Bat benefits

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

A vampire bat flies through the night. Credit: Uwe Schmidt, CC BY-SA 4.0 <

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?

Underside of oak leaf showing caterpillars hard at work. Credit: Lis Kernan.

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. 

Researchers set up a bat exclosure in the forest. A series of ropes and pulleys allowed them to raise the netting each evening and take it down in the morning. Credit: Elizabeth Beilke.

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.

Basic experimental design. Control plots allowed both bats and birds, while experimental plots allowed birds but excluded bats.

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.

Mean number of insects (+95% confidence intervals) per seedling at the beginning of the field season (left Figure a) and at the end of the field season (right Figure B). Gray dots represent data generated by a statistical model.

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. 

(Figure a – left) Mean defoliation when bats were permitted (top) and excluded (bottom). (Figure b – middle) Mean (+95% Confidence interval) defoliation when bats were permitted (control) or excluded from oak trees. (Figure c – right) Mean (+95% Confidence interval) defoliation when bats were permitted (control) or excluded from hickory trees.

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. Ecology104(2): e3903 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.

Fungal fiasco for furry flying friends

Because they are nocturnal, relatively quiet (to our ears), and in general, not very large, most people don’t realize how abundant and diverse bats are. Bats make up about 20% of all mammal species. They are ecologically critical in their roles as insect predators, pollinators and seed dispersers. Unfortunately, bats in the eastern United States and Canada are under siege by the fungus Pseudogymnoascus destructans (Pd), which has killed several million bats in the eastern United States and Canada since its emergence in 2006.


Bats hibernating in Aeolus Cave (Vermont) in 2009, prior to fungal induced die-off. Credit: Joel Flewelling.


The same location one year later. Credit: Joel Flewelling.

Bats are infected when they return to their caves and mines to hibernate. The fungus invades their skin, creating white fungal patches on the muzzle and ears, and disrupting hibernation patterns with consequent high overwintering mortality for several species. The disease is called white-nose syndrome (WNS)


Two little brown bats (Myotis lucifugous) with white-nose syndrome. Credit: Alan C. Hicks.

Winifred Frick has been studying bats for 17 years. She and her colleagues are trying to determine the long-term prognosis for WNS in North American bat populations. They are interested in several related questions. First, how is WNS spreading in North America? Second, are some individuals, or species, tolerant of the fungus, and thus able to sustain infections without dying? Third, is there any evidence for the evolution of resistance, in which some individuals can fight off the infection, and thus carry reduced fungal loads?

Thirty ecologists and even more research assistants throughout the United States and Canada collaborated in this study, collecting tissue from many thousands of bats, and suspected fungal samples from 79 cave walls. This team of researchers used molecular biology techniques (quantitative PCR) to estimate fungal loads. The map and data below summarize some of the findings.


The map on the left shows the spread of WNS over the past 9 years (see key below map). The eight graphs show colony size in red, using the left y-axis, and Pd load in log10 attograms (1 attogram (ag) = 10-18grams), using the right y-axis. Thus, for example, a Pd load value of 5 = 100,000 ag, while a Pd load value of 4 = 10,000 ag.

The first point is that WNS was first detected in New York (black patch with arrow # 1), and quickly spread throughout the Appalachian Mountains in New England, north into Canada, and south into Virginia and West Virginia. More recently WNS has spread further west, and most disturbingly (not pictured) it was also found in the state of Washington in 2016.

On a slightly brighter note, populations of two species, Myotis lucifugus and Perimyotis subflavus, are showing evidence of resistance. For example, two Myotis lucifugus populations (1 and 2 on the map and graph) have reversed their initial sharp declines and are showing significant recovery (red dots). While all sampled individuals still have numerous Pd parasites (open circles on the graphs), the average fungal load has dropped sharply in several populations in recent years (blue dots), indicating the development of resistance.

But there is still a huge reason for concern. For example, consider the northern long-eared bat, Myotis septentrionalis.  WNS spreads very rapidly and fungal loads climb to unsustainable levels among individuals of this species, usually leading to complete extirpation within three years of the first Pd infection at any site. This bat has disappeared from 69% of its caves, and is now endangered in Canada, and is being considered for protection under the United States endangered species act.


Lone Myotis septentrionalis with WNS. Credit: Alan C. Hicks.

The question becomes, what can we do about white-nose syndrome? This disease is particularly pernicious, because samples from cave walls indicate that the fungus can persist outside the host for extended periods of time. So even if populations crash, there is still a reservoir of infection waiting to attack any bats that might move into a cave. Frick suggests that we need to think broadly about conservation efforts that might help the bats, particularly in areas where they are developing tolerance or resistance. She recommends identifying and protecting habitat that contains suitable hibernacula during the winter, and rich foraging sites and appropriate roosts for the rest of the year.

note: the paper that describes this research is from the journal Ecology. The reference is Frick, Winifred F., Tina L. Cheng, Kate E. Langwig, Joseph R. Hoyt, Amanda F. Janicki, Katy L. Parise, Jeffrey T. Foster, and A. Marm Kilpatrick (2017). Pathogen dynamics during invasion and establishment of white‐nose syndrome explain mechanisms of host persistence. Ecology 98(3): 624-631.Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.