Caretakers versus Exploiters: Impacting Biodiversity in the Age of Humans

The Plight of the Little Brown Bat (a Case Study)

Unfortunately for the approximately 1,300 bat species around the world, there are a growing number of bat species that are in a state of decline for various reasons, primarily habitat loss, changing climate, or the emergence of WNS in North America. “The International Union for the Conservation of Nature (IUCN) currently lists 24 bat species as Critically Endangered, meaning they face an imminent risk of extinction. Fifty-three others are Endangered, and 104 bat species are considered Vulnerable. Bats also are among the most under-studied of mammals. The IUCN lists 226 bat species as “Data Deficient”– there is simply too little information available to determine their conservation status. Of the 1,296 bat species that have been assessed by the IUCN almost a third are considered either threatened (vulnerable, endangered, or critically endangered) or data deficient, indicating the need for more conservation attention to these species.”⁷ “Scientists predict that the once common little brown bat (Myotis lucifugus), will be reduced to just 1% of its pre-WNS population numbers by 2030.”⁸ Events that kill large portions of populations, including natural or human induced disasters, increasingly threaten biodiversity.  Assuming that biodiversity is a good thing and we want to 1) do no harm to an animal species or its habitat and 2) repair and restore the natural state of a species, then we first need to examine the threats that many of these endangered bats have in common.

Habitat Loss

Habitat loss and destruction caused by human activity has had adverse effects on many of the endangered bat species.  Forests around the world have been reduced to make room for farms and cities, the timber harvested for wood.  Mining operations have disturbed bat hibernation zones.  Unlike the little brown bat, some species of bats have very small geographic ranges – some even limited to unique islands.  These species are even more susceptible to habitat loss due to human encroachment.  Not all human and bat interactions are harmful, and much like coyotes and raccoons in the Northeast United States, some animals tend to live side by side with humans quite well.  The big and little brown bats of Connecticut once had a thriving population despite living near humans. 

Climate Change

Bats are sensitive to temperature and any change in climate will undoubtfully result in a shift in the geographic ranges of some bat species.  A study of vampire bats in Latin America has shown that these bats are highly sensitive to low temperatures and as overall temperatures increase, their range will continue to move northward. The movement of common vampire bats into the United States would present ecological, commercial, medical, and educational challenges, including probable impacts on other bat species, the livestock industry and public health concerns. Perhaps the most important impact might be increasingly negative views among the public about all bats; countless beneficial bats are killed throughout their range in mistaken efforts to eliminate vampire bats.

Understanding how and when these bats might expand into the United States should give us time to prepare and educate before problems arrive.⁹  The vampire bats’ response to the changing climate should also give us hints about how the ranges of some other bats might be altered. We expect to see changes in the range and migration timing of some bat species and fear that some species may be lost. The next few decades will be extremely challenging for scientists trying to discern critical impacts of climate change and the rapid alterations in species distributions and interactions that it will cause.¹⁰

In 2002, “overheated flying foxes, panting and frantically fanning themselves with their wings, fell from the trees in New South Wales, Australia.  Up to 3,500 black and grey-headed flying foxes died on the ground beneath their roosts, victims, researchers believe, of heat waves that pushed temperatures to 108 degrees F (42º C). In this era of looming climate change, such scorching temperatures are occurring more often. Tragically so: since 1994, more than 30,000 flying foxes have died in New South Wales, apparently because of at least 19 episodes of extreme heat.”¹¹  Another consideration due to climate change is the impact it has on microbes that exist around bats.  As temperatures change, so can the resistances of potential microbial pathogens. 

Invasive species can be a major contributor to population declines, including invasive pathogens, against which native species can experience high mortality due to a lack of evolved immunity and other defenses.  Introduced fungal pathogens can be particularly dangerous as they can frequently survive in the environment for extended periods, affect a relatively broad range of hosts, and can be highly virulent, thereby driving mass-mortalities of native species of plants and animals.¹² 

White-Nose Syndrome

Figure 4 Little brown bats with visible white fungus syndrome on their noses.

Herein lies the problem for the little brown bat.  They are not endangered necessarily due to habitat destruction, predation, or climate change (although these issues do play a role in species survival).  Despite living in close conditions where the spread of disease is likely, bats have incredibly robust immune systems.  The reason for their drastic decline is due to a fungal disease that has been contact sourced to a cave outside of Albany, NY in 2006 and is spreading across the continent, largely via infected bats.  Pseudogymnoascus destructans (abbreviated P. destructans) is a highly destructive fungal pathogen that has decimated populations of bats, with twelve out of 49 known North American bat species currently affected, and some populations, such as the little brown bat, experiencing observed losses of 90–100% in places where colonies hibernate. The fungal infection is commonly known as white-nose syndrome (WNS) due to the small, white fungal spots that appear around the nose, muzzle, and wings of the bat’s body.¹³  P. destructans is a cold loving fungus that grows within the temperature range 10°C to 20°C (50°F to 68°F), and little brown bats hibernate in caves and structures in that same temperature range. The exact mechanism of death is still being studied, but bats apparently die from secondary physiological complications, mainly depleted fat reserves, associated with too frequent disturbances in their hibernation.  In other words, they wake up more frequently due to irritations caused by the fungus, which in turn depletes their stored energy and they basically starve to death.  Bats affected by WNS register a slightly higher body temperature, indicating an energy depleting fever response.  The fungus also depletes collagen in the bat’s skin and as the fungus grows it replaces the living skin underneath. 

The P. destructans fungus is nothing new.  Evidence from researchers of the P. destructans genome have found different fungal variants in Europe and Asia; however, the variant found in North America seems to have originated in Europe.  So why don’t bats in Europe suffer the same fate with WNS as in North America?  A study published in March 2020 suggests that differences in environmental reservoir dynamics play an important role in population growth rates of bats in both North America and Eurasia. “Data from both native regions where bat populations have coexisted with P. destructans for thousands of years reveal that a crucial difference between stable and declining bat populations is the extent of the environmental reservoir in early winter. The seasonal decay of P. destructans in the environment over the summer in Eurasia leads to a reduced reservoir of P. destructans in early winter, delayed infection, a shorter period of pathogen growth on bats, and lower fungal loads at the end of winter. This means that while most bats eventually become infected by the end of winter across Eurasia, they survive until spring, when they can emerge from hibernation and clear infection. In contrast, bats in WNS-established areas of North America become rapidly infected when they return to an extensive and heavily contaminated environmental reservoir in early winter and, given these longer periods of infection, have higher fungal burdens (and greatly increased mortality).”¹⁴