Optimizing Walking Pace to Maximize Snake Detection Rate: A Visual Encounter Survey Experiment

Visual encounter survey efforts can be defined and constrained by duration, distance, or both duration and distance simultaneously. This study examines the optimal walking pace that will maximize the number of animal detections within a limited time frame. We predicted that animal sighting rate per unit of distance would decline with increasing pace, but that maximal sighting rate per unit of time would occur at some intermediate pace because walking faster means covering more ground and passing more animals that could potentially be detected. We conducted a controlled experiment during which we searched for Brown Treesnakes (Boiga irregularis) while walking 220-m-long transects at three different speeds. For transects inside the forest, detection rate per unit distance decreased (nonlinearly) with increasing pace. When considering catch per unit time, however, we found 5% more snakes at the medium pace compared with the slow pace, and 63% more snakes at the fast pace compared with the slow pace. The pattern appeared different for surveys along forest edges with higher vegetation density, in which snakes might be more difficult to spot. Surprisingly, pace had no detectable effect on the body sizes or perch heights of snakes successfully detected. Finding the optimal search pace for a particular study organism in a focal habitat has the potential to substantially increase survey cost-efficacy.