Auditory Sampling of Frogs: Detection Efficiency in Relation to Survey Duration

Abstract

Call surveys are used widely to assess distribution and abundance of anurans. The durations of these surveys often are based on convenience rather than on empirical analysis. Knowing how frog detection varies with survey duration is valuable for designing sampling schemes, yet few studies have examined the relationship between survey duration and detection efficiency. We conducted call surveys for frogs in central Texas to assess effects of survey duration on detection efficiency. We controlled analytically for temporal and environmental covariates that had the potential to confound our assessment of survey duration. Cumulative detection efficiency of all species was 94% for 15-min surveys and did not increase appreciably with longer durations up to 30 min. Detection efficiency for number of species was significantly higher for 15-min surveys than it was for 5-min surveys, and the variability of detection efficiency decreased with increasing survey duration. Detection efficiency for number of calling individuals of Acris crepitans and Rana sphenocephala did not differ among 5-, 10-, and 15-min surveys. Of the temporal and environmental covariates examined, only the year in which a survey was conducted was significantly associated with detection efficiency for number of species. None of the covariates was significantly related to detection efficiency for A. crepitans or R. sphenocephala. When sampling resources such as time and personnel are limited, knowledge about detection efficiencies is essential for allocating survey effort.