Meeting Abstract

15.6  Jan. 4  Lessons in history: colony size and population decline of Brazilian free-tailed bats at Carlsbad Caverns HRISTOV, N.I.*; BETKE, M; KUNZ, T.H.; Boston University; Boston University; Boston University hristov@bu.edu

The colony of Brazilian free-tailed bats (Tadarida brasiliensis) at Carlsbad Caverns is one of several well-known colonies of this highly gregarious and conspicuous species in North America. For over 80 years researchers have attempted to estimate the size of this colony with mixed results. Primitive methods and lack of repeatability have resulted in questionable estimates giving rise to poorly understood but highly popularized long-term trends. In this study, we present the most accurate and complete, seasonal, colony size estimates to date, based on a new census method – advanced thermal infrared imaging and computer vision processing. The size of the colony was estimated monthly from March through October in 2005 and 2006. Our results indicate large changes in the size of the colony within the same season and between seasons. Colony size estimates range from 67,602 to 793,838 bats, values that are lower than historical estimates for this location. In addition, consecutive daily estimates show large fluctuations in the size of the colony by as many as 290,000 individuals indicating that the colony is considerably more dynamic than previously suggested. Using realistic 3D simulations, paired with additional quantitative analyses of bat emergence behavior, we raise questions about the validity of early historic estimates that millions of bats once roosted in this cave, and prompt a reevaluation of the long-term pattern of decline that has been suggested for this species. The answer to these questions requires accurate, base-line data that incorporate seasonal and long-term observations. Thermal infrared imaging and computer vision processing provide a highly effective and reliable method for the accumulation of such data as has been demonstrated in the present study.