliage and under bark, and fires can both create and destroy bat roosts. For example, two species threatened by White Nose Syndrome in the Eastern US preferentially roost in fire killed snags. In the Western US, we still know little about bat roosting ecology and almost nothing about how roosting opportunities are influenced by fire, however evidence suggests that one species, the long-eared myotis (Myotis evotis) avoids burned areas when roosting. Another previously unexamined question is how the configuration of burned patches in the landscape influences bats. Patchy, or mixed-severity fire may create high quality edge foraging habitat for a range of bat species, but the extensive high severity burned patches may only be suitable for open-adapted species. Finally, the entirety of peer-reviewed studies concerning bats and fire are from only two countries: Australia and the United States. Further support for studies in other regions where fire prone landscapes coincide with high bat diversity are needed.

 

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