Changing Night Skies: How an Exotic Disease is Decimating America’s Backyard Bats

By David Jachowski 

    Across the eastern half of the US, the bats you see in the night skies this spring and summer have dramatically changed from the ones you saw less than a decade ago.  The little brown bat, the once common dusk-time companion that harmlessly fluttered in millions over American homes has undergone a 91% decline.  Other species, like the endangered Indiana bat, have now gone from modest steps toward recovery back to the doorstep of extinction.

    The culprit is a disease, white-nose syndrome.  First documented in New York in 2006, over the short span of seven years the disease has been spreading south and west across the US, reaching 25 US states and 5 Canadian provinces and already killing more than 5.7 million bats according to the U.S. Fish and Wildlife Service.

    The disease is caused by a fungus that was introduced and now resides within infected caves where it is acquired when the bats enter for the winter.  And because bats roost communally in caves, they often are all exposed once the disease arrives.  The disease itself, in addition to causing the tell-tale white-nose on infected bats, causes damage to wings and reduction in overwinter energy stores.  The end result is massive (often over 90%) overwinter mortality in bat populations in infected caves within as little as a single year. 

    But what about the half of North American bat species that are non-cave roosters?  These species that overwinter in trees or attics or migrate south for the winter and otherwise avoid caves have yet to be found carrying the disease.   Are these non-cave roosters taking advantage of the night skies that formerly were dominated by little brown bats and other cave-roosting species? 

    Rather than a free-for-all, researchers have already shown that the night skies are highly structured and that bats typically avoid other bats while out foraging.  Bats tend to feed in other areas or become active at different times in the evening to avoid competition and interfering with each other’s echolocation calls (termed acoustical jamming).  For example, little brown bats like to swarm around wetlands and to be active at dusk, eating mayflies and other small, often soft-bodied insects; big browns forage deeper in the forest on harder-shelled beetles and moths.  Each bat species in the night skies has a unique set of adaptations allowing it to exploit a separate potential niche.  To see this you only have to open your field guide so you can you can see the different wing morphologies that allow the bats to forage in different ways, similar to the design of aircraft wings, some ideal for maneuvering dense forests while others ideal for speed in open wetlands.  

    I recently worked with researchers at Virginia Tech, U.S. Forest Service and Ft. Drum Military Installation in New York where they have been monitoring bats for over a decade to address the question:  What happens when the cave-roosting species die off and competition for space in the night skies of New York has gone away?  Results from our investigations, soon to be published in the journal, Diversity and Distributions reveal that following the arrival of the disease and crash in formerly dominant little brown bat populations, remnant bats primarily composed of non-cave roosters were shifting their activity to the early evening hours.  Also, several bat species were more active in the wetland areas formerly dominated by little brown bats.  It was evident that a massive re-shifting of bats was occurring in the night skies of a post-white-nose syndrome world. 

    However, despite the passage of over 5 years since arrival of the disease, we are not seeing more non-cave roosters on the landscape than prior to the disease arriving.  The non-cave roosters have not yet translated the behavioral shift in using areas where resources are likely more plentiful into higher survival and reproductive success.  So the non-impacted, non-cave roosting bat species are shifting when and where they are active, but so far they are not responding in sufficient numbers to have the same impact that the cave-roosters were having on the landscape.  This means that millions of insects will no longer be consumed, ranging from moths to garden pests and mosquitos. 

    Perhaps more troublesome, the non-cave roosting, often migratory bats are increasingly threatened by their own set of factors, including a recent push for wind energy development.  These non-cave roosters have to make long seasonal movements north and south to avoid the cold and take advantage of the summer insect bonanza like so many migratory birds.  But with increases in wind turbine development on their migratory paths, we could be seeing a wicked problem emerging: disease is nearly extirpating cave-roosting bats, and energy development is knocking back the migratory non-cave roosters.  The end result is a dramatic decline and uncertain future for bats in the night skies of North America.

Further reading

Jachowski, D., Dobony, C., Coleman, L., Ford, W., Britzke, E., & Rodrigue, J. (2014). Disease and community structure: white-nose syndrome alters spatial and temporal niche partitioning in sympatric bat species Diversity and Distributions DOI: 10.1111/ddi.12192

Kunz, T. H., Braun de Torrez, E., Bauer, D., Lobova, T., & Fleming, T. H. (2011). Ecosystem services provided by bats. Annals of the New York Academy of Sciences, 1223(1), 1-38.

Thogmartin, W. E., Sanders-Reed, C. A., Szymanski, J. A., McKann, P. C., Pruitt, L., King, R. A., ... & Russell, R. E. (2013). White-nose syndrome is likely to extirpate the endangered Indiana bat over large parts of its range.  Biological Conservation, 160, 162-172.

(Photos courtesy Ryan von Linden/New York Department of Environmental Conservation)