Big Bend Bats: Diversity and Community Structure

Looking out across the high elevation forests of the Chisos Mountains, Big Bend National Park.

K. Demere

From the snaking riparian areas of the Rio Grande, to the arid lowlands of the Chihuahuan Desert and the high elevation forests of the Chisos Mountains, Big Bend National Park is home to a unique assortment of mammalian life, including 22 species of bats that take full advantage of the habitat gradient (Ammerman et al. 2012).

Early efforts to determine the true diversity of these nocturnal winged occupants of Big Bend were led by Dr. David Easterla, Northwest Missouri State University, from 1967 to 1971 and not only targeted the 801,200 acres of the national park, but extended into the surrounding areas of Mexico. During these leading surveys, Easterla was able to capture 2,670 bats and document a total of 18 species providing a strong foundation for describing the chiropteran diversity within the park (Easterla 1973 a, b).

 

Dr. Loren Ammerman (middle) leads net setup along Terlingua Creek in Big Bend National Park.

Image provided by L. Ammerman.

Twenty-five years after Easterla’s initial surveys, Dr. Loren Ammerman of Angelo State University also took an interest in the bats of Texas and the Big Bend region. As a result, Ammerman has led annual excursions, spanning all months, to Big Bend National Park with the objective of expanding upon our general knowledge of bats, their genetics, roosting ecology, and diet since 1996.

 

Three species of bats (Left Townsends Big-eared Bat, Corynorhinus townsendii; top right: Hoary Bat, Lasiurus cinereus; bottom right: Big Brown Bat, Eptesicus fuscus) known from Big Bend National Park.

Images provided by K. Demere and L. Ammerman

Current Research

Analysis of data collected from 1996 to 2013 demonstrates an extensive survey effort by Ammerman and her students, whom at that point had captured approximately 5,700 bats of 21 species over 258 nights and across 31 survey sites in Big Bend National Park and 10 sites outside of park boundaries (Ammerman and Adams). When supplemented with the initial species diversity logged by Easterla, the number of bats known to occur in the park was officially raised from 18 to 22 species, just 5 shy of the total 27 species found within the greater Chihuahuan Desert region (Ammerman et al. 2012). Species that were reported in the park for the first time by Ammerman and her team include the Western Yellow Bat (Lasiurus xanthinus), the Silver Haired Bat (Lasionycteris noctivagans), and the Tri-colored Bat (Perimyotis subflavus). A full list of bat species documented within the park can be found in the Mammal Checklist (Big Bend Natural History Association 2019).

Although new species have been documented since Easterla’s initial surveys, statistical review of the two survey efforts indicate a high level of bat community similarity spanning from 1967 to 2013 and suggest a relatively stable population for species commonly found within the park (Ammerman and Adams).

Efforts to survey bats and their overall community structure within Big Bend National Park have continued since 2013 and are on-going. Not only will the vast amount data collected by Easterla and Ammerman be a valuable resource for understanding bats within the park, but the active continuation of survey efforts could prove to be our best tool for understanding the impact of a disease deadly to bats, White-nose Syndrome (WNS).

 

White-nose syndrome occurrence map by year (2019). Data last updated 8.30.19.

www.whitenosesyndrome.org

White-Nose Syndrome

The fungus, Pseudogymnoascus destructans, known to cause WNS was detected in Texas for the first time in early 2017 within 6 counties in the Texas panhandle (Texas Parks and Wildlife Department 2017). Since initial discovery, the fungus has now been detected in 21 counties and on four species including the Cave Myotis (Myotis velifer), Tri-colored Bat (Perimyotis subflavus), Townsend’s Big-eared Bat (Corynorhinus townsendii), and Mexican Free-tailed Bats (Tadarida brasiliensis) – all bats that occur in Big Bend National Park. To date, detections of the fungus in Frio and Victoria Counties are the most southern detections of the fungus in the country and detection within Val Verde County is the closest known proximity of the fungus to Big Bend National Park.

Uncertainty and concern for Texas bats is high. Since the discovery of the disease in 2006, WNS has continued to spread at an alarming rate across the eastern and central portions of the United States killing an estimated 6.7 million bats between 2006 and 2011 alone. No diagnostic symptoms or casualties of WNS have been documented within Texas; however, the initial lag in the disease itself is not unexpected as the symptoms could take several years to develop. Five species of bats found within the park, the Big Brown Bat (Eptesicus fuscus), Tri-colored Bat, Cave Myotis, Long-legged Bat (Myotis volans), and the Yuma Myotis (Myotis yumanensis) are known to experience full symptoms of the disease, while other relevant species within the park like the Silver-haired Bat (Lasionycteris noctivagans), Western small-footed Bat (Myotis ciliolabrum), Townsend’s Big-eared Bat and the Mexican Free-tailed bat have all been found with low levels of fungus on fur or wings but they are not known to succumb to WNS. The risks for endangered Mexican long-nosed bats (Leptonycteris nivalis) if the cold-loving fungus were to be introduced into their cave roost is unpredictable. The long-term monitoring of community bat structure and the documentation of the relative abundance of these species, and other Myotis species that are not found within the current WNS infected areas, in Big Bend could prove to be the best tool for managers in Texas to assess pre- and post- WNS population counts and species composition should the disease and its impacts make its way into Big Bend National Park.

Literature Cited

Ammerman, L. K. and E. R. Adams. 2014 Community Structure and Population Trends for Bats in Big Bend National Park, Texas Over the Last 18 Years. Southwestern Association of Naturalists, Poster presentation.

Ammerman, L. K., C. L. Hice and D. J. Schmidly. 2012. Bats of Texas. Texas A&M University Press, College Station, Texas.

Easterla, D. A. 1973a. Ecology of the 18 species of Chiroptera at Big Bend National Park, Texas. Part I. The Northwest Missouri State University Studies, 34: 1-53.

Easterla, D. A. 1973b. Ecology of the 18 species of Chiroptera at Big Bend National Park, Texas. Part II. The Northwest Missouri State University Studies, 34:54-165.

Mammal Checklist: Big Bend National Park, Rio Grande Wild, and Scenic River. Revised May 2019. Big Bend National History Association in cooperation with National Park Service.

Texas Parks and Wildlife Department. 2017. Fungus that Causes White-nose Syndrome in Bats Detected in Texas https://tpwd.texas.gov/newsmedia/releases/?req=20170323c

Article written by Krysta Demere
Photos by Krysta Demere and Loren Ammerman
Angelo State University, Texas