Chiroptophobia, the fear of bats, is widespread throughout the world, but also subject to the cultural biases of different regions.  In Europe, bats were historically associated with the Devil, evil spirits and witchcraft. Dante’s Inferno describes the Devil’s wings as being very much like bat wings in form and texture. Vampirism was well established in Eastern European folklore before Bram Stoker’s depiction of Count Dracula routinely transforming himself into a huge vampire bat. Other regions of the world are historically more nuanced in their perspectives. For example, in Madurai, India, worshippers of the Muni god revere the Indian Flying Fox, Pteropus medius, and protect bat colonies from harm.  In Pudukkottai, Pteropus bats are guardians of sacred groves, while in Bihar these same bats bring wealth. But in the Punjab region of India magicians use bat blood to do malevolent magic, and across the border in some regions of Pakistan, bats are associated with evil witchcraft. This is only the tip of the humans/bats cultural iceberg.  For a thorough consideration, you should go to https://www.intechopen.com/chapters/80107.

Can ecologists help us resolve this conundrum? The answer is also nuanced.  On the negative side, bats live in dense colonies, are very social and relatively long-lived.  Taken together, these traits allow them to harbor many pathogens, including rabies and coronaviruses, which may be passed on to humans.  On the positive side, bats consume many insects including those that carry diseases.  Recent research has also shown that bats consume insects that eat crops.  Thus, in agricultural ecosystems, there exists a trophic cascade in which bats reduce insect abundance, which leads to an increase in crop production.  Armed with this knowledge, and a recent finding by Tim Divoll that some bats eat insects that defoliate oak and hickory trees, Elizabeth Beilke and Joy O’Keefe decided to explore how important bats were in forested ecosystems.  Does a similar trophic cascade exist, in which bats reduce herbivorous insect abundance, which leads to an increase in tree production?

To explore the trophic cascade hypothesis, Beilke and O’Keefe set up a three-year experiment (during 2018 – 2020) in the Yellowwood State Forest in Indiana, USA. They built 6 X 7 X 7 meter exclosures that were covered with nylon-mesh netting large enough to allow most insects but small enough to exclude bats. 

Each experimental unit was a control exclosure without netting, and an experimental exclosure in which the netting was raised during the night to exclude bats, and lowered during the day so that birds could forage.  This allowed the researchers to attribute any treatment effects exclusively to nocturnal animals – basically bats.  They set up seven pairs of exclosures each year; unfortunately one exclosure was destroyed when three trees fell on it during a violent storm. Within each exclosure Beilke and O’Keefe monitored 9 or 10 oak and hickory seedlings during the treatment period.  They counted the number of insects on oak and hickory leaves in May, when the enclosures were set up, and August, when they were taken down.

Did bat exclusion increase insect density?  The answer is a resounding “yes” with bat exclusion associated with a 300% increase in insect density in comparison to control plots.

Most important, did this increase in insect density lead to greater defoliation of the trees?  Beilke and O’Keefe found that both oaks and hickories suffered greater defoliation when bats were excluded.  The impact on oaks was substantially greater than the impact on  hickories. 

There is some evidence that bats tend to eat more insects that feed on oak trees than insects that feed on hickories. For example, the most common bat in the forest, the eastern red bat, consumed three times more oak-defoliating than hickory-defoliating insect species. Thus bats could be affecting forest composition by preferentially protecting oaks over hickories.  However, given recent declines in bat abundance from white-nose fungus and habitat destruction by humans, losing this protection may be contributing to oak declines in the Eastern United States.

Beilke and O’Keefe point out that bats can negatively influence herbivorous insects directly or indirectly.  Direct effects involve eating herbivorous insects, which are positioned directly on leaves.  Indirect effects can include eating the non-herbivorous adult insects (e.g. butterflies and moths) that produce the herbivorous caterpillars. In addition, some insects are sensitive to the ultrasonic sounds emitted by echolocating bats and may tend to avoid areas populated by bats. Overall, the bat/herbivorous insect/tree trophic cascade results in forests benefitting bats by providing food and places to roost, while bats benefit forested ecosystems by protecting them from herbivory. We now have one more reason to embrace our local bats.

note: the paper that describes this research is from the journal Ecology. The reference is Beilke, E.A. and O’Keefe, J.M., 2023. Bats reduce insect density and defoliation in temperate forests: An exclusion experiment. Ecology, 104(2): e3903. https://doi.org/10.1002/ecy.3903. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2023 by the Ecological Society of America. All rights reserved.