Evaluation of Predator Attendance at Pitfall Traps in Texas

Researchers using drift-fence sampling with associated pitfall traps have analyzed a number of problems associated with this technique. One unquantified problem is the effect foraging vertebrate predators might have on animals captured in pitfall traps. We used Deer CamH cameras and track-monitoring stations to estimate amount and variety of vertebrate predators attending pitfall arrays in Bastrop and Guadalupe counties, Texas. We recorded 316 photographs of 19 species of vertebrates over 327 camera days among 16 drift-fence arrays. During 1,838 trap nights, we documented 679 individual track sets at track-monitoring stations established on 50 individual pitfall traps and 8 control plots. Ten potential vertebrate predators were documented visiting pitfall arrays with the raccoon (Procyon lotor) being the most frequently recorded species. Statistical analyses indicate that presence of predators at track-monitoring stations or pitfall traps did not influence detectability or capture success of small vertebrates. However, these results could be confounded by the low effect size and reduced power due to low number of animals captured in pitfall traps during the study. Consistent and frequent visits by predators to pitfall traps indicate that risks exist for confined animals and the potential consequences increase for rare or endangered taxa, which potentially could be exposed to higher levels of predation when confined to pitfall traps. RESUMEN—Investigadores que usan muestreos de cercas de deriva y trampas de hoyo han analizado algunos problemas con la técnica. Un problema no cuantificado es el efecto que los vertebrados depredadores pueden tener en los animales capturados en las trampas. Utilizamos cámaras Deer CamH y estaciones de huellas para estimar la cantidad y diversidad de depredadores que visitaron las trampas en los condados de Bastrop y Guadalupe, Texas. Registramos 19 especies de vertebrados en 316 fotografı́as durante 327 cámara-noches en 16 series de cercas de deriva. Durante 1838 trampa-noches, documentamos 679 juegos de huellas individuales en 50 trampas y en 8 sitios control. Se documentaron 10 depredadores potenciales visitando las trampas, siendo el mapache (Procyon lotor) la especie más frecuente. El análisis estadı́stico indica que la presencia de los depredadores en las trampas o en las estaciones de huellas no afectó significativamente a la detectabilidad o al éxito de captura de pequeños vertebrados. Sin embargo, estos resultados pueden estar sesgados debido al bajo tamaño del efecto y el poder estadı́stico reducido por la muestra pequeña de animales capturados durante el estudio. La frecuente y consistente visita de depredadores a las trampas de hoyo indica un riesgo potencial para los animales capturados. Las posibles consecuencias son más graves cuando los taxones capturados son raros o están en peligro de extinción, ya que pueden estar expuestos a niveles mayores de depredación durante su confinamiento en las trampas. Pitfall traps with or without associated terrestrial drift fences remain a commonly applied technique in sampling small terrestrial vertebrates (Shoop, 1965; Stenhouse, 1985; Sutton et al., 1999; Jenkins et al., 2003). Several studies have evaluated their effectiveness and reported problems with the method (Brown, 1997; Crosswhite et al., 1999) including mortality of trapped animals (Yunger et al., 1992; Enge, 2001). Mortality factors associated with pitfall trapping include desiccation (Jenkins et al., 2003), drowning (Aubry and Stringer, 2000), starvation (Yunger et al., 1992), exposure (Padget-Flohr and Jennings, 2001), and predation among animals within the trap (Dodd and Scott, 1994). However, predation upon animals caught in pitfall traps by foraging vertebrate predators rarely is mentioned. Most direct predation events mentioned in the literature are anecdotal and the majority address predation within pitfall traps by trapped mammals such as shrews (Jenkins et al., 2003) or minor disturbances to The Southwestern Naturalist swna-53-04-04.3d 9/10/08 13:12:53 450 Cust # CLG-05 THE SOUTHWESTERN NATURALIST 53(4):450–457 DECEMBER 2008