Influence of Surface Area, Water Level and Adjacent Vegetation on Bat Use of Artificial Water Sources

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ABSTRACT.

Reliable natural water sources often are unavailable for bats in semi-arid regions such as the Texas Panhandle. Metal stock tanks commonly are used to supply water to livestock and are used by bats as a water source. It is unknown how surface area, water level and adjacent vegetation influence use of tanks by bats. Infrared video cameras and supplemental infrared lights were used to video tape bat behavior and use of stock tanks in Palo Duro Canyon State Park, Texas. Treatment tanks were set out in pairs approximately 80 m apart in a cross-over design to account for influence of location on use of tanks by bats. Treatments included three sizes of tanks (1.2 m, 1.8 m and 3.0 m in diameter), three levels of cover of adjacent vegetation (no vegetation, light vegetation and heavy vegetation) and two water levels (full and ½ full). The number of bats that passed over 3.0 m and 1.2 m tanks was similar; however, bats drank from large tanks more than small tanks. Passes were similar between the tanks surrounded by light vegetation and tanks with no vegetation, but bats drank more from tanks surrounded by light vegetation. Tanks surrounded by heavy vegetation experienced fewer passes and fewer drinks than tanks without vegetation. Water level had no effect on the number of passes by bats but ½ full tanks were used for drinking less frequently than full tanks. Our research indicates that size of tank, water level in tanks and characteristics of adjacent vegetation influence use of metal livestock tanks by bats. Use of larger tanks, keeping tanks full and managing vegetation around tanks increases use of tanks by bats.

INTRODUCTION

Bats rely on water sources for foraging, reproductive and basic physiological requirements. Water requirements vary depending on the environment. An individual resting bat inhabiting a low humidity region can experience daily evaporative water loss equivalent to 30% of its body mass (Webb and Speakman, 1995). Consequently, a large number of bat species inhabiting drier climates select diurnal roosting sites near water (Rabe et al, 1998). Bats inhabiting wetter regions do not appear to select habitat based on proximity to water (Betts, 1998; Walthen et al, 2000). In addition to environment, water requirements also may vary widely depending on reproductive status. Daily water flux values double in lactating big brown bats compared to pregnant individuals (Kurta et ai, 1990). Adams and Hayes (2008) observed lactating female fringed myotis drinking from water sources an average of 21.3 times per night, whereas non-reproductive females visited only 3.7 times per night.

Large bat colonies thrive throughout arid and semi-arid habitats of the southwestern United States despite their dependence on increasingly unreliable water sources in harsh environments. Climate change models predict increasing temperatures and decreased precipitation throughout much of the southern Great Plains will result in lower soil water content and extensive drought (Morgan et ai, 2008). These trends predicted for the southern Great Plains are further exacerbated by direct extraction of water sources. The greatest amount of water withdrawn from surface and groundwater sources in the Great Plains states occurs in Texas. In one of the drier regions of the state, the semi-arid Texas Panhandle, the natural surface water sources (playa lakes) are declining due to agriculture. The Llano Estacado lies above the Ogallala Aquifer, however water for crop irrigation can be pumped less expensively from playas than the aquifer (Haukos and Smith, 1992; Cunfer, 2005) . Over 70% of playas greater than 4 ha have been modified by deepening pits to concentrate water into a smaller surface area (Haukos and Smith, 1992). The shallow depressions of the playas are filled by eroding top soil as well as deliberate filling to increase the amount of land that can be cultivated (Proctor, 1990).

Although natural water sources in these regions are becoming increasingly scarce, over 265 million acres or 70% of grasslands in the Great Plains are grazed (Cunfer, 2005). …