Estimates and Effects of Vertebrate Predation on Drift Fence Associated Pitfall Traps
Ferguson, Adam W.
Drift fence sampling with associated pitfall traps is a technique used to assess small vertebrate communities among a variety of taxa. Researchers using this trapping technique have analyzed a number of factors from drift fence design to associated problems. One potential problem often overlooked is the effect foraging vertebrate predators might have on animals captured in pitfall traps. Removal of animals from pitfalls by vertebrate predators might lead researchers to inaccurate conclusions regarding the composition of small vertebrate animal communities and their populations. Motion-sensitive cameras and track monitoring stations were used to estimate the amount and variety of vertebrate predators attending a series of 18 pitfall arrays monitoring the endangered Houston toad (Bufo houstonensis) on the Griffith League Ranch in Bastrop County, Texas. Ten potential vertebrate predators were documented visiting pitfall arrays with the northern raccoon (Procyon lotor) being the most frequently recorded visitor for both the motion-sensitive cameras (41.8% of total photographs) and track monitoring stations (64.5% of total tracks recorded). Proportions of pitfalls with tracks and animals present were compared to the proportion of pitfall traps without tracks and animals using a 95% confidence interval of two proportions and nominal logistic regression analyses. No statistical difference was detected in number of animals captured in pitfalls visited by predators verse number of animals captured in pitfalls without predator activity (95% Cl = -0.026 < Pr P2 < 0.035; r2= 0.0012, p = 0.2712). Although the presence of predators did not seem to affect the overall capture success of pitfall traps, the frequency of predator visits to pitfalls indicated risks still existed for confined animals. This risk increases when dealing with rare or endangered taxa, which could potentially be exposed to higher levels of predation when confined to pitfall traps. The generally low capture success of 18 pitfall arrays during my study and the difficulty in addressing direct predation might have limited my ability to detect the true effects of predators on drift fence sampling. Future studies should focus on different monitoring techniques such as infrared video camera systems and other ways to estimate the loss of individual animals from pitfall traps. Additionally, precautionary measures should be encouraged for researchers using pitfall sampling to survey rare or endangered taxa.
predation, drift fences, vertebrates, Guadalupe County
Ferguson, A. W. (2005). Estimates and effects of vertebrate predation on drift fence associated pitfall traps (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.